[page i. ]

THE

POWER OF MOVEMENT

IN

PLANTS. 

[page ii. ]

[page iii. ]

THE

POWER OF MOVEMENT

IN

PLANTS. 

 BY CHARLES DARWIN, LL. D. , F. R. S. 

ASSISTED BY

FRANCIS DARWIN. 

[page iv. ]

[page v. ]

CONTENTS. 

-----

INTRODUCTION... Page 1-9. 

CHAPTER I. 

THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS. 

Brassica oleracea, circumnutation of the radicle, of the arched hypocotylwhilst still buried beneath the ground, whilst rising above the ground andstraightening itself, and when erect--Circumnutation of the cotyledons--Rate of movement--Analogous observations on various organs in species ofGithago, Gossypium, Oxalis, Tropaeolum, Citrus, Aesculus, of severalLeguminous and Cucurbitaceous genera, Opuntia, Helianthus, Primula, Cyclamen, Stapelia, Cerinthe, Nolana, Solanum, Beta, Ricinus, Quercus, Corylus, Pinus, Cycas, Canna, Allium, Asparagus, Phalaris, Zea, Avena, Nephrodium, and Selaginella... 10-66

CHAPTER II. 

GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS. 

Generality of the circumnutating movement--Radicles, their circumnutationof service--Manner in which they penetrate the ground--Manner in whichhypocotyls and other organs break through the ground by being arched--Singular manner of germination in Megarrhiza, etc. --Abortion of cotyledons--Circumnutation of hypocotyls and epicotyls whilst still buried and arched--Their power of straightening themselves--Bursting of the seed-coats--Inherited effect of the arching process in hypo-[page vi. ]gean hypocotyls--Circumnutation of hypocotyls and epicotyls when erect--Circumnutation of cotyledons--Pulvini or joints of cotyledons, duration oftheir activity, rudimentary in Oxalis corniculata, their development--Sensitiveness of cotyledons to light and consequent disturbance of theirperiodic movements--Sensitiveness of cotyledons to contact... Page 67-128

CHAPTER III. 

SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS. 

Manner in which radicles bend when they encounter an obstacle in the soil--Vicia faba, tips of radicles highly sensitive to contact and otherirritants--Effects of too high a temperature--Power of discriminatingbetween objects attached on opposite sides--Tips of secondary radiclessensitive--Pisum, tips of radicles sensitive--Effects of such sensitivenessin overcoming geotropism--Secondary radicles--Phaseolus, tips of radicleshardly sensitive to contact, but highly sensitive to caustic and to theremoval of a slice--Tropaeolum--Gossypium--Cucurbita--Raphanus--Aesculus, tip not sensitive to slight contact, highly sensitive to caustic--Quercus, tip highly sensitive to contact--Power of discrimination--Zea, tip highlysensitive, secondary radicles--Sensitiveness of radicles to moist air--Summary of chapter... 129-200

CHAPTER IV. 

THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS. 

Circumnutation of stems: concluding remarks on--Circumnutation of stolons:aid thus afforded in winding amongst the stems of surrounding plants--Circumnutation of flower-stems--Circumnutation of Dicotyledonous leaves--Singular oscillatory movement of leaves of Dionaea--Leaves of Cannabis sinkat night--Leaves of Gymnosperms--Of Monocotyledons--Cryptogams--Concludingremarks on the circumnutation of leaves; generally rise in the evening andsink in the morning... 201-262[page vii. ]

CHAPTER V. 

MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTICMOVEMENTS. 

Circumnutation modified through innate causes or through the action ofexternal conditions--Innate causes--Climbing plants; similarity of theirmovements with those of ordinary plants; increased amplitude; occasionalpoints of difference--Epinastic growth of young leaves--Hyponastic growthof the hypocotyls and epicotyls of seedlings--Hooked tips of climbing andother plants due to modified circumnutation--Ampelopsis tricuspidata--Smithia Pfundii--Straightening of the tip due to hyponasty--Epinasticgrowth and circumnutation of the flower-peduncles of Trifolium repens andOxalis carnosa... Page 263-279

CHAPTER VI. 

MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEPOF COTYLEDONS. 

Preliminary sketch of the sleep or nyctitropic movements of leaves--Presence of pulvini--The lessening of radiation the final cause ofnyctitropic movements--Manner of trying experiments on leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea and on the cotyledons ofMimosa--Concluding remarks on radiation from leaves--Small differences inthe conditions make a great difference in the result - Description of thenyctitropic position and movements of the cotyledons of various plants--List of species--Concluding remarks--Independence of the nyctitropicmovements of the leaves and cotyledons of the same species--Reasons forbelieving that the movements have been acquired for a specialpurpose... 280-316

CHAPTER VII. 

MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES. 

Conditions necessary for these movements--List of Genera and Families, which include sleeping plants--Description of the movements in the severalGenera--Oxalis: leaflets folded at[page viii. ]night--Averrhoa: rapid movements of the leaflets--Porlieria: leaflets closewhen plant kept very dry--Tropaeolum: leaves do not sleep unless wellilluminated during day--Lupinus: various modes of sleeping--Melilotus:singular movements of terminal leaflet--Trifolium--Desmodium: rudimentarylateral leaflets, movements of, not developed on young plants, state oftheir pulvini--Cassia: complex movements of the leaflets--Bauhinia: leavesfolded at night--Mimosa pudica: compounded movements of leaves, effect ofdarkness--Mimosa albida, reduced leaflets of--Schrankia: downward movementof the pinnae--Marsilea: the only cryptogam known to sleep--Concludingremarks and summary--Nyctitropism consists of modified circumnutation, regulated by the alternations of light and darkness--Shape of first trueleaves... Page 317-417

CHAPTER VIII. 

MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT. 

Distinction between heliotropism and the effects of light on theperiodicity of the movements of leaves--Heliotropic movements of Beta, Solanum, Zea, and Avena--Heliotropic movements towards an obscure light inApios, Brassica, Phalaris, Tropaeolum, and Cassia--Apheliotropic movementsof tendrils of Bignonia--Of flower-peduncles of Cyclamen--Burying of thepods--Heliotropism and apheliotropism modified forms of circumnutation--Steps by which one movement is converted into the other--Transversal-heliotropismus or diaheliotropism influenced by epinasty, theweight of the part and apogeotropism--Apogeotropism overcome during themiddle of the day by diaheliotropism--Effects of the weight of the bladesof cotyledons--So called diurnal sleep--Chlorophyll injured by intenselight--Movements to avoid intense light... 418-448

CHAPTER IX. 

SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS. 

Uses of heliotropism--Insectivorous and climbing plants not heliotropic--Same organ heliotropic at one age and not at another--Extraordinarysensitiveness of some plants to light--The effects[page ix. ]of light do not correspond with its intensity--Effects of previousillumination--Time required for the action of light--After-effects oflight--Apogeotropism acts as soon as light fails--Accuracy with whichplants bend to the light--This dependent on the illumination of one wholeside of the part--Localised sensitiveness to light and its transmittedeffects--Cotyledons of Phalaris, manner of bending--Results of theexclusion of light from their tips--Effects transmitted beneath the surfaceof the ground--Lateral illumination of the tip determines the direction ofthe curvature of the base--Cotyledons of Avena, curvature of basal part dueto the illumination of upper part--Similar results with the hypocotyls ofBrassica and Beta--Radicles of Sinapis apheliotropic, due to thesensitiveness of their tips--Concluding remarks and summary of chapter--Means by which circumnutation has been converted into heliotropism orapheliotropism... Page 449-492

CHAPTER X. 

MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION. 

Means of observation--Apogeotropism--Cytisus--Verbena--Beta--Gradualconversion of the movement of circumnutation into apogeotropism in Rubus, Lilium, Phalaris, Avena, and Brassica--Apogeotropism retarded byheliotropism--Effected by the aid of joints or pulvini--Movements offlower-peduncles of Oxalis--General remarks on apogeotropism--Geotropism--Movements of radicles--Burying of seed-capsules--Use of process--Trifoliumsubterraneum--Arachis--Amphicarpaea--Diageotropism--Conclusion... 493-522

CHAPTER XI. 

LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS. 

General considerations--Vicia faba, effects of amputating the tips of theradicles--Regeneration of the tips--Effects of a short exposure of the tipsto geotropic action and their subsequent amputation--Effects of amputatingthe tips obliquely--Effects of cauterising the tips--Effects of grease onthe tips--Pisum[page x. ]sativum, tips of radicles cauterised transversely, and on their upper andlower sides--Phaseolus, cauterisation and grease on the tips--Gossypium--Cucurbita, tips cauterised transversely, and on their upper and lowersides--Zea, tips cauterised--Concluding remarks and summary of chapter--Advantages of the sensibility to geotropism being localised in the tips ofthe radicles... Page 523-545

CHAPTER XII. 

SUMMARY AND CONCLUDING REMARKS. 

Nature of the circumnutating movement--History of a germinating seed--Theradicle first protrudes and circumnutates--Its tip highly sensitive--Emergence of the hypocotyl or of the epicotyl from the ground under theform of an arch--Its circumnutation and that of the cotyledons--Theseedling throws up a leaf-bearing stem--The circumnutation of all the partsor organs--Modified circumnutation--Epinasty and hyponasty--Movements ofclimbing plants--Nyctitropic movements--Movements excited by light andgravitation--Localised sensitiveness--Resemblance between the movements ofplants and animals--The tip of the radicle acts like a brain... 546-573

INDEX... 574-593

[page 1]

THE MOVEMENTS OF PLANTS. 

INTRODUCTION. 

THE chief object of the present work is to describe and connect togetherseveral large classes of movement, common to almost all plants. The mostwidely prevalent movement is essentially of the same nature as that of thestem of a climbing plant, which bends successively to all points of thecompass, so that the tip revolves. This movement has been called by Sachs"revolving nutation;" but we have found it much more convenient to use theterms circumnutation and circumnutate. As we shall have to say much aboutthis movement, it will be useful here briefly to describe its nature. If weobserve a circumnutating stem, which happens at the time to be bent, wewill say towards the north, it will be found gradually to bend more andmore easterly, until it faces the east; and so onwards to the south, thento the west, and back again to the north. If the movement had been quiteregular, the apex would have described a circle, or rather, as the stem isalways growing upwards, a circular spiral. But it generally describesirregular elliptical or oval figures; for the apex, after pointing in anyone direction, commonly moves back to the opposite side, not, however, returning along the same line. Afterwards other irregular ellipses or ovalsare successively described, with their longer[page 2]axes directed to different points of the compass. Whilst describing suchfigures, the apex often travels in a zigzag line, or makes smallsubordinate loops or triangles. In the case of leaves the ellipses aregenerally narrow. 

Until recently the cause of all such bending movements was believed to bedue to the increased growth of the side which becomes for a time convex;that this side does temporarily grow more quickly than the concave side hasbeen well established; but De Vries has lately shown that such increasedgrowth follows a previously increased state of turgescence on the convexside. * In the case of parts provided with a so-called joint, cushion orpulvinus, which consists of an aggregate of small cells that have ceased toincrease in size from a very early age, we meet with similar movements; andhere, as Pfeffer has shown** and as we shall see in the course of thiswork, the increased turgescence of the cells on opposite sides is notfollowed by increased growth. Wiesner denies in certain cases the accuracyof De Vries' conclusion about turgescence, and maintains*** that theincreased extensibility of the cell-walls is the more important element. That such extensibility must accompany increased turgescence in order thatthe part may bend is manifest, and this has been insisted on by severalbotanists; but in the case of unicellular plants it can hardly fail to bethe more important element. On the whole we may at present conclude thatin-

* Sachs first showed ('Lehrbuch, ' etc. , 4th edit. P. 452) the intimateconnection between turgescence and growth. For De Vries' interesting essay, 'Wachsthumskrümmungen mehrzelliger Organe, ' see 'Bot. Zeitung, ' Dec. 19, 1879, p. 830. 

** 'Die Periodischen Bewegungen der Blattorgane, ' 1875. 

*** 'Untersuchungen über den Heliotropismus, ' Sitzb. Der K. Akad. DerWissenschaft. (Vienna), Jan. 1880. 

[page 3]creased growth, first on one side and then on another, is a secondaryeffect, and that the increased turgescence of the cells, together with theextensibility of their walls, is the primary cause of the movement ofcircumnutation. *

In the course of the present volume it will be shown that apparently everygrowing part of every plant is continually circumnutating, though often ona small scale. Even the stems of seedlings before they have broken throughthe ground, as well as their buried radicles, circumnutate, as far as thepressure of the surrounding earth permits. In this universally presentmovement we have the basis or groundwork for the acquirement, according tothe requirements of the plant, of the most diversified movements. Thus, thegreat sweeps made by the stems of twining plants, and by the tendrils ofother climbers, result from a mere increase in the amplitude of theordinary movement of circumnutation. The position which young leaves andother organs ultimately assume is acquired by the circumnutating movementbeing increased in some one direction. The leaves of various plants aresaid to sleep at night, and it will be seen that their blades then assume avertical position through modified circumnutation, in order to protecttheir upper surfaces from being chilled through radiation. The movementsof various organs to the light, which are so general throughout thevegetable kingdom, and occasionally from the light, or transversely withrespect to it, are all modified

* See Mr. Vines' excellent discussion ('Arbeiten des Bot. Instituts inWürzburg, ' B. II. Pp. 142, 143, 1878) on this intricate subject. Hofmeister's observations ('Jahreschrifte des Vereins für Vaterl. Naturkunde in Würtemberg, ' 1874, p. 211) on the curious movements ofSpirogyra, a plant consisting of a single row of cells, are valuable inrelation to this subject. 

[page 4]forms of circumnutation; as again are the equally prevalent movements ofstems, etc. , towards the zenith, and of roots towards the centre of theearth. In accordance with these conclusions, a considerable difficulty inthe way of evolution is in part removed, for it might have been asked, howdid all these diversified movements for the most different purposes firstarise? As the case stands, we know that there is always movement inprogress, and its amplitude, or direction, or both, have only to bemodified for the good of the plant in relation with internal or externalstimuli. 

Besides describing the several modified forms of circumnutation, some othersubjects will be discussed. The two which have interested us most are, firstly, the fact that with some seedling plants the uppermost part aloneis sensitive to light, and transmits an influence to the lower part, causing it to bend. If therefore the upper part be wholly protected fromlight, the lower part may be exposed for hours to it, and yet does notbecome in the least bent, although this would have occurred quickly if theupper part had been excited by light. Secondly, with the radicles ofseedlings, the tip is sensitive to various stimuli, especially to veryslight pressure, and when thus excited, transmits an influence to the upperpart, causing it to bend from the pressed side. On the other hand, if thetip is subjected to the vapour of water proceeding from one side, the upperpart of the radicle bends towards this side. Again it is the tip, as statedby Ciesielski, though denied by others, which is sensitive to theattraction of gravity, and by transmission causes the adjoining parts ofthe radicle to bend towards the centre of the earth. These several cases ofthe effects of contact, other irritants, vapour, light, and the[page 5]attraction of gravity being transmitted from the excited part for somelittle distance along the organ in question, have an important bearing onthe theory of all such movements. 

[Terminology. --A brief explanation of some terms which will be used, musthere be given. With seedlings, the stem which supports the cotyledons (i. E. The organs which represent the first leaves) has been called by manybotanists the hypocotyledonous stem, but for brevity sake we will speak ofit merely as the hypocotyl: the stem immediately above the cotyledons willbe called the epicotyl or plumule. The radicle can be distinguished fromthe hypocotyl only by the presence of root-hairs and the nature of itscovering. The meaning of the word circumnutation has already beenexplained. Authors speak of positive and negative heliotropism, *--that is, the bending of an organ to or from the light; but it is much moreconvenient to confine the word heliotropism to bending towards the light, and to designate as apheliotropism bending from the light. There is anotherreason for this change, for writers, as we have observed, occasionally dropthe adjectives positive and negative, and thus introduce confusion intotheir discussions. Diaheliotropism may express a position more or lesstransverse to the light and induced by it. In like manner positivegeotropism, or bending towards the centre of the earth, will be called byus geotropism; apogeotropism will mean bending in opposition to gravity orfrom the centre of the earth; and diageotropism, a position more or lesstransverse to the radius of the earth. The words heliotropism andgeotropism properly mean the act of moving in relation to the light or theearth; but in the same manner as gravitation, though defined as "the act oftending to the centre, " is often used to express the cause of a bodyfalling, so it will be found convenient occasionally to employ heliotropismand geotropism, etc. , as the cause of the movements in question. 

The term epinasty is now often used in Germany, and implies that the uppersurface of an organ grows more quickly than the

* The highly useful terms of Heliotropism and Geotropism were first used byDr. A. B. Frank: see his remarkable 'Beiträge zur Pflanzenphysiologie, '1868. [page 6]lower surface, and thus causes it to bend downwards. Hyponasty is thereverse, and implies increased growth along the lower surface, causing thepart to bend upwards. *

Methods of Observation. --The movements, sometimes very small and sometimesconsiderable in extent, of the various organs observed by us, were tracedin the manner which after many trials we found to be best, and which mustbe described. Plants growing in pots were protected wholly from the light, or had light admitted from above, or on one side as the case might require, and were covered above by a large horizontal sheet of glass, and withanother vertical sheet on one side. A glass filament, not thicker than ahorsehair, and from a quarter to three-quarters of an inch in length, wasaffixed to the part to be observed by means of shellac dissolved inalcohol. The solution was allowed to evaporate, until it became so thickthat it set hard in two or three seconds, and it never injured the tissues, even the tips of tender radicles, to which it was applied. To the end ofthe glass filament an excessively minute bead of black sealing-wax wascemented, below or behind which a bit of card with a black dot was fixed toa stick driven into the ground. The weight of the filament was so slightthat even small leaves were not perceptibly pressed down. Another method ofobservation, when much magnification of the movement was not required, willpresently be described. The bead and the dot on the card were viewedthrough the horizontal or vertical glass-plate (according to the positionof the object), and when one exactly covered the other, a dot was made onthe glass-plate with a sharply pointed stick dipped in thick Indian-ink. Other dots were made at short intervals of time and these were afterwardsjoined by straight lines. The figures thus traced were therefore angular;but if dots had been made every 1 or 2 minutes, the lines would have beenmore curvilinear, as occurred when radicles were allowed to trace their owncourses on smoked glass-plates. To make the dots accurately was the soledifficulty, and required some practice. Nor could this be done quiteaccurately, when the movement was much magnified, such as 30 times andupwards; yet even in this case the general course may be trusted. To testthe accuracy of the above method of observation, a filament was fixed to an

* These terms are used in the sense given them by De Vries, 'WürzburgArbeiten, ' Heft ii 1872, p. 252. 

[page 7]inanimate object which was made to slide along a straight edge and dotswere repeatedly made on a glass-plate; when these were joined, the resultought to have been a perfectly straight line, and the line was very nearlystraight. It may be added that when the dot on the card was placedhalf-an-inch below or behind the bead of sealing-wax, and when theglass-plate (supposing it to have been properly curved) stood at a distanceof 7 inches in front (a common distance), then the tracing represented themovement of the bead magnified 15 times. 

Whenever a great increase of the movement was not required, another, and insome respects better, method of observation was followed. This consisted infixing two minute triangles of thin paper, about 1/20 inch in height, tothe two ends of the attached glass filament; and when their tips werebrought into a line so that they covered one another, dots were made asbefore on the glass-plate. If we suppose the glass-plate to stand at adistance of seven inches from the end of the shoot bearing the filament, the dots when joined, will give nearly the same figure as if a filamentseven inches long, dipped in ink, had been fixed to the moving shoot, andhad inscribed its own course on the plate. The movement is thusconsiderably magnified; for instance, if a shoot one inch in length werebending, and the glass-plate stood at the distance of seven inches, themovement would be magnified eight times. It would, however, have been verydifficult to have ascertained in each case how great a length of the shootwas bending; and this is indispensable for ascertaining the degree to whichthe movement is magnified. 

After dots had been made on the glass-plates by either of the abovemethods, they were copied on tracing paper and joined by ruled lines, witharrows showing the direction of the movement. The nocturnal courses arerepresented by straight broken lines. The first dot is always made largerthan the others, so as to catch the eye, as may be seen in the diagrams. The figures on the glass-plates were often drawn on too large a scale to bereproduced on the pages of this volume, and the proportion in which theyhave been reduced is always given. * Whenever it could be approximately toldhow much the movement had been magnified, this is stated. We have perhaps

* We are much indebted to Mr. Cooper for the care with which he has reducedand engraved our diagrams. 

[page 8]introduced a superfluous number of diagrams; but they take up less spacethan a full description of the movements. Almost all the sketches of plantsasleep, etc. , were carefully drawn for us by Mr. George Darwin. 

As shoots, leaves, etc. , in circumnutating bend more and more, first in onedirection and then in another, they were necessarily viewed at differenttimes more or less obliquely; and as the dots were made on a flat surface, the apparent amount of movement is exaggerated according to the degree ofobliquity of the point of view. It would, therefore, have been a muchbetter plan to have used hemispherical glasses, if we had possessed them ofall sizes, and if the bending part of the shoot had been distinctly hingedand could have been placed so as to have formed one of the radii of thesphere. But even in this case it would have been necessary afterwards tohave projected the figures on paper; so that complete accuracy could nothave been attained. From the distortion of our figures, owing to the abovecauses, they are of no use to any one who wishes to know the exact amountof movement, or the exact course pursued; but they serve excellently forascertaining whether or not the part moved at all, as well as the generalcharacter of the movement. ]

In the following chapters, the movements of a considerable number of plantsare described; and the species have been arranged according to the systemadopted by Hooker in Le Maout and Decaisne's 'Descriptive Botany. ' No onewho is not investigating the present subject need read all the details, which, however, we have thought it advisable to give. To save the readertrouble, the conclusions and most of the more important parts have beenprinted in larger type than the other parts. He may, if he thinks fit, readthe last chapter first, as it includes a summary of the whole volume; andhe will thus see what points interest him, and on which he requires thefull evidence. 

Finally, we must have the pleasure of returning our[page 9]sincere thanks to Sir Joseph Hooker and to Mr. W. Thiselton Dyer for theirgreat kindness, in not only sending us plants from Kew, but in procuringothers from several sources when they were required for our observations;also, for naming many species, and giving us information on various points. [page 10]

CHAPTER I. 

THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS. 

Brassica oleracea, circumnutation of the radicle, of the arched hypocotylwhilst still buried beneath the ground, whilst rising above the ground andstraightening itself, and when erect--Circumnutation of the cotyledons--Rate of movement--Analogous observations on various organs in species ofGithago, Gossypium, Oxalis, Tropaeolum, Citrus, Aesculus, of severalLeguminous and Cucurbitaceous genera, Opuntia, Helianthus, Primula, Cyclamen, Stapelia, Cerinthe, Nolana, Solanum, Beta, Ricinus, Quercus, Corylus, Pinus, Cycas, Canna, Allium, Asparagus, Phalaris, Zea, Avena, Nephrodium, and Selaginella. 

THE following chapter is devoted to the circumnutating movements of theradicles, hypocotyls, and cotyledons of seedling plants; and, when thecotyledons do not rise above the ground, to the movements of the epicotyl. But in a future chapter we shall have to recur to the movements of certaincotyledons which sleep at night. 

[Brassica oleracea (Cruciferae)'. --Fuller details will be given withrespect to the movements in this case than in any other, as space and timewill thus ultimately be saved. 

Radicle. --A seed with the radicle projecting . 05 inch was fastened withshellac to a little plate of zinc, so that the radicle stood up vertically;and a fine glass filament was then fixed near its base, that is, close tothe seed-coats. The seed was surrounded by little bits of wet sponge, andthe movement of the bead at the end of the filament was traced (Fig. 1)during sixty hours. In this time the radicle increased in length from . 05to . 11 inch. Had the filament been attached at first close to the apex ofthe radicle, and if it could have remained there all the time, the movementexhibited would have[page 11]been much greater, for at the close of our observations the tip, instead ofstanding vertically upwards, had become bowed downwards through geotropism, so as almost to touch the zinc plate. As far as we could roughly ascertainby measurements made with compasses on other seeds, the tip alone, for alength of only 2/100 to 3/100 of an inch, is acted on by geotropism. Butthe tracing shows that the basal part of the radicle continued tocircumnutate irregularly during the whole time. The actual extreme amountof movement of the bead at the end of the filament was nearly . 05 inch, butto what extent the movement of the radicle was magnified by the filament, which was nearly 3/4 inch in length, it was impossible to estimate. 

Fig. 1. Brassica oleracea: circumnutation of radicle, traced on horizontalglass, from 9 A. M. Jan. 31st to 9 P. M. Feb. 2nd. Movement of bead at end offilament magnified about 40 times. 

Another seed was treated and observed in the same manner, but the radiclein this case protruded . 1 inch, and was notFig. 2. Brassica oleracea: circumnutating and geotropic movement ofradicle, traced on horizontal glass during 46 hours. 

fastened so as to project quite vertically upwards. The filament wasaffixed close to its base. The tracing (Fig. 2, reduced by half) shows themovement from 9 A. M. Jan. 31st to 7 A. M. Feb. 2nd; but it continued to moveduring the whole of the[page 12]2nd in the same general direction, and in a similar zigzag manner. From theradicle not being quite perpendicular when the filament was affixedgeotropism came into play at once; but the irregular zigzag course showsthat there was growth (probably preceded by turgescence), sometimes on oneand sometimes on another side. Occasionally the bead remained stationaryfor about an hour, and then probably growth occurred on the side oppositeto that which caused the geotropic curvature. In the case previouslydescribed the basal part of the very short radicle from being turnedvertically upwards, was at first very little affected by geotropism. Filaments were affixed in two other instances to rather longer radiclesprotruding obliquely from seeds which had been turned upside down; and inthese cases the lines traced on the horizontal glasses were only slightlyzigzag, and the movement was always in the same general direction, throughthe action of geotropism. All these observations are liable to severalcauses of error, but we believe, from what will hereafter be shown withrespect to the movements of the radicles of other plants, that they may belargely trusted. 

Hypocotyl. --The hypocotyl protrudes through the seed-coats as a rectangularprojection, which grows rapidly into an arch like the letter U turnedupside down; the cotyledons being still enclosed within the seed. Inwhatever position the seed may be embedded in the earth or otherwise fixed, both legs of the arch bend upwards through apogeotropism, and thus risevertically above the ground. As soon as this has taken place, or evenearlier, the inner or concave surface of the arch grows more quickly thanthe upper or convex surface; and this tends to separate the two legs andaids in drawing the cotyledons out of the buried seed-coats. By the growthof the whole arch the cotyledons are ultimately dragged from beneath theground, even from a considerable depth; and now the hypocotyl quicklystraightens itself by the increased growth of the concave side. 

Even whilst the arched or doubled hypocotyl is still beneath the ground, itcircumnutates as much as the pressure of the surrounding soil will permit;but this was difficult to observe, because as soon as the arch is freedfrom lateral pressure the two legs begin to separate, even at a very earlyage, before the arch would naturally have reached the surface. Seeds wereallowed to germinate on the surface of damp earth, and after they had fixedthemselves by their radicles, and after the, as yet, only[page 13]slightly arched hypocotyl had become nearly vertical, a glass filament wasaffixed on two occasions near to the base of the basal leg (i. E. The one inconnection with the radicle), and its movements were traced in darkness ona horizontal glass. The result was that long lines were formed running innearly the plane of the vertical arch, due to the early separation of thetwo legs now freed from pressure; but as the lines were zigzag, showinglateral movement, the arch must have been circumnutating, whilst it wasstraightening itself by growth along its inner or concave surface. 

A somewhat different method of observation was next followed:Fig. 3. Brassica oleracea: circumnutating movement of buried and archedhypocotyl (dimly illuminated from above), traced on horizontal glass during45 hours. Movement of bead of filament magnified about 25 times, and herereduced to one-half of original scale. 

as soon as the earth with seeds in a pot began to crack, the surface wasremoved in parts to the depth of . 2 inch; and a filament was fixed to thebasal leg of a buried and arched hypocotyl, just above the summit of theradicle. The cotyledons were still almost completely enclosed within themuch-cracked seed-coats; and these were again covered up with damp adhesivesoil pressed pretty firmly down. The movement of the filament was traced(Fig. 3) from 11 A. M. Feb. 5th till 8 A. M. Feb. 7th. By this latter periodthe cotyledons had been dragged from beneath the pressed-down earth, butthe upper part of the hypocotyl still formed nearly a right angle with thelower part. The tracing shows that the arched hypocotyl tends at this early[page 14]age to circumnutate irregularly. On the first day the greater movement(from right to left in the figure) was not in the plane of the vertical andarched hypocotyl, but at right angles to it, or in the plane of the twocotyledons, which were still in close contact. The basal leg of the arch atthe time when the filament was affixed to it, was already bowedconsiderably backwards, or from the cotyledons; had the filament beenaffixed before this bowing occurred, the chief movement would have been atright angles to that shown in the figure. A filament was attached toanother buried hypocotyl of the same age, and it moved in a similar generalmanner, but the line traced was not so complex. This hypocotyl becamealmost straight, and the cotyledons were dragged from beneath the ground onthe evening of the second day. 

Fig. 4. Brassica oleracea: circumnutating movement of buried and archedhypocotyl, with the two legs of the arch tied together, traced onhorizontal glass during 33 ½ hours. Movement of the bead of filamentmagnified about 26 times, and here reduced to one-half original scale. 

Before the above observations were made, some arched hypocotyls buried atthe depth of a quarter of an inch were uncovered; and in order to preventthe two legs of the arch from beginning to separate at once, they were tiedtogether with fine silk. This was done partly because we wished toascertain how long the hypocotyl, in its arched condition, would continueto move, and whether the movement when not masked and disturbed by thestraightening process, indicated circumnutation. Firstly a filament wasfixed to the basal leg of an arched hypocotyl close above the summit of theradicle. The cotyledons were still partially enclosed within theseed-coats. The movement was traced (Fig. 4) from 9. 20 A. M. On Dec. [page 15]23rd to 6. 45 A. M. On Dec. 25th. No doubt the natural movement was muchdisturbed by the two legs having been tied together; but we see that it wasdistinctly zigzag, first in one direction and then in an almost oppositeone. After 3 P. M. On the 24th the arched hypocotyl sometimes remainedstationary for a considerable time, and when moving, moved far slower thanbefore. Therefore, on the morning of the 25th, the glass filament wasremoved from the base of the basal leg, and was fixed horizontally on thesummit of the arch, which, from the legs having been tied, had grown broadand almost flat. The movement was now traced during 23 hours (Fig. 5), andwe

Fig. 5. Brassica oleracea: circumnutating movement of the crown of a buriedand arched hypocotyl, with the two legs tied together, traced on ahorizontal glass during 23 hours. Movement of the bead of the filamentmagnified about 58 times, and here reduced to one-half original scale. 

see that the course was still zigzag, which indicates a tendency tocircumnutation. The base of the basal leg by this time had almostcompletely ceased to move. 

As soon as the cotyledons have been naturally dragged from beneath theground, and the hypocotyl has straightened itself by growth along the inneror concave surface, there is nothing to interfere with the free movementsof the parts; and the circumnutation now becomes much more regular andclearly displayed, as shown in the following cases:--A seedling was placedin front and near a north-east window with a line joining the[page 16]two cotyledons parallel to the window. It was thus left the whole day so asto accommodate itself to the light. On the following morning a filament wasfixed to the midrib of the larger and taller cotyledon (which enfolds theother and smaller one, whilst still within the seed), and a mark beingplaced close behind, the movement of the whole plant, that is, of thehypocotyl and cotyledon, was traced greatly magnified on a vertical glass. At first the plant bent so much towards the light that it was useless toattempt to trace the movement; but at 10 A. M. Heliotropism almost whollyceased and the first dot was

Fig. 6. Brassica oleracea: conjoint circumnutation of the hypocotyl andcotyledons during 10 hours 45 minutes. Figure here reduced to one-halforiginal scale. 

made on the glass. The last was made at 8. 45 P. M. ; seventeen dots beingaltogether made in this interval of 10 h. 45 m. (see Fig. 6). It should benoticed that when I looked shortly after 4 P. M. The bead was pointing offthe glass, but it came on again at 5. 30 P. M. , and the course during thisinterval of 1 h. 30 m. Has been filled up by imagination, but cannot be farfrom correct. The bead moved seven times from side to side, and thusdescribed 3 ½ ellipses in 10 3/4 h. ; each being completed on an average in3 h. 4 m. 

On the previous day another seedling had been observed under similarconditions, excepting that the plant was so[page 17]placed that a line joining the two cotyledons pointed towards the window;and the filament was attached to the smaller cotyledon on the side furthestfrom the window. Moreover the plant was now for the first time placed inthis position. The cotyledons bowed themselves greatly towards the lightfrom 8 to 10. 50 A. M. , when the first dot was made (Fig. 7). During the

Fig. 7. Brassica oleracea: conjoint circumnutation of the hypocotyl andcotyledons, from 10. 50 A. M. To 8 A. M. On the following morning. Tracingmade on a vertical glass. 

next 12 hours the bead swept obliquely up and down 8 times and described 4figures representing ellipses; so that it travelled at nearly the same rateas in the previous case. During the night it moved upwards, owing to thesleep-movement of the cotyledons, and continued to move in the samedirection till 9 A. M. On the following morning; but this latter movementwould not have occurred with seedlings under their natural conditions fullyexposed to the light. 

By 9. 25 A. M. On this second day the same cotyledon had[page 18]begun to fall, and a dot was made on a fresh glass. The movement was traceduntil 5. 30 P. M. As shown in (Fig. 8), which is given, because the coursefollowed was much more irregular than on the two previous occasions. Duringthese 8 hours the bead changed its course greatly 10 times. The upwardmovement of the cotyledon during the afternoon and early part of the nightis here plainly shown. 

Fig. 8. Brassica oleracea: conjoint circumnutation of the hypocotyl andcotyledons during 8 hours. Figure here reduced to one-third of the originalscale, as traced on a vertical glass. 

As the filaments were fixed in the three last cases to one of thecotyledons, and as the hypocotyl was left free, the tracings show themovement of both organs conjoined; and we now wished to ascertain whetherboth circumnutated. Filaments were therefore fixed horizontally to twohypocotyls close beneath the petioles of their cotyledons. These seedlingshad stood for two days in the same position before a north-east window. Inthe morning, up to about 11 A. M. , they moved in zigzag lines towards thelight; and at night they again became almost upright through apogeotropism. After about 11 A. M. They moved a little back from the light, often crossingand recrossing their former path in zigzag lines. The sky on this dayvaried much in brightness, and these observations merely proved that thehypocotyls were continually moving in a manner resembling circumnutation. On a previous day which was uniformly cloudy, a hypocotyl was firmlysecured to a little stick, and a filament was fixed to the larger of thetwo cotyledons, and its movement was traced on a vertical glass. It fellgreatly from 8. 52 A. M. , when the first dot was made, till 10. 55 A. M. ; itthen rose greatly until 12. 17 P. M. Afterwards it fell a little and made aloop, but by 2. 22 P. M. It had risen a little and continued rising till 9. 23P. M. , when it made another loop, and at 10. 30 P. M. Was again rising. Theseobservations show that the cotyledons move[page 19]vertically up and down all day long, and as there was some slight lateralmovement, they circumnutated. 

Fig. 9. Brassica oleracea: circumnutation of hypocotyl, in darkness, tracedon a horizontal glass, by means of a filament with a bead fixed across itssummit, between 9. 15 A. M. And 8. 30 A. M. On the following morning. Figurehere reduced to one-half of original scale. 

The cabbage was one of the first plants, the seedlings of which wereobserved by us, and we did not then know how far the circumnutation of thedifferent parts was affected by light. Young seedlings were therefore keptin complete darkness except for a minute or two during each observation, when they were illuminated by a small wax taper held almost verticallyabove them. During the first day the hypocotyl of one changed its course 13times (see Fig. 9); and it deserves notice that the longer axes of thefigures described often cross one another at right or nearly right angles. Another seedling was observed in the same manner, but it was much older, for it had formed a true leaf a quarter of an inch in length, and thehypocotyl was 1 3/8 inch in height. The figure traced was a very complexone, though the movement was not so great in extent as in the last case. 

The hypocotyl of another seedling of the same age was secured to a littlestick, and a filament having been fixed to the midrib of one of thecotyledons, the movement of the bead was traced during 14 h. 15 m. (seeFig. 10) in darkness. It should be noted that the chief movement of thecotyledons, namely, up and down, would be shown on a horizontal glass-plateonly by the lines in the direction of the midrib (that is, [page 20]up and down, as Fig. 10 here stands) being a little lengthened orshortened; whereas any lateral movement would be well exhibited. Thepresent tracing shows that the cotyledon did thus move laterally (that is, from side to side in the tracing) 12 times in the 14 h. 15 m. Ofobservation. Therefore the cotyledons certainly circumnutated, though thechief movement was up and down in a vertical plane. 

Fig 10. Brassica oleracea: circumnutation of a cotyledon, the hypocotylhaving been secured to a stick, traced on a horizontal glass, in darkness, from 8. 15 A. M. To 10. 30 P. M. Movement of the bead of the filament magnified13 times. 

Rate of Movement. --The movements of the hypocotyls and cotyledons ofseedling cabbages of different ages have now been sufficiently illustrated. With respect to the rate, seedlings were placed under the microscope withthe stage removed, and with a micrometer eye-piece so adjusted that eachdivision equalled 1/500 inch; the plants were illuminated by light passingthrough a solution of bichromate of potassium so as to eliminateheliotropism. Under these circumstances it was interesting to observe howrapidly the circumnutating apex of a cotyledon passed across the divisionsof the micrometer. Whilst travelling in any direction the apex generallyoscillated backwards and forwards to the extent of 1/500 and sometimes ofnearly 1/250 of an inch. These oscillations were quite different from thetrembling caused by any disturbance in the same room or by the shutting ofa distant door. The first seedling observed was nearly two inches in heightand had been etiolated by having been grown in darkness. The tip of thecotyledon passed across 10 divisions of the micrometer, that is, 1/50 of aninch, in 6 m. 40 s. Short glass filaments were then fixed vertically to thehypocotyls of several seedlings so as to project a little above thecotyledons, thus exaggerating the rate of movement; but only a few of theobservations thus made are worth giving. The most remarkable fact was theoscillatory movement above described, and the difference of rate at whichthe point crossed the divisions of the micrometer, after short intervals oftime. For instance, a tall not-etiolated seedling had been kept for 14 h. In darkness; it was exposed before a north-east window for only[page 21]two or three minutes whilst a glass filament was fixed vertically to thehypocotyl; it was then again placed in darkness for half an hour andafterwards observed by light passing through bichromate of potassium. Thepoint, oscillating as usual, crossed five divisions of the micrometer (i. E. 1/100 inch) in 1 m. 30 s. The seedling was then left in darkness for anhour, and now it required 3 m. 6 s. To cross one division, that is, 15 m. 30 s. To have crossed five divisions. Another seedling, after beingoccasionally observed in the back part of a northern room with a very dulllight, and left in complete darkness for intervals of half an hour, crossedfive divisions in 5 m. In the direction of the window, so that we concludedthat the movement was heliotropic. But this was probably not the case, forit was placed close to a north-east window and left there for 25 m. , afterwhich time, instead of moving still more quickly towards the light, asmight have been expected, it travelled only at the rate of 12 m. 30 s. Forfive divisions. It was then again left in complete darkness for 1 h. , andthe point now travelled in the same direction as before, but at the rate of3 m. 18 s. For five divisions. 

We shall have to recur to the cotyledons of the cabbage in a futurechapter, when we treat of their sleep-movements. The circumnutation, also, of the leaves of fully-developed plants will hereafter be described. 

Fig. 11. Githago segetum: circumnutation of hypocotyl, traced on ahorizontal glass, by means of a filament fixed transversely across itssummit, from 8. 15 A. M. To 12. 15 P. M. On the following day. Movement of beadof filament magnified about 13 times, here reduced to one-half the originalscale. 

Githago segetum (Caryophylleae). --A young seedling was dimly illuminatedfrom above, and the circumnutation of the hypo-[page 22]cotyl was observed during 28 h. , as shown in Fig. 11. It moved in alldirections; the lines from right and to left in the figure being parallelto the blades of the cotyledons. The actual distance travelled from side toside by the summit of the hypocotyl was about . 2 of an inch; but it wasimpossible to be accurate on this head, as the more obliquely the plant wasviewed, after it had moved for some time, the more the distances wereexaggerated. 

We endeavoured to observe the circumnutation of the cotyledons, but as theyclose together unless kept exposed to a moderately bright light, and as thehypocotyl is extremely heliotropic, the necessary arrangements were tootroublesome. We shall recur to the nocturnal or sleep-movements of thecotyledons in a future chapter. 

Fig. 12. Gossypium: circumnutation of hypocotyl, traced on a horizontalglass, from 10. 30 A. M. To 9. 30 A. M. On following morning, by means of afilament fixed across its summit. Movement of bead of filament magnifiedabout twice; seedling illuminated from above. 

Gossypium (var. Nankin cotton) (Malvaceae). --The circumnutation of ahypocotyl was observed in the hot-house, but the movement was so muchexaggerated that the bead twice passed for a time out of view. It was, however, manifest that two somewhat irregular ellipses were nearlycompleted in 9 h. Another seedling, 1 ½ in. In height, was then observedduring 23 h. ; but the observations were not made at sufficiently shortintervals, as shown by the few dots in Fig. 12, and the tracing was not nowsufficiently enlarged. Nevertheless there could be no doubt about thecircumnutation of the hypocotyl, which described in 12 h. A figurerepresenting three irregular ellipses of unequal sizes. 

 The cotyledons are in constant movement up and down during the whole day, and as they offer the unusual case of moving downwards late in the eveningand in the early part of the night, many observations were made on them. Afilament was fixed along the middle of one, and its movement traced on avertical glass; but the tracing is not given, as the hypocotyl was notsecured, so that it was impossible to distinguish clearly between itsmovement and that of the cotyledon. The cotyledons rose from 10. 30 A. M. Toabout 3 P. M. ; they then sank till 10 P. M. , rising, however, greatly in thelatter part of the night. [page 23]The angles above the horizon at which the cotyledons of another seedlingstood at different hours is recorded in the following short table: --

Oct. 20 2. 50 P. M... 25o above horizon. Oct. 20 4. 20 P. M... 22o above horizon. Oct. 20 5. 20 P. M... 15o above horizon. Oct. 20 10. 40 P. M... 8o above horizon. Oct. 21 8. 40 A. M... 28o above horizon. Oct. 21 11. 15 A. M... 35o above horizon. Oct. 21 9. 11 P. M... 10o below horizon. 

The position of the two cotyledons was roughly sketched at various hourswith the same general result. 

In the following summer, the hypocotyl of a fourth seedling was secured toa little stick, and a glass filament with triangles of paper having beenfixed to one of the cotyledons, its movements were traced on a verticalglass under a double skylight in the house. The first dot was made at 4. 20P. M. June 20th; and the cotyledon fell till 10. 15 P. M. In a nearly straightline. Just past midnight it was found a little lower and somewhat to oneside. By the early morning, at 3. 45 A. M. , it had risen greatly, but by 6. 20A. M. Had fallen a little. During the whole of this day (21st) it fell in aslightly zigzag line, but its normal course was disturbed by the want ofsufficient illumination, for during the night it rose only a little, andtravelled irregularly during the whole of the following day and night ofJune 22nd. The ascending and descending lines traced during the three daysdid not coincide, so that the movement was one of circumnutation. Thisseedling was then taken back to the hot-house, and after five days wasinspected at 10 P. M. , when the cotyledons were found hanging so nearlyvertically down, that they might justly be said to have been asleep. On thefollowing morning they had resumed their usual horizontal position. 

Oxalis rosea (Oxalideae). --The hypocotyl was secured to a little stick, andan extremely thin glass filament, with two triangles of paper, was attachedto one of the cotyledons, which was . 15 inch in length. In this and thefollowing species the end of the petiole, where united to the blade, isdeveloped into a pulvinus. The apex of the cotyledon stood only 5 inchesfrom the vertical glass, so that its movement was not greatly exaggeratedas long as it remained nearly horizontal; but in the course of the day itboth rose considerably above and fell beneath a horizontal position, andthen of course the movement was much exaggerated. [page 24]In Fig. 13 its course is shown from 6. 45 A. M. On June 17th, to 7. 40 A. M. Onthe following morning; and we see that during the daytime, in the course of11 h. 15 m. , it travelled thrice down and twice up. After 5. 45 P. M. Itmoved rapidly downwards, and in an hour or two depended vertically; it thusremained all night asleep. This position could not be represented on thevertical glass nor in the figure here given. By 6. 40 A. M. On the followingmorning (18th) both cotyledons had risen greatly, and they continued torise until 8 A. M. , when they stood almost horizontally. Their movement wastraced during the whole of this day and until the next morning; but atracing is not given, as it was closely similar to Fig. 13, excepting thatthe lines were more zigzag. The cotyledons moved 7 times, either upwards ordownwards; and at about 4 P. M. The great nocturnal sinking movementcommenced. 

Fig. 13. Oxalis rosea: circumnutation of cotyledons, the hypocotyl beingsecured to a stick; illuminated from above. Figure here given one-half oforiginal scale. 

Another seedling was observed in a similar manner during nearly 24 h. , butwith the difference that the hypocotyl was left free. The movement also wasless magnified. Between 8. 12 A. M. And 5 P. M. On the 18th, the apex of thecotyledon moved 7 times upwards or downwards (Fig. 14). The nocturnalsinking movement, which is merely a great increase of one of the diurnaloscillations, commenced about 4 P. M. 

Oxalis Valdiviana. --This species is interesting, as the coty-[page 25]ledons rise perpendicularly upwards at night so as to come into closecontact, instead of sinking vertically downwards, as in the case of O. Rosea. A glass filament was fixed to a cotyledon, . 17 of an inch in length, and the hypocotyl was left free. On

Fig. 14. Oxalis rosea: conjoint circumnutation of the cotyledons andhypocotyl, traced from 8. 12 A. M. On June 18th to 7. 30 A. M. 19th. The apexof the cotyledon stood only 3 3/4 inches from the vertical glass. Figurehere given one-half of original scale. 

Fig. 15. Oxalis Valdiviana: conjoint circumnutation of a cotyledon and thehypocotyl, traced on vertical glass, during 24 hours. Figure here givenone-half of original scale; seedling illuminated from above. 

the first day the seedling was placed too far from the vertical glass; sothat the tracing was enormously exaggerated and the movement could not betraced when the cotyledon either rose or sank much; but it was clearly seenthat the cotyledons rose thrice and fell twice between 8. 15 A. M. And 4. 15P. M. Early on the following morning (June 19th) the apex of a cotyledon was[page 26]placed only 1 7/8 inch from the vertical glass. At 6. 40 A. M. It stoodhorizontally; it then fell till 8. 35, and then rose. Altogether in thecourse of 12 h. It rose thrice and fell thrice, as may be seen in Fig. 15. The great nocturnal rise of the cotyledons usually commences about 4 or 5P. M. , and on the following morning they are expanded or stand horizontallyat about 6. 30 A. M. In the present instance, however, the great nocturnalrise did not commence till 7 P. M. ; but this was due to the hypocotyl havingfrom some unknown cause temporarily bent to the left side, as is shown inthe tracing. To ascertain positively that the hypocotyl circumnutated, amark was placed at 8. 15 P. M. Behind the two now closed and verticalcotyledons; and the movement of a glass filament fixed upright to the topof the hypocotyl was traced until 10. 40 P. M. During this time it moved fromside to side, as well as backwards and forwards, plainly showingcircumnutation; but the movement was small in extent. Therefore Fig. 15represents fairly well the movements of the cotyledons alone, with theexception of the one great afternoon curvature to the left. 

Oxalis corniculata (var. Cuprea). --The cotyledons rise at night to avariable degree above the horizon, generally about 45o: those on someseedlings between 2 and 5 days old were found to be in continued movementall day long; but the movements were more simple than in the last twospecies. This may have partly resulted from their not being sufficientlyilluminated whilst being observed, as was shown by their not beginning torise until very late in the evening. 

Oxalis (Biophytum) sensitiva. --The cotyledons are highly remarkable fromthe amplitude and rapidity of their movements during the day. The angles atwhich they stood above or beneath the horizon were measured at shortintervals of time; and we regret that their course was not traced duringthe whole day. We will give only a few of the measurements, which were madewhilst the seedlings were exposed to a temperature of 22 1/2o to 24 ½decrees C. One cotyledon rose 70o in 11 m. ; another, on a distinctseedling, fell 80o in 12 m. Immediately before this latter fall the samecotyledon had risen from a vertically downward to a vertically upwardposition in 1 h. 48 m. , and had therefore passed through 180o in under 2 h. We have met with no other instance of a circumnutating movement of suchgreat amplitude as 180o; nor of such rapidity of movement as the passagethrough 80o in 12 m. The cotyledons of this plant sleep at night by rising[page 27]vertically and coming into close contact. This upward movement differs fromone of the great diurnal oscillations above described only by the positionbeing permanent during the night and by its periodicity, as it alwayscommences late in the evening. 

Tropaeolum minus (?) (var. Tom Thumb) (Tropaeoleae). --The cotyledons arehypogean, or never rise above the ground. By removing the soil a buriedepicotyl or plumule was found, with its summit arched abruptly downwards, like the arched hypocotyl of the cabbage previously described. A glassfilament with a bead at its end was affixed to the basal half or leg, justabove the hypogean cotyledons, which were again almost surrounded by looseearth. The tracing (Fig. 16) shows the course of the bead during 11 h. After the last dot given in the figure, the bead moved to a great distance, and finally off the glass, in the direction indicated by the broken line. This great movement, due to increased growth along the concave surface ofthe arch, was caused by the basal leg bending backwards from the upperpart, that is in a direction opposite to the dependent tip, in the samemanner as occurred with the hypocotyl of the cabbage. Another buried andarched epicotyl was observed in the same manner, excepting that the twolegs of the arch were tied together with fine silk for the sake ofpreventing the great movement just mentioned. It moved, however, in theevening in the same direction as before, but the line followed was not sostraight. During the morning the tied arch moved in an irregularlycircular, strongly zigzag course, and to a greater distance than in theprevious case, as was shown in a tracing, magnified 18 times. The movementsof a young plant bearing a few leaves and of a mature plant, will hereafterbe described. 

Fig. 16. Tropaeolum minus (?): circumnutation of buried and archedepicotyl, traced on a horizontal glass, from 9. 20 A. M. To 8. 15 P. M. Movement of bead of filament magnified 27 times. [page 28]

Citrus aurantium (Orange) (Aurantiaceae). --The cotyledons are hypogean. Thecircumnutation of an epicotyl, which at the close of our observations was. 59 of an inch (15 mm. ) in height above the ground, is shown in the annexedfigure (Fig. 17), as observed during a period of 44 h. 40 m. 

Fig. 17. Citrus aurantium: circumnutation of epicotyl with a filament fixedtransversely near its apex, traced on a horizontal glass, from 12. 13 P. M. On Feb. 20th to 8. 55 A. M. On 22nd. The movement of the bead of the filamentwas at first magnified 21 times, or 10 1/2, in figure here given, andafterwards 36 times, or 18 as here given; seedling illuminated from above. 

Aesculus hippocastanum (Hippocastaneae). --Germinating seeds were placed ina tin box, kept moist internally, with a sloping bank of damp argillaceoussand, on which four smoked glass-plates rested, inclined at angles of 70oand 65o with the horizon. The tips of the radicles were placed so as justto touch the upper end of the glass-plates, and, as they grew downwardsthey pressed lightly, owing to geotropism, on the smoked surfaces, and lefttracks of their course. In the middle part of each track the glass wasswept clean, but the margins were much blurred and irregular. Copies of twoof these tracks (all four being nearly alike) were made on tracing paperplaced over the glass-plates after they had been varnished; and they are asexact as possible considering the nature of the margins (Fig. 18). Theysuffice to show that there was some lateral, almost serpentine movement, and that the tips in their downward course pressed with unequal force onthe plates, as[page 29]the tracks varied in breadth. The more perfectly serpentine tracks made bythe radicles of Phaseolus multiflorus and Vicia faba (presently to bedescribed), render it almost certain that the radicles of the present plantcircumnutated. 

Fig. 18. Aesculus hippocastanum: outlines of tracks left on inclinedglass-plates by tips of radicles. In A the plate was inclined at 70o withthe horizon, and the radicle was 1. 9 inch in length, and . 23 inch indiameter at base. In B the plate was inclined 65o with the horizon, and theradicle was a trifle larger. 

Phaseolus multiflorus (Leguminosae). --Four smoked glass-plates werearranged in the same manner as described under Aesculus, and the tracksleft by the tips of four radicles of the present plant, whilst growingdownwards, were photographed as transparent objects. Three of them are hereexactly copied (Fig. 19). Their serpentine courses show that the tips movedregularly from side to side; they also pressed alternately with greater orless force on the plates, sometimes rising up and leaving them altogetherfor a very short distance; but this was better seen on the original platesthan in the copies. These radicles therefore were continually moving in alldirections--that is, they circumnutated. The distance between the extremeright and left positions of the radicle A, in its lateral movement, was 2mm. , as ascertained by measurement with an eye-piece micrometer. 

Fig. 19. Phaseolus multiflorus: tracks left on inclined smoked glass-platesby tips of radicles in growing downwards. A and C, plates inclined at 60o, B inclined at 68o with the horizon. 

Vicia faba (Common Bean) (Leguminosae). --Radicle. --Some beans were allowedto germinate on bare sand, and after one had protruded its radicle to alength of . 2 of an inch, it was turned upside down, so that the radicle, which was kept in damp air, now stood upright. A filament, nearly an inchin length, was affixed obliquely near its tip; and the movement of theterminal bead was traced from 8. 30 A. M. To 10. 30 P. M. , as shown in Fig. 18. The radicle at first changed its course twice[page 30]abruptly, then made a small loop and then a larger zigzag curve. During thenight and till 11 A. M. On the following

Fig. 20. Vicia faba: circumnutation of a radicle, at first pointingvertically upwards, kept in darkness, traced on a horizontal glass, during14 hours. Movement of bead of filament magnified 23 times, here reduced toone-half of original scale. 

morning, the bead moved to a great distance in a nearly straight line, inthe direction indicated by the broken line in the figure. This resultedfrom the tip bending quickly downwards, as it had now become much declined, and had thus gained a position highly favourable for the action ofgeotropism. Fig. 21. Vicia faba: tracks left on inclined smoked glass-plates, by tipsof radicles in growing downwards. Plate C was inclined at 63o, plates A andD at 71o, plate B at 75o, and plate E at a few degrees beneath the horizon. [page 31]

We next experimented on nearly a score of radicles by allowing them to growdownwards over inclined plates of smoked glass, in exactly the same manneras with Aesculus and Phaseolus. Some of the plates were inclined only a fewdegrees beneath the horizon, but most of them between 60o and 75o. In thelatter cases the radicles in growing downwards were deflected only a littlefrom the direction which they had followed whilst germinating in sawdust, and they pressed lightly on the glass-plates (Fig. 21). Five of the mostdistinct tracks are here copied, and they are all slightly sinuous, showingcircumnutation. Moreover, a close examination of almost every one of thetracks clearly showed that the tips in their downward course hadalternately pressed with greater or less force on the plates, and hadsometimes risen up so as nearly to leave them for short intervals. Thedistance between the extreme right and left positions of the radicle A was0. 7 mm. , ascertained in the same manner as in the case of Phaseolus. 

Epicotyl. --At the point where the radicle had protruded from a bean laid onits side, a flattened solid lump projected . 1 of an inch, in the samehorizontal plane with the bean. This protuberance consisted of the convexsummit of the arched epicotyl; and as it became developed the two legs ofthe arch curved themselves laterally upwards, owing to apogeotropism, atsuch a rate that the arch stood highly inclined after 14 h. , and verticallyin 48 h. A filament was fixed to the crown of the protuberance before anyarch was visible, but the basal half grew so quickly that on the secondmorning the end of the filament was bowed greatly downwards. It wastherefore removed and fixed lower down. The line traced during these twodays extended in the same general direction, and was in parts nearlystraight, and in others plainly zigzag, thus giving some evidence ofcircumnutation. 

As the arched epicotyl, in whatever position it may be placed, bendsquickly upwards through apogeotropism, and as the two legs tend at a veryearly age to separate from one another, as soon as they are relieved fromthe pressure of the surrounding earth, it was difficult to ascertainpositively whether the epicotyl, whilst remaining arched, circumnutated. Therefore some rather deeply buried beans were uncovered, and the two legsof the arches were tied together, as had been done with the epicotyl ofTropaeolum and the hypocotyl of the Cabbage. The movements of the tiedarches were traced in the usual manner on[page 32]two occasions during three days. But the tracings made under such unnaturalconditions are not worth giving; and it need only be said that the lineswere decidedly zigzag, and that small loops were occasionally formed. Wemay therefore conclude that the epicotyl circumnutates whilst still archedand before it has grown tall enough to break through the surface of theground. 

In order to observe the movements of the epicotyl at a somewhat moreadvanced age, a filament was fixed near the base of one which was no longerarched, for its upper half now formed a right angle with the lower half. This bean had germinated on bare damp sand, and the epicotyl began tostraighten itself much sooner than would have occurred if it had beenproperly planted. The course pursued during 50 h. (from 9 A. M. Dec. 26th, to 11 A. M. 28th) is here shown (Fig. 22); and we seeFig. 22. Vicia faba: circumnutation of young epicotyl, traced in darknessduring 50 hours on a horizontal glass. Movement of bead of filamentmagnified 20 times, here reduced to one-half of original scale. 

that the epicotyl circumnutated during the whole time. Its basal part grewso much during the 50 h. That the filament at the end of our observationswas attached at the height of . 4 inch above the upper surface of the bean, instead of close to it. If the bean had been properly planted, this part ofthe epicotyl would still have been beneath the soil. 

Late in the evening of the 28th, some hours after the above observationswere completed, the epicotyl had grown much straighter, for the upper partnow formed a widely open angle with the lower part. A filament was fixed tothe upright basal part, higher up than before, close beneath the lowestscale-like process or homologue of a leaf; and its movement was traced[page 33]during 38 h. (Fig. 23). We here again have plain evidence of continuedcircumnutation. Had the bean been properly planted, the part of theepicotyl to which the filament was attached, the

Fig. 23. Vicia faba: circumnutation of the same epicotyl as in Fig. 22, alittle more advanced in age, traced under similar conditions as before, from 8. 40 A. M. Dec. 28th, to 10. 50 A. M. 30th. Movement of bead heremagnified 20 times. 

movement of which is here shown, would probably have just risen above thesurface of the ground. 

Lathyrus nissolia (Leguminosae). --This plant was selected for observationfrom being an abnormal form with grass-like leaves. 

Fig. 24. Lathyrus nissolia: circumnutation of stem of young seedling, traced in darkness on a horizontal glass, from 6. 45 A. M. Nov. 22nd, to 7A. M. 23rd. Movement of end of leaf magnified about 12 times, here reducedto one-half of original scale. 

The cotyledons are hypogean, and the epicotyl breaks through the ground inan arched form. The movements of a stem, 1. 2 inch in height, consisting ofthree internodes, the lower one almost wholly subterranean, and the upperone bearing a short, [page 34]narrow leaf, is shown during 24 h. , in Fig. 24. No glass filament wasemployed, but a mark was placed beneath the apex of the leaf. The actuallength of the longer of the two ellipses described by the stem was about. 14 of an inch. On the previous day the chief line of movement was nearlyat right angles to that shown in the present figure, and it was moresimple. 

Cassia tora* (Leguminosae). --A seedling was placed before a

Fig. 25. Cassia tora: conjoint circumnutation of cotyledons and hypocotyl, traced on vertical glass, from 7. 10 A. M. Sept. 25th to 7. 30 A. M. 26th. Figure here given reduced to one-half of original scale. 

* Seeds of this plant, which grew near the sea-side, were sent to us byFritz Müller from S. Brazil. The seedlings did not flourish or flower wellwith us; they were sent to Kew, and were pronounced not to bedistinguishable from C. Tora. [page 35]

north-east window; it bent very little towards it, as the hypocotyl whichwas left free was rather old, and therefore not highly heliotropic. Afilament had been fixed to the midrib of one of the cotyledons, and themovement of the whole seedling was traced during two days. Thecircumnutation of the hypocotyl is quite insignificant compared with thatof the cotyledons. These rise up vertically at night and come into closecontact; so that they may be said to sleep. This seedling was so old that avery small true leaf had been developed, which at night was completelyhidden by the closed cotyledons. On Sept. 24th, between 8 A. M. And 5 P. M. , the cotyledons moved five times up and five times down; they thereforedescribed five irregular ellipses in the course of the 9 h. The greatnocturnal rise commenced about 4. 30 P. M. 

On the following morning (Sept. 25th) the movement of the same cotyledonwas again traced in the same manner during 24 h. ; and a copy of the tracingis here given (Fig. 25). The morning was cold, and the window had beenaccidentally left open for a short time, which must have chilled the plant;and this probably prevented it from moving quite as freely as on theprevious day; for it rose only four and sank only four times during theday, one of the oscillations being very small. At 7. 10 A. M. , when the firstdot was made, the cotyledons were not fully open or awake; they continuedto open till about 9 A. M. , by which time they had sunk a little beneath thehorizon: by 9. 30 A. M. They had risen, and then they oscillated up and down;but the upward and downward lines never quite coincided. At about 4. 30 P. M. The great nocturnal rise commenced. At 7 A. M. On the following morning(Sept. 26th) they occupied nearly the same level as on the previousmorning, as shown in the diagram: they then began to open or sink in theusual manner. The diagram leads to the belief that the great periodicaldaily rise and fall does not differ essentially, excepting in amplitude, from the oscillations during the middle of the day. 

Lotus Jacoboeus (Leguminosae). --The cotyledons of this plant, after the fewfirst days of their life, rise so as to stand almost, though rarely quite, vertically at night. They continue to act in this manner for a long timeeven after the development of some of the true leaves. With seedlings, 3inches in height, and bearing five or six leaves, they rose at night about45o. They continued to act thus for about an additional fortnight. Subsequently they remained horizontal at night, though still green[page 36]and at last dropped off. Their rising at night so as to stand almostvertically appears to depend largely on temperature; for when the seedlingswere kept in a cool house, though they still continued to grow, thecotyledons did not become vertical at night. It is remarkable that thecotyledons do not generally rise at night to any conspicuous extent duringthe first four or five days after germination; but the period was extremelyvariable with seedlings kept under the same conditions; and many wereobserved. Glass filaments with minute triangles of paper were fixed to thecotyledons (1 ½ mm. In breadth) of two seedlings, only 24 h. Old, and thehypocotyl was secured to a stick; their movements greatly magnified weretraced, and they certainly circumnutated the whole time on a small scale, but they did not exhibit any distinct nocturnal and diurnal movement. Thehypocotyls, when left free, circumnutated over a large space. 

Another and much older seedling, bearing a half-developed leaf, had itsmovements traced in a similar manner during the three first days and nightsof June; but seedlings at this age appear to be very sensitive to adeficiency of light; they were observed under a rather dim skylight, at atemperature of between 16o to 17 1/2o C. ' and apparently, in consequence ofthese conditions, the great daily movement of the cotyledons ceased on thethird day. During the first two days they began rising in the earlyafternoon in a nearly straight line, until between 6 and 7 P. M. , when theystood vertically. During the latter part of the night, or more probably inthe early morning, they began to fall or open, so that by 6. 45 A. M. Theystood fully expanded and horizontal. They continued to fall slowly for sometime, and during the second day described a single small ellipse, between 9A. M. And 2 P. M. , in addition to the great diurnal movement. The coursepursued during the whole 24 h. Was far less complex than in the foregoingcase of Cassia. On the third morning they fell very much, and thencircumnutated on a small scale round the same spot; by 8. 20 P. M. Theyshowed no tendency to rise at night. Nor did the cotyledons of any of themany other seedlings in the same pot rise; and so it was on the followingnight of June 5th. The pot was then taken back into the hot-house, where itwas exposed to the sun, and on the succeeding night all the cotyledons roseagain to a high angle, but did not stand quite vertically. On each of theabove days the line representing the great nocturnal[page 37]rise did not coincide with that of the great diurnal fall, so that narrowellipses were described, as is the usual rule with circumnutating organs. The cotyledons are provided with a pulvinus, and its development willhereafter be described. 

Mimosa pudica (Leguminosae). --The cotyledons rise up vertically at night, so as to close together. Two seedlings were observed in the greenhouse(temp. 16o to 17o C. Or 63o to 65o F. ). Their hypocotyls were secured tosticks, and glass filaments bearing little triangles of paper were affixedto the cotyledons of both. Their movements were traced on a vertical glassduring 24 h. On November 13th. The pot had stood for some time in the sameposition, and they were chiefly illuminated through the glass-roof. Thecotyledons of one of these seedlings moved downward in the morning till11. 30 A. M. , and then rose, moving rapidly in the evening until they stoodvertically, so that in this case there was simply a single great daily falland rise. The other seedling behaved rather differently, for it fell in themorning until 11. 30 A. M. , and then rose, but after 12. 10 P. M. Again fell;and the great evening rise did not begin until 1. 22 P. M. On the followingmorning this cotyledon had fallen greatly from its vertical position by8. 15 A. M. Two other seedlings (one seven and the other eight days old) hadbeen previously observed under unfavourable circumstances, for they hadbeen brought into a room and placed before a north-east window, where thetemperature was between only 56o and 57o F. They had, moreover, to beprotected from lateral light, and perhaps were not sufficientlyilluminated. Under these circumstances the cotyledons moved simplydownwards from 7 A. M. Till 2 P. M. , after which hour and during a large partof the night they continued to rise. Between 7 and 8 A. M. On the followingmorning they fell again; but on this second and likewise on the third daythe movements became irregular, and between 3 and 10. 30 P. M. Theycircumnutated to a small extent about the same spot; but they did not riseat night. Nevertheless, on the following night they rose as usual. 

Cytisus fragrans (Leguminosae). --Only a few observations were made on thisplant. The hypocotyl circumnutated to a considerable extent, but in asimple manner--namely, for two hours in one direction, and then much moreslowly back again in a zigzag course, almost parallel to the first line, and beyond the starting-point. It moved in the same direction all night, but next morning began to return. The cotyledons continually[page 38]move both up and down and laterally; but they do not rise up at night in aconspicuous manner. 

Lupinus luteus (Leguminosae). --Seedlings of this plant were observedbecause the cotyledons are so thick (about . 08 of an inch) that it seemedunlikely that they would move. Our observations were not very successful, as the seedlings are strongly heliotropic, and their circumnutation couldnot be accurately observed near a north-east window, although they had beenkept during the previous day in the same position. A seedling was thenplaced in darkness with the hypocotyl secured to a stick; both cotyledonsrose a little at first, and then fell during the rest of the day; in theevening between 5 and 6 P. M. They moved very slowly; during the night onecontinued to fall and the other rose, though only a little. The tracing wasnot much magnified, and as the lines were plainly zigzag, the cotyledonsmust have moved a little laterally, that is, they must have circumnutated. 

The hypocotyl is rather thick, about . 12 of inch; nevertheless itcircumnutated in a complex course, though to a small extent. The movementof an old seedling with two true leaves partially developed, was observedin the dark. As the movement was magnified about 100 times it is nottrustworthy and is not given; but there could be no doubt that thehypocotyl moved in all directions during the day, changing its course 19times. The extreme actual distance from side to side through which theupper part of the hypocotyl passed in the course of 14 ½ hours was only1/60 of an inch; it sometimes travelled at the rate of 1/50 of an inch inan hour. 

Cucurbita ovifera (Cucurbitaceae). --Radicle: a seed which had

Fig. 26. Cucurbita ovifera: course followed by a radicle in bendinggeotropically downwards, traced on a horizontal glass, between 11. 25 A. M. And 10. 25 P. M. ; the direction during the night is indicated by the brokenline. Movement of bead magnified 14 times. 

germinated on damp sand was fixed so that the slightly curved radicle, which was only . 07 inch in length, stood almost vertically[page 39]upwards, in which position geotropism would act at first with little power. A filament was attached near to its base, and projected at about an angleof 45o above the horizon. The general course followed during the 11 hoursof observation and during the following night is shown in the accompanyingdiagram (Fig. 26), and was plainly due to geotropism; but it was also clearthat the radicle circumnutated. By the next morning the tip had curved somuch downwards that the filament, instead of projecting at 45o above thehorizon, was nearly horizontal. Another germinating seed was turned upsidedown and covered with damp sand; and a filament was fastened to the radicleso as to project at an angle of about 50o above the horizon; this radiclewas . 35 of an inch in length and a little curved. The course pursued wasmainly governed, as in the last case, by geotropism, but the line tracedduring 12 hours and magnified as before was more strongly zigzag, againshowing circumnutation. 

Four radicles were allowed to grow downwards over plates of smoked glass, inclined at 70o to the horizon, under the

Fig. 27. Cucurbita ovifera: tracks left by tips of radicles in growingdownwards over smoked glass-plates, inclined at 70o to the horizon. 

Fig. 28. Cucurbita ovifera: circumnutation of arched hypocotyl at a veryearly age, traced in darkness on a horizontal glass, from 8 A. M. To 10. 20A. M. On the following day. The movement of the bead magnified 20 times, here reduced to one-half of original scale. 

same conditions as in the cases of Aesculus, Phaseolus, and Vicia. Facsimiles are here given (Fig. 27) of two of these tracks; and a thirdshort one was almost as plainly serpentine as that at A. It was alsomanifest by a greater or less amount of soot having been swept off theglasses, that the tips had[page 40]pressed alternately with greater and less force on them. There must, therefore, have been movement in at least two planes at right angles to oneanother. These radicles were so delicate that they rarely had the power tosweep the glasses quite clean. One of them had developed some lateral orsecondary rootlets, which projected a few degrees beneath the horizon; andit is an important fact that three of them left distinctly serpentinetracks on the smoked surface, showing beyond doubt that they hadcircumnutated like the main or primary radicle. But the tracks were soslight that they could not be traced and copied after the smoked surfacehad been varnished. 

Fig. 29. Cucurbita ovifera: circumnutation of straight and verticalhypocotyl, with filament fastened transversely across its upper end, tracedin darkness on a horizontal glass, from 8. 30 A. M. To 8. 30 P. M. The movementof the terminal bead originally magnified about 18 times, here only 4 ½times. 

Hypocotyl. --A seed lying on damp sand was firmly fixed by two crossed wiresand by its own growing radicle. The cotyledons were still enclosed withinthe seed-coats; and the short hypocotyl, between the summit of the radicleand the cotyledons, was as yet only slightly arched. A filament (. 85 ofinch in length) was attached at an angle of 35o above the horizon to theside of the arch adjoining the cotyledons. This part would ultimately formthe upper end of the hypocotyl, after it had grown straight and vertical. Had the seed been properly planted, the hypocotyl at this stage of growthwould have been deeply buried beneath the surface. The course followed bythe bead of the filament is shown in Fig. 28. The chief lines of movementfrom left to right in the figure were parallel to the plane of the twounited cotyledons and of the flattened seed; and this movement would aid indragging them out of the seed-coats, which are held down by a specialstructure hereafter to be described. The movement at right angles to theabove lines was due to the arched hypocotyl becoming more arched as itincreased in height. The foregoing observations apply to the leg of thearch next to the cotyledons, but[page 41]the other leg adjoining the radicle likewise circumnutated at an equallyearly age. 

The movement of the same hypocotyl after it had become straight andvertical, but with the cotyledons only partially expanded, is shown in Fig. 29. The course pursued during 12 h. Apparently represents four and a halfellipses or ovals, with the longer axis of the first at nearly right anglesto that of the others. The longer axes of all were oblique to a linejoining the opposite cotyledons. The actual extreme distance from side toside over which the summit of the tall hypocotyl passed in the course of 12h. Was . 28 of an inch. The original figure was traced on a large scale, andfrom the obliquity of the line of view the outer parts of the diagram aremuch exaggerated. 

Cotyledons. --On two occasions the movements of the cotyledons were tracedon a vertical glass, and as the ascending and descending lines did notquite coincide, very narrow ellipses were formed; they thereforecircumnutated. Whilst young they rise vertically up at night, but theirtips always remain reflexed; on the following morning they sink down again. With a seedling kept in complete darkness they moved in the same manner, for they sank from 8. 45 A. M. To 4. 30 P. M. ; they then began to rise andremained close together until 10 P. M. , when they were last observed. At 7A. M. On the following morning they were as much expanded as at any hour onthe previous day. The cotyledons of another young seedling, exposed to thelight, were fully open for the first time on a certain day, but were foundcompletely closed at 7 A. M. On the following morning. They soon began toexpand again, and continued doing so till about 5 P. M. ; they then began torise, and by 10. 30 P. M. Stood vertically and were almost closed. At 7 A. M. On the third morning they were nearly vertical, and again expanded duringthe day; on the fourth morning they were not closed, yet they opened alittle in the course of the day and rose a little on the following night. By this time a minute true leaf had become developed. Another seedling, still older, bearing a well-developed leaf, had a sharp rigid filamentaffixed to one of its cotyledons (85 mm. In length), which recorded its ownmovements on a revolving drum with smoked paper. The observations were madein the hot-house, where the plant had lived, so that there was no change intemperature or light. The record commenced at 11 A. M. On February 18th; andfrom this hour till 3 P. M. The[page 42]cotyledon fell; it then rose rapidly till 9 P. M. , then very gradually till3 A. M. February 19th, after which hour it sank gradually till 4. 30 P. M. ;but the downward movement was interrupted by one slight rise or oscillationabout 1. 30 P. M. After 4. 30 P. M. (19th) the cotyledon rose till 1 A. M. (inthe night of February 20th) and then sank very gradually till 9. 30 A. M. , when our observations ceased. The amount of movement was greater on the18th than on the 19th or on the morning of the 20th. 

Cucurbita aurantia. --An arched hypocotyl was found buried a little beneaththe surface of the soil; and in order to prevent it straightening itselfquickly, when relieved from the surrounding pressure of the soil, the twolegs of the arch were tied together. The seed was then lightly covered withloose damp earth. A filament with a bead at the end was affixed to thebasal leg, the movements of which were observed during two days in theusual manner. On the first day the arch moved in a zigzag line towards theside of the basal leg. On the next day, by which time the dependentcotyledons had been dragged above the surface of the soil, the tied archchanged its course greatly nine times in the course of 14 ½ h. It swept alarge, extremely irregular, circular figure, returning at night to nearlythe same spot whence it had started early in the morning. The line was sostrongly zigzag that it apparently represented five ellipses, with theirlonger axes pointing in various directions. With respect to the periodicalmovements of the cotyledons, those of several young seedlings formedtogether at 4 P. M. An angle of about 60o, and at 10 P. M. Their lower partsstood vertically and were in contact; their tips, however, as is usual inthe genus, were permanently reflexed. These cotyledons, at 7 A. M. On thefollowing morning, were again well expanded. 

Lagenaria vulgaris (var. Miniature Bottle-gourd) (Cucurbitaceae). --Aseedling opened its cotyledons, the movements of which were alone observed, slightly on June 27th and closed them at night: next day, at noon (28th), they included an angle of 53o, and at 10 P. M. They were in close contact, so that each had risen 26 1/2o. At noon, on the 29th, they included anangle of 118o, and at 10 P. M. An angle of 54o, so each had risen 32o. On thefollowing day they were still more open, and the nocturnal rise wasgreater, but the angles were not measured. Two other seedlings wereobserved, and behaved during three days in a closely similar manner. Thecotyledons, therefore, [page 43]open more and more on each succeeding day, and rise each night about 30o;consequently during the first two nights of their life they standvertically and come into contact. 

Fig. 30. Lagenaria vulgaris: circumnutation of a cotyledon, 1 ½ inch inlength, apex only 4 3/4 inches from the vertical glass, on which itsmovements were traced from 7. 35 A. M. July 11th to 9. 5 A. M. On the 14th. Figure here given reduced to one-third of original scale. 

In order to ascertain more accurately the nature of these movements, thehypocotyl of a seedling, with its cotyledons well expanded, was secured toa little stick, and a filament with triangles of paper was affixed to oneof the cotyledons. The observations were made under a rather dim skylight, and the temperature during the whole time was between 17 1/2o to 18o C. (63oto 65o F. ). Had the temperature been higher and the light brighter, themovements would probably have been greater. On July 11th (see Fig. 30), thecotyledon fell from 7. 35 A. M. Till 10 A. M. ; it then rose (rapidly after 4P. M. ) till it stood quite vertically at 8. 40 P. M. During the early morningof the next day (12th) it fell, and continued to fall till 8 A. M. , afterwhich hour it rose, then fell, and again rose, so that by 10. 35 P. M. Itstood much higher than it did in the morning, but was not vertical as onthe preceding night. During the following early morning and whole day(13th) it fell and circumnutated, but had not risen when observed late inthe evening; and this was probably due to the deficiency of heat or light, or of both. We thus see that the cotyledons became more widely open at noonon each succeeding day; and that they rose considerably each night, thoughnot acquiring a vertical position, except during the first two nights. 

Cucumis dudaim (Cucurbitaceae). --Two seedlings had opened[page 44]their cotyledons for the first time during the day, --one to the extent of90o and the other rather more; they remained in nearly the same positionuntil 10. 40 P. M. ; but by 7 A. M. On the following morning the one which hadbeen previously open to the extent of 90o had its cotyledons vertical andcompletely shut; the other seedling had them nearly shut. Later in themorning they opened in the ordinary manner. It appears therefore that thecotyledons of this plant close and open at somewhat different periods fromthose of the foregoing species of the allied genera of Cucurbita andLagenaria. 

Fig. 31. Opuntia basilaris: conjoint circumnutation of hypocotyl andcotyledon; filament fixed longitudinally to cotyledon, and movement tracedduring 66 h. On horizontal glass. Movement of the terminal bead magnifiedabout 30 times, here reduced to one-third scale. Seedling kept inhot-house, feebly illuminated from above. 

Opuntia basilaris (Cacteae). --A seedling was carefully observed, because, considering its appearance and the nature of the mature plant, it seemedvery unlikely that either the hypocotyl or cotyledons would circumnutate toan appreciable extent. The cotyledons were well developed, being . 9 of aninch in length, . 22 in breadth, and . 15 in thickness. The almostcylindrical hypocotyl, now bearing a minute spinous bud on its summit, wasonly . 45 of an inch in height, and . 19 in diameter. The tracing (Fig. 31)shows the combined movement of the hypocotyl and of one of the cotyledons, from 4. 45 P. M. On May 28th to 11 A. M. On the 31st. On the 29th a nearlyperfect ellipse was completed. On the 30th the hypocotyl moved, from someunknown cause, in the same general direction in a zigzag line; but between4. 30 and 10 P. M. Almost completed a second small ellipse. The cotyledonsmove only a little up and down: thus at 10. 15 P. M. They stood only 10ohigher than at noon. The chief seat of movement therefore, at least whenthe cotyledons are rather old as in the present case, lies in thehypocotyl. The ellipse described on the 29th had its longer axis directedat nearly right angles to a line joining the two cotyledons. The actualamount of movement of the bead at the end of the[page 45]filament was, as far as could be ascertained, about . 14 of an inch. 

Fig. 32. Helianthus annuus: circumnutation of hypocotyl, with filamentfixed across its summit, traced on a horizontal glass in darkness, from8. 45 A. M. To 10. 45 P. M. , and for an hour on following morning. Movement ofbead magnified 21 times, here reduced to one-half of original scale. 

Helianthus annuus (Compositae). --The upper part of the hypocotyl movedduring the day-time in the course shown in the annexed figure (Fig. 32). Asthe line runs in various directions, crossing itself several times, themovement may be considered as one of circumnutation. The extreme actualdistance travelled was at least . 1 of an inch. The movements of thecotyledons of two seedlings were observed; one facing a north-east window, and the other so feebly illuminated from above us as to be almost indarkness. They continued to sink till about noon, when they began to rise;but between 5 and 7 or 8 P. M. They either sank a little, or movedlaterally, and then again began to rise. At 7 A. M. On the following morningthose on the plant before the north-east window had opened so little thatthey stood at an angle of 73o above the horizon, and were not observed anylonger. Those on the seedling which had been kept in almost completedarkness, sank during the whole day, without rising about mid-day, but roseduring the night. On the third and fourth days they continued sinkingwithout any alternate ascending movement; and this, no doubt, was due tothe absence of light. 

Primula Sinensis (Primulaceae). --A seedling was placed with the twocotyledons parallel to a north-east window on a day when the light wasnearly uniform, and a filament was affixed to one of them. Fromobservations subsequently made on another seedling with the stem secured toa stick, the greater part of the movement shown in the annexed figure (Fig. 33), must have been that of the hypocotyl, though the cotyledons certainlymove up and down to a certain extent both during the day and night. Themovements of the same seedling were traced[page 46]on the following day with nearly the same result; and there can be no doubtabout the circumnutation of the hypocotyl. 

Fig. 33. Primula Sinensis: conjoint circumnutation of hypocotyl andcotyledon, traced on vertical glass, from 8. 40 A. M. To 10. 45 P. M. Movementsof bead magnified about 26 times. 

Cyclamen Persicum (Primulaceae). --This plant is generally supposed toproduce only a single cotyledon, but Dr. H. Gressner* has shown that asecond one is developed after a long interval of time. The hypocotyl isconverted into a globular corm, even before the first cotyledon has brokenthrough the ground with its blade closely enfolded and with its petiole inthe form of an arch, like the arched hypocotyl or epicotyl of any ordinarydicotyledonous plant. A glass filament was affixed to a cotyledon, . 55 ofan inch in height, the petiole of which had straightened itself and stoodnearly vertical, but with the blade not as yet fully expanded. Itsmovements were traced during 24 ½ h. On a horizontal glass, magnified 50times; and in this interval it described two irregular small circles; ittherefore circumnutates, though on an extremely small scale. 

Fig. 34. Stapelia sarpedon: circumnutation of hypocotyl, illuminated fromabove, traced on horizontal glass, from 6. 45 A. M. June 26th to 8. 45 A. M. 28th. Temp. 23-24o C. Movement of bead magnified 21 times. 

Stapelia sarpedon (Asclepiadeae). --This plant, when mature, resembles acactus. The flattened hypocotyl is fleshy, enlarged in the upper part, andbears two rudimentary cotyledons. It breaks through the ground in an archedform, with the rudimentary cotyledons closed or in contact. A filament wasaffixed almost

* 'Bot. Zeitung, ' 1874, p. 837. [page 47]

vertically to the hypocotyl of a seedling half an inch high; and itsmovements were traced during 50 h. On a horizontal glass (Fig. 34). Fromsome unknown cause it bowed itself to one side, and as this was effected bya zigzag course, it probably circumnutated; but with hardly any otherseedling observed by us was this movement so obscurely shown. 

Ipomoea caerulea vel Pharbitis nil (Convolvulaceae). --Seedlings of thisplant were observed because it is a twiner, the upper internodes of whichcircumnutate conspicuously; but like other twining plants, the first fewinternodes which rise above the ground are stiff enough to supportthemselves, and therefore do not circumnutate in any plainly recognisablemanner. * In this particular instance the fifth internode (including thehypocotyl) was the first which plainly circumnutated and twined round astick. We therefore wished to learn whether circumnutation could beobserved in the hypocotyl if carefully observed in our usual manner. Twoseedlings were kept in the dark with filaments fixed to the upper part oftheir hypocotyls; but from circumstances not worth explaining theirmovements were traced for only a short time. One moved thrice forwards andtwice backwards in nearly opposite directions, in the course of 3 h. 15 m. ;and the other twice forwards and twice backwards in 2 h. 22 m. Thehypocotyl therefore circumnutated at a remarkably rapid rate. It may herebe added that a filament was affixed transversely to the summit of thesecond internode above the cotyledons of a little plant 3 ½ inches inheight; and its movements were traced on a horizontal glass. Itcircumnutated, and the actual distance travelled from side to side was aquarter of an inch, which was too small an amount to be perceived withoutthe aid of marks. 

The movements of the cotyledons are interesting from their complexity andrapidity, and in some other respects. The hypocotyl (2 inches high) of avigorous seedling was secured to a stick, and a filament with triangles ofpaper was affixed to one of the cotyledons. The plant was kept all day inthe hot-house, and at 4. 20 P. M. (June 20th) was placed under a skylight inthe house, and observed occasionally during the evening and night. It fellin a slightly zigzag line to a moderate extent from 4. 20 P. M. Till 10. 15P. M. When looked at shortly after midnight (12. 30 P. M. ) it had risen a verylittle, and considerably by

* 'Movements and Habits of Climbing Plants, ' p. 33, 1875. [page 48]

3. 45 A. M. When again looked at, at 6. 10 A. M. (21st), it had fallen largely. A new tracing was now begun (see Fig. 35), and soon afterwards, at 6. 42A. M. , the cotyledon had risen a little. During the forenoon it was observedabout every hour; but between 12. 30 and 6 P. M. Every half-hour. If theobservations had been made at these short intervals during the whole day, the figure would have been too intricate to have been copied. As it was, the cotyledon moved up and down in the course of 16 h. 20 m. (i. E. Between6. 10 A. M. And 10. 30 P. M. ) thirteen times. 

Fig 35. Ipomoea caerulea: circumnutation of cotyledon, traced on verticalglass, from 6. 10 A. M. June 21st to 6. 45 A. M. 22nd. Cotyledon with petiole1. 6 inch in length, apex of blade 4. 1 inch from the vertical glass; somovement not greatly magnified; temp. 20o C. 

The cotyledons of this seedling sank downwards during both evenings and theearly part of the night, but rose during the latter part. As this is anunusual movement, the cotyledons of twelve other seedlings were observed;they stood almost or quite horizontally at mid-day, and at 10 P. M. Were alldeclined at various angles. The most usual angle was between 30o and 35o;but three stood at about 50o and one at even 70o beneath the horizon. Theblades of all these cotyledons had attained almost their full size, viz. From 1 to 1 ½ inches in length, measured along their midribs. It is aremarkable fact that whilst young--that is, when less than half an inch inlength, measured in the same manner--they do not sink[page 49]downwards in the evening. Therefore their weight, which is considerablewhen almost fully developed, probably came into play in originallydetermining the downward movement. The periodicity of this movement is muchinfluenced by the degree of light to which the seedlings have been exposedduring the day; for three kept in an obscure place began to sink aboutnoon, instead of late in the evening; and those of another seedling werealmost paralysed by having been similarly kept during two whole days. Thecotyledons of several other species of Ipomoea likewise sink downwards latein the evening. 

Cerinthe major (Boragineae). --The circumnutation of the hypocotyl of ayoung seedling with the cotyledons hardly

Fig. 36. Cerinthe major: circumnutation of hypocotyl, with filament fixedacross its summit, illuminated from above, traced on horizontal glass, from9. 26 A. M. To 9. 53 P. M. On Oct. 25th. Movement of the bead magnified 30times, here reduced to one-third of original scale. 

expanded, is shown in the annexed figure (Fig. 36), which apparentlyrepresents four or five irregular ellipses, described in the course of alittle over 12 hours. Two older seedlings were similarly observed, excepting that one of them was kept in the dark; their hypocotyls alsocircumnutated, but in a more simple manner. The cotyledons on a seedlingexposed to the light fell from the early morning until a little after noon, and then continued to rise until 10. 30 P. M. Or later. The cotyledons ofthis same seedling acted in the same general manner during the twofollowing days. It had previously been tried in the dark, and after beingthus kept for only 1 h. 40 m. The cotyledons began at 4. 30 P. M. To sink, instead of continuing to rise till late at night. [page 50]

Nolana prostrata (Nolaneae). --The movements were not traced, but a pot withseedlings, which had been kept in the dark for an hour, was placed underthe microscope, with the micrometer eye-piece so adjusted that eachdivision equalled 1/500th of an inch. The apex of one of the cotyledonscrossed rather obliquely four divisions in 13 minutes; it was also sinking, as shown by getting out of focus. The seedlings were again placed indarkness for another hour, and the apex now crossed two divisions in 6 m. 18 s. ; that is, at very nearly the same rate as before. After anotherinterval of an hour in darkness, it crossed two divisions in 4 m. 15 s. , therefore at a quicker rate. In the afternoon, after a longer interval inthe dark, the apex was motionless, but after a time it recommenced moving, though slowly; perhaps the room was too cold. Judging from previous cases, there can hardly be a doubt that this seedling was circumnutating. 

Fig. 37. Solanum lycopersicum: circumnutation of hypocotyl, with filamentfixed across its summit, traced on horizontal glass, from 10 A. M. To 5 P. M. Oct. 24th. Illuminated obliquely from above. Movement of bead magnifiedabout 35 times, here reduced to one-third of original scale. 

Solanum lycopersicum (Solaneae). --The movements of the hypocotyls of twoseedling tomatoes were observed during seven hours, and there could be nodoubt that both circumnutated. They were illuminated from above, but by anaccident a little light entered on one side, and in the accompanying figure(Fig. 37) it may be seen that the hypocotyl moved to this side (the upperone in the figure), making small loops and zigzagging in its course. Themovements of the cotyledons were also traced both on vertical andhorizontal glasses; their angles with the horizon were likewise measured atvarious hours. They fell from 8. 30 A. M. (October 17th) to about noon; thenmoved laterally in a zigzag line, and at about 4 P. M. Began to rise; theycontinued to do so until 10. 30 P. M. , by which hour they stood verticallyand were asleep. At what hour of the night or early morning they began tofall was not ascertained. Owing to the lateral movement shortly aftermid-day, the descending and ascending lines did not coincide, and irregularellipses were described during each 24 h. The regular periodicity of thesemovements is destroyed, as we shall hereafter see, if the seedlings arekept in the dark. [page 51]

Solanum palinacanthum. --Several arched hypocotyls rising nearly . 2 of aninch above the ground, but with the cotyledons still buried beneath thesurface, were observed, and the tracings showed that they circumnutated. Moreover, in several cases little open circular spaces or cracks in theargillaceous sand which surrounded the arched hypocotyls were visible, andthese appeared to have been made by the hypocotyls having bent first to oneand then to another side whilst growing upwards. In two instances thevertical arches were observed to move to a considerable distance backwardsfrom the point where the cotyledons lay buried; this movement, which hasbeen noticed in some other cases, and which seems to aid in extracting thecotyledons from the buried seed-coats, is due to the commencement of thestraightening of the hypocotyl. In order to prevent this latter movement, the two legs of an arch, the

Fig. 38. Solanum palinacanthum: circumnutation of an arched hypocotyl, justemerging from the ground, with the two legs tied together, traced indarkness on a horizontal glass, from 9. 20 A. M. Dec. 17th to 8. 30 A. M. 19th. Movement of bead magnified 13 times; but the filament, which was affixedobliquely to the crown of the arch, was of unusual length. 

summit of which was on a level with the surface of the soil, were tiedtogether; the earth having been previously removed to a little depth allround. The movement of the arch during 47 hours under these unnaturalcircumstances is exhibited in the annexed figure. 

The cotyledons of some seedlings in the hot-house were horizontal aboutnoon on December 13th; and at 10 P. M. Had risen to an angle of 27o abovethe horizon; at 7 A. M. On the following[page 52]morning, before it was light, they had risen to 59o above the horizon; inthe afternoon of the same day they were found again horizontal. 

Beta vulgaris (Chenopodeae). --The seedlings are excessively sensitive tolight, so that although on the first day they were uncovered only duringtwo or three minutes at each observation, they all moved steadily towardsthe side of the room whence the light proceeded, and the tracings consistedonly of slightly zigzag lines directed towards the light. On the next daythe plants were placed in a completely darkened room, and at eachobservation were illuminated as much as possible from vertically above by asmall wax taper. The annexed figure (Fig. 39) shows the movement of thehypocotyl during 9 h. Under these circumstances. A second seedling wassimilarly observed at the same time, and the tracing had the same peculiarcharacter, due to the hypocotyl often moving and returning in nearlyparallel lines. The movement of a third hypocotyl differed greatly. 

Fig. 39. Beta vulgaris: circumnutation of hypocotyl, with filament fixedobliquely across its summit, traced in darkness on horizontal glass, from8. 25 A. M. To 5. 30 P. M. Nov. 4th. Movement of bead magnified 23 times, herereduced to one-third of original scale. 

We endeavoured to trace the movements of the cotyledons, and for thispurpose some seedlings were kept in the dark, but they moved in an abnormalmanner; they continued rising from 8. 45 A. M. To 2 P. M. , then movedlaterally, and from 3 to 6 P. M. Descended; whereas cotyledons which havebeen exposed all the day to the light rise in the evening so as to standvertically at night; but this statement applies only to young seedlings. For instance, six seedlings in the greenhouse had their cotyledonspartially open for the first time on the morning of November 15th, and at8. 45 P. M. All were completely closed, so that they might properly be saidto be asleep. Again, on the morning of November 27th, the cotyledons offour other seedlings, which were surrounded by a collar of brown paper sothat they received light only from above, were open to the extent of 39o;at 10 P. M. They were completely closed; next morning (November 28th) at6. 45 A. M. Whilst it was still dark, two of them[page 53]were partially open and all opened in the course of the morning; but at10. 20 P. M. All four (not to mention nine others which had been open in themorning and six others on another occasion) were again completely closed. On the morning of the 29th they were open, but at night only one of thefour was closed, and this only partially; the three others had theircotyledons much more raised than during the day. On the night of the 30ththe cotyledons of the four were only slightly raised. 

Ricinus Borboniensis (Euphorbiaceae). --Seeds were purchased under the abovename--probably a variety of the common castor-oil plant. As soon as anarched hypocotyl had risen clear above the ground, a filament was attachedto the upper leg bearing the cotyledons which were still buried beneath thesurface, and the movement of the bead was traced on a horizontal glassduring a period of 34 h. The lines traced were strongly zigzag, and as thebead twice returned nearly parallel to its former course in two differentdirections, there could be no doubt that the arched hypocotylcircumnutated. At the close of the 34 h. The upper part began to rise andstraighten itself, dragging the cotyledons out of the ground, so that themovements of the bead could no longer be traced on the glass. 

Quercus (American sp. ) (Cupuliferae). --Acorns of an American oak which hadgerminated at Kew were planted in a pot in the greenhouse. Thistransplantation checked their growth; but after a time one grew to a heightof five inches, measured to the tips of the small partially unfolded leaveson the summit, and now looked vigorous. It consisted of six very thininternodes of unequal lengths. Considering these circumstances and thenature of the plant, we hardly expected that it would circumnutate; but theannexed figure (Fig. 40) shows that it did so in a conspicuous manner, changing its course many times and travelling in all directions during the48 h. Of observation. The figure seems to represent 5 or 6 irregular ovalsor ellipses. The actual amount of movement from side to side (excluding onegreat bend to the left) was about . 2 of an inch; but this was difficult toestimate, as owing to the rapid growth of the stem, the attached filamentwas much further from the mark beneath at the close than at thecommencement of the observations. It deserves notice that the pot wasplaced in a north-east room within a deep box, the top of which was not atfirst covered up, so that the inside facing[page 54]the windows was a little more illuminated than the opposite side; andduring the first morning the stem travelled to a greater distance in thisdirection (to the left in the figure) than it did afterwards when the boxwas completely protected from light. 

Fig. 40. Quercus (American sp. ): circumnutation of young stem, traced onhorizontal glass, from 12. 50 P. M. Feb. 22nd to 12. 50 P. M. 24th. Movement ofbead greatly magnified at first, but slightly towards the close of theobservations--about 10 times on an average. 

Quercus robur. --Observations were made only on the movements of theradicles from germinating acorns, which were allowed to grow downwards inthe manner previously described, over plates of smoked glass, inclined atangles between 65o and 69o to the horizon. In four cases the tracks leftwere almost straight, but the tips had pressed sometimes with more andsometimes with less force on the glass, as shown by the varying thicknessof the tracks and by little bridges of soot left across them. In the fifthcase the track was slightly serpentine, that is, the tip had moved a littlefrom side to side. In the sixth case (Fig. 41, A) it was plainlyserpentine, and the tip had pressed almost equably on the glass in itswhole course. In the seventh case (B) the tip had moved both laterally andhad pressed[page 55]alternately with unequal force on the glass; so that it had moved a littlein two planes at right angles to one another. In the eighth and last case(C) it had moved very little laterally, but had alternately left the glassand come into contact with it again. There can be no doubt that in the lastfour cases the radicle of the oak circumnutated whilst growing downwards. 

Fig. 41. Quercus robur: tracks left on inclined smoked glass-plates by tipsof radicles in growing downwards. Plates A and C inclined at 65o and plateB at 68o to the horizon. 

Corylus avellana (Corylaceae). --The epicotyl breaks through the ground inan arched form; but in the specimen which was first examined, the apex hadbecome decayed, and the epicotyl grew to some distance through the soil, ina tortuous, almost horizontal direction, like a root. In consequence ofthis injury it had emitted near the hypogean cotyledons two secondaryshoots, and it was remarkable that both of these were arched, like thenormal epicotyl in ordinary cases. The soil was removed from around one ofthese arched secondary shoots, and a glass filament was affixed to thebasal leg. The whole was kept damp beneath a metal-box with a glass lid, and was thus illuminated only from above. Owing apparently to the lateralpressure of the earth being removed, the terminal and bowed-down part ofthe shoot began at once to move upwards, so that after 24 h. It formed aright angle with the lower part. This lower part, to which the filament wasattached, also straightened itself, and moved a little backwards from theupper part. Consequently a long line was traced on the horizontal glass;and[page 56]this was in parts straight and in parts decidedly zigzag, indicatingcircumnutation. 

On the following day the other secondary shoot was observed; it was alittle more advanced in age, for the upper part, instead of dependingvertically downwards, stood at an angle of 45o above the horizon. The tipof the shoot projected obliquely . 4 of an inch above the ground, but by theclose of our observations, which lasted 47 h. , it had grown, chieflytowards its base, to a height of . 85 of an inch. The filament was fixedtransversely to the basal and almost upright half of the shoot, closebeneath the lowest scale-like appendage. The circumnutating course pursuedis shown in the accompanying figure (Fig. 42). The actual distancetraversed from side to side was about . 04 of an inch. 

Fig. 42. Corylus avellana: circumnutation of a young shoot emitted from theepicotyl, the apex of which had been injured, traced on a horizontal glass, from 9 A. M. Feb. 2nd to 8 A. M. 4th. Movement of bead magnified about 27times. 

Pinus pinaster (Coniferae). --A young hypocotyl, with the tips of thecotyledons still enclosed within the seed-coats, was at first only . 35 ofan inch in height; but the upper part grew so rapidly that at the end ofour observations it was . 6 in height, Fig. 43. Pinus pinaster: circumnutation of hypocotyl, with filament fixedacross its summit, traced on horizontal glass, from 10 A. M. March 21st to 9A. M. 23rd. Seedling kept in darkness. Movement of bead magnified about 35times. [page 57]

and by this time the filament was attached some way down the little stem. From some unknown cause, the hypocotyl moved far towards the left, butthere could be no doubt (Fig. 43) that it circumnutated. Another hypocotylwas similarly observed, and it likewise moved in a strongly zigzag line tothe same side. This lateral movement was not caused by the attachment ofthe glass filaments, nor by the action of light; for no light was allowedto enter when each observation was made, except from vertically above. 

The hypocotyl of a seedling was secured to a little stick; it bore nine inappearance distinct cotyledons, arranged in a circle. The movements of twonearly opposite ones were observed. The tip of one was painted white, witha mark placed below, and the figure described (Fig. 44, A) shows that itmade an irregular

Fig. 44. Pinus pinaster: circumnutation of two opposite cotyledons, tracedon horizontal glass in darkness, from 8. 45 A. M. To 8. 35 P. M. Nov. 25th. Movement of tip in A magnified about 22 times, here reduced to one-half oforiginal scale. 

circle in the course of about 8 h. During the night it travelled to aconsiderable distance in the direction indicated by the broken line. Aglass filament was attached longitudinally to the other cotyledon, and thisnearly completed (Fig, 44, B) an irregular circular figure in about 12hours. During the night it also moved to a considerable distance, in thedirection indicated by the broken line. The cotyledons thereforecircumnutate independently of the movement of the hypocotyl. Although theymoved much during the night, they did not approach each other so as tostand more vertically than during the day. [page 58]

Cycas pectinata (Cycadeae). --The large seeds of this plant in germinatingfirst protrude a single leaf, which breaks through the ground with thepetiole bowed into an arch and with the leaflets involuted. A leaf in thiscondition, which at the close of our observations was 2 ½ inches in height, had its movements traced in a warm greenhouse by means of a glass filamentbearing paper triangles attached across its tip. The tracing (Fig. 45)shows how large, complex, and rapid were the circum-

Fig. 45. Cycas pectinata: circumnutation of young leaf whilst emerging fromthe ground, feebly illuminated from above, traced on vertical glass, from 5P. M. May 28th to 11 A. M. 31st. Movement magnified 7 times, here reduced totwo-thirds of original scale. 

nutating movements. The extreme distance from side to side which it passedover amounted to between . 6 and . 7 of an inch. 

Canna Warscewiczii (Cannaceae). --A seedling with the plumule projecting oneinch above the ground was observed, but not under fair conditions, as itwas brought out of the hot-house and kept in a room not sufficiently warm. Nevertheless the tracing (Fig. 46) shows that it made two or threeincomplete irregular circles or ellipses in the course of 48 hours. Theplumule is straight; and this was the first instance observed[page 59]by us of the part that first breaks through the ground not being arched. 

Fig. 46. Canna Warscewiczii: circumnutation of plumule with filamentaffixed obliquely to outer sheath-like leaf, traced in darkness onhorizontal glass from 8. 45 A. M. Nov. 9th to 8. 10 A. M. 11th. Movement ofbead magnified 6 times. 

Allium cepa (Liliaceae). --The narrow green leaf, which protrudes from theseed of the common onion as a cotyledon, * breaks through the ground in theform of an arch, in the same manner as the hypocotyl or epicotyl of adicotyledonous plant. Long after the arch has risen above the surface theapex remains within the seed-coats, evidently absorbing the still abundantcontents. The summit or crown of the arch, when it first protrudes from theseed and is still buried beneath the ground, is simply rounded; but beforeit reaches the surface it is developed into a conical protuberance of awhite colour (owing to the absence of chlorophyll), whilst the adjoiningparts are green, with the epidermis apparently rather thicker and tougherthan elsewhere. We may therefore conclude that this conical protuberance isa special adaptation for breaking through the ground, ** and answers thesame end as the knife-like white crest on the summit of the straightcotyledon of the Gramineae. 

* This is the expression used by Sachs in his 'Text-book of Botany. '

** Haberlandt has briefly described ('DieSchutzeinrichtungen... Keimpflanze, ' 1877, p. 77) this curious structure andthe purpose which it subserves. He states that good figures of thecotyledon of the onion have been given by Tittmann and by Sachs in his'Experimental Physiologie, ' p. 93. [page 60]

After a time the apex is drawn out of the empty seed-coats, and rises up, forming a right angle, or more commonly a still larger angle with the lowerpart, and occasionally the whole becomes nearly straight. The conicalprotuberance, which originally formed the crown of the arch, is now seatedon one side, and appears like a joint or knee, which from acquiringchlorophyll becomes green, and increases in size. In rarely or neverbecoming perfectly straight, these cotyledons differ remarkably from theultimate condition of the arched hypocotyls or epicotyls of dicotyledons. It is, also, a singular circumstance that the attenuated extremity of theupper bent portion invariably withers and dies. 

A filament, 1. 7 inch in length, was affixed nearly upright beneath the kneeto the basal and vertical portion of a cotyledon; and its movements weretraced during 14 h. In the usual manner. The tracing here given (Fig. 47)indicates circumnutation. The movement of the upper part above the knee ofthe same cotyledon, which projected at about an angle of 45o above thehorizon, was observed at the same time. A filament was not affixed to it, but a mark was placed beneath the apex, which was almost white frombeginning to wither, and its movements were thus traced. The figuredescribed resembled pretty closely that above given; and this shows thatthe chief seat of movement is in the lower or basal part of the cotyledon. 

Fig. 47. Allium cepa: circumnutation of basal half of arched cotyledon, traced in darkness on horizontal glass, from 8. 15 A. M. To 10 P. M. Oct. 31st. Movement of bead magnified about 17 times. 

Asparagus officinalis (Asparageae). --The tip of a straight plumule orcotyledon (for we do not know which it should be called) was found at adepth of . 1 inch beneath the surface, and the earth was then removed allround to the dept of . 3 inch. A glass filament was affixed obliquely to it, and the movement of the bead, magnified 17 times, was traced in darkness. During the first 1 h. 15 m. The plumule moved to the right, and during thenext two hours it returned in a roughly parallel but strongly zigzagcourse. From some unknown cause it had grown up through the soil in aninclined direction, and now through apogeotropism it moved during nearly 24h. In[page 61]the same general direction, but in a slightly zigzag manner, until itbecame upright. On the following morning it changed its course completely. There can therefore hardly be a doubt that the plumule circumnutates, whilst buried beneath the ground, as much as the pressure of thesurrounding earth will permit. The surface of the soil in the pot was nowcovered with a thin layer of very fine argillaceous sand, which was keptdamp; and after the tapering seedlings had grown a few tenths of an inch inheight, each was found surrounded by a little open space or circular crack;and this could be accounted for only by their having circumnutated and thuspushed away the sand on all sides; for there was no vestige of a crack inany other part. 

In order to prove that there was circumnutation, the move-

Fig. 48. Asparagus officinalis: circumnutation of plumules with tipswhitened and marks placed beneath, traced on a horizontal glass. A, youngplumule; movement traced from 8. 30 A. M. Nov. 30th to 7. 15 A. M. Nextmorning; magnified about 35 times. B, older plumule; movement traced from10. 15 A. M. To 8. 10 P. M. Nov. 29th; magnified 9 times, but here reduced toone-half of original scale. 

ments of five seedlings, varying in height from . 3 inch to 2 inches, weretraced. They were placed within a box and illuminated from above; but inall five cases the longer axes of the figures described were directed tonearly the same point; so that more light seemed to have come through theglass roof of the greenhouse on one side than on any other. All fivetracings resembled each other to a certain extent, and it will suffice togive two of them. In A (Fig. 48) the seedling was only . 45 of an[page 62]inch in height, and consisted of a single internode bearing a bud on itssummit. The apex described between 8. 30 A. M. And 10. 20 P. M. (i. E. Duringnearly 14 hours) a figure which would probably have consisted of 3 ½ellipses, had not the stem been drawn to one side until 1 P. M. , after whichhour it moved backwards. On the following morning it was not far distantfrom the point whence it had first started. The actual amount of movementof the apex from side to side was very small, viz. About 1/18th of an inch. The seedling of which the movements are shown in Fig. 48, B, was 1 3/4 inchin height, and consisted of three internodes besides the bud on the summit. The figure, which was described during 10 h. , apparently represents twoirregular and unequal ellipses or circles. The actual amount of movement ofthe apex, in the line not influenced by the light, was . 11 of an inch, andin that thus influenced . 37 of an inch. With a seedling 2 inches in heightit was obvious, even without the aid of any tracing, that the uppermostpart of the stem bent successively to all points of the compass, like thestem of a twining plant. A little increase in the power of circumnutatingand in the flexibility of the stem, would convert the common asparagus intoa twining plant, as has occurred with one species in this genus, namely, A. Scandens. 

Phalaris Canariensis (Gramineae). --With the Gramineae the part which firstrises above the ground has been called by some authors the pileole; andvarious views have been expressed on its homological nature. It isconsidered by some great authorities to be a cotyledon, which term we willuse without venturing to express any opinion on the subject. * It consistsin the present case of a slightly flattened reddish sheath, terminatingupwards in a sharp white edge; it encloses a true green leaf, whichprotrudes from the sheath through a slit-like orifice, close beneath and atright angles to the sharp edge on the summit. The sheath is not arched whenit breaks through the ground. 

The movements of three rather old seedlings, about 1 ½ inch in height, shortly before the protrusion of the leaves, were first traced. They wereilluminated exclusively from above; for, as will hereafter be shown, theyare excessively sensitive to the* We are indebted to the Rev. G. Henslow for an abstract of the views whichhave been held on this subject, together with references. [page 63]

action of light; and if any enters even temporarily on one side, theymerely bend to this side in slightly zigzag lines. Of the three tracingsone alone (Fig. 49) is here given. Had the observations been more frequentduring the 12 h. Two oval figures would have been described with theirlonger axes at right angles to one another. The actual amount of movementof the apex from side to side was about . 3 of an inch. The figuresdescribed by the other two seedlings resembled to a certain extent the onehere given. 

Fig. 49. Phalaris Canariensis: circumnutation of a cotyledon, with a markplaced below the apex, traced on a horizontal glass, from 8. 35 A. M. Nov. 26th to 8. 45 A. M. 27th. Movement of apex magnified 7 times, here reduced toone-half scale. 

A seedling which had just broken through the ground and projected only1/20th of an inch above the surface, was next observed in the same manneras before. It was necessary to clear away the earth all round the seedlingto a little depth in order to place a mark beneath the apex. The figure(Fig. 50) shows that the apex moved to one side, but changed its course tentimes in the course of the ten hours of observation; so that there can beno doubt about its circumnutation. The cause of the general movement in onedirection could hardly be attributed to the entrance of lateral light, asthis was carefully guarded against; and we suppose it was in some mannerconnected with the removal of the earth round the little seedling. 

Fig. 50. Phalaris Canariensis: circumnutation of a very young cotyledon, with a mark placed below the apex, traced on a horizontal glass, from 11. 37A. M. To 9. 30 P. M. Dec. 13th. Movement of apex greatly magnified, herereduced to one-fourth of original scale. 

Lastly, the soil in the same pot was searched with the aid of a lens, andthe white knife-like apex of a seedling was found on an exact level withthat of the surrounding surface. The soil was removed all round the apex tothe depth of a quarter of an inch, the seed itself remaining covered. Thepot, protected from lateral light, was placed under the micro-[page 64]scope with a micrometer eye-piece, so arranged that each division equalled1/500th of an inch. After an interval of 30 m. The apex was observed, andit was seen to cross a little obliquely two divisions of the micrometer in9 m. 15 s. ; and after a few minutes it crossed the same space in 8 m. 50s. The seedling was again observed after an interval of three-quarters of anhour, and now the apex crossed rather obliquely two divisions in 10 m. Wemay therefore conclude that it was travelling at about the rate of 1/50thof an inch in 45 minutes. We may also conclude from these and the previousobservations, that the seedlings of Phalaris in breaking through thesurface of the soil circumnutate as much as the surrounding pressure willpermit. This fact accounts (as in the case before given of the asparagus)for a circular, narrow, open space or crack being distinctly visible roundseveral seedlings which had risen through very fine argillaceous sand, keptuniformly damp. 

Fig. 51. Zea mays: circumnutation of cotyledon, traced on horizontal glass, from 8. 30 A. M. Feb. 4th to 8 A. M. 6th. Movement of bead magnified on anaverage about 25 times. 

Zea mays (Gramineae). --A glass filament was fixed obliquely to the summitof a cotyledon, rising . 2 of an inch above the ground; but by the thirdmorning it had grown to exactly thrice this height, so that the distance ofthe bead from the mark below was greatly increased, consequently thetracing (Fig. 51) was much more magnified on the first than on the secondday. The upper part of the cotyledon changed its course by at least as muchas a rectangle six times on each of the two days. The plant was illuminatedby an obscure light from vertically above. This was a necessary precaution, as on the previous day we had traced the movements of cotyledons placed ina deep box, the inner side of which was feebly illuminated on one side froma distant north-east window, and at each observation by a wax taper heldfor a minute or two on the same side; and the result was that thecotyledons travelled all day long to this side, though making in theircourse some conspicuous flexures, from which fact alone we might have[page 65]concluded that they were circumnutating; but we thought it advisable tomake the tracing above given. 

Radicles. --Glass filaments were fixed to two short radicles, placed so asto stand almost upright, and whilst bending downwards through geotropismtheir courses were strongly zigzag; from this latter circumstancecircumnutation might have been inferred, had not their tips become slightlywithered after the first 24 h. , though they were watered and the air keptvery damp. Nine radicles were next arranged in the manner formerlydescribed, so that in growing downwards they left tracks on smokedglass-plates, inclined at various angles between 45o and 80o beneath thehorizon. Almost every one of these tracks offered evidence in their greateror less breadth in different parts, or in little bridges of soot beingleft, that the apex had come alternately into more and less close contactwith the glass. In the accompanying figure (Fig. 52) we have an accuratecopy of one such track. In two instances alone (and in these the plateswere highly inclined) there was some evidence of slight lateral movement. We presume therefore that the friction of the apex on the smoked surface, little as this could have been, sufficed to check the movement from side toside of these delicate radicles. 

Fig. 52. Zea mays: track left on inclined smoked glass-plate by tip ofradicle in growing downwards. 

Avena sativa (Gramineae). --A cotyledon, 1 ½ inch in height, was placed infront of a north-east window, and the movement of the apex was traced on ahorizontal glass during two days. It moved towards the light in a slightlyzigzag line from 9 to 11. 30 A. M. On October 15th; it then moved a littlebackwards and zigzagged much until 5 P. M. , after which hour, and curing thenight, it continued to move towards the window. On the following morningthe same movement was continued in a nearly straight line until 12. 40 P. M. , when the sky remained until 2. 35 extraordinarily dark from thunder-clouds. During this interval of 1 h. 55 m. , whilst the light was obscure, it wasinteresting to observe how circumnutation overcame heliotropism, for theapex, instead of continuing to move towards the window in a slightly zigzagline, reversed its course four times, making two small narrow ellipses. Adiagram of this case will be given in the chapter on Heliotropism. [page 66]

A filament was next fixed to a cotyledon only 1/4 of an inch in height, which was illuminated exclusively from above, and as it was kept in a warmgreenhouse, it grew rapidly; and now there could be no doubt about itscircumnutation, for it described a figure of 8 as well as two smallellipses in 5 ½ hours. 

Nephrodium molle (Filices). --A seedling fern of this species came up bychance in a flowerpot near its parent. The frond, as yet only slightlylobed, was only . 16 of an inch in length and . 2 in breadth, and wassupported on a rachis as fine as a hair and . 23 of an inch in height. Avery thin glass filament, which projected for a length of . 36 of an inch, was fixed to the end of the frond. The movement was so highly magnifiedthat the figure (Fig. 53) cannot be fully trusted; but the frond wasconstantly moving in a complex manner, and the bead greatly changed itscourse eighteen times in the 12 hours of observation. Within half an hourit often returned in a line almost parallel to its former course. Thegreatest amount of movement occurred between 4 and 6 P. M. Thecircumnutation of this plant is interesting, because the species in thegenus Lygodium are well known to circumnutate conspicuously and to twineround any neighbouring object. 

Fig. 53. Nephrodium molle: circumnutation of very young frond, traced indarkness on horizontal glass, from 9 A. M. To 9 P. M. Oct. 30th. Movement ofbead magnified 48 times. 

Selaginella Kraussii (?) (Lycopodiaceae). --A very young plant, only . 4 ofan inch in height, had sprung up in a pot in the hot-house. An extremelyfine glass filament was fixed to the end of the frond-like stem, and themovement of the bead traced on a horizontal glass. It changed its courseseveral times, as shown in Fig. 54, whilst observed during 13 h. 15 m. , andreturned at night to a point not far distant from that whence it hadstarted in the morning. There can be no doubt that this little plantcircumnutated. 

Fig. 54. Selaginella Kraussii (?): circumnutation of young plant, kept indarkness, traced from 8. 45 A. M. To 10 P. M. Oct. 31st. [page 67]

CHAPTER II. 

GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS. 

Generality of the circumnutating movement--Radicles, their circumnutationof service--Manner in which they penetrate the ground--Manner in whichhypocotyls and other organs break through the ground by being arched--Singular manner of germination in Megarrhiza, etc. --Abortion of cotyledons--Circumnutation of hypocotyls and epicotyls whilst still buried and arched--Their power of straightening themselves--Bursting of the seed-coats--Inherited effect of the arching process in hypogean hypocotyls--Circumnutation of hypocotyls and epicotyls when erect--Circumnutation ofcotyledons--Pulvini or joints of cotyledons, duration of their activity, rudimentary in Oxalis corniculata, their development--Sensitiveness ofcotyledons to light and consequent disturbance of their periodic movements--Sensitiveness of cotyledons to contact. 

THE circumnutating movements of the several parts or organs of aconsiderable number of seedling plants have been described in the lastchapter. A list is here appended of the Families, Cohorts, Sub-classes, etc. , to which they belong, arranged and numbered according to theclassification adopted by Hooker. * Any one who will consider this list willsee that the young plants selected for observation, fairly represent thewhole vegetable series excepting the lowest cryptogams, and the movementsof some of the latter when mature will hereafter be described. As all theseedlings which were observed, including Conifers, Cycads and Ferns, whichbelong to the most ancient

* As given in the 'General System of Botany, ' by Le Maout and Decaisne, 1873. [page 68]

types amongst plants, were continually circumnutating, we may infer thatthis kind of movement is common to every seedling species. 

SUB-KINGDOM I. --Phaenogamous Plants. 

Class I. --DICOTYLEDONS. 

Sub-class I. --Angiosperms. Family. Cohort. 14. Cruciferae. II. PARIETALES. 26. Caryophylleae. IV. CARYOPHYLLALES. 36. Malvaceae. VI MALVALES. 41. Oxalideae. VII. GERANIALES. 49. Tropaeoleae. DITTO52. Aurantiaceae. DITTO70. Hippocastaneae. X. SAPINDALES. 75. Leguminosae. XI. ROSALES. 106. Cucurbitaceae. XII. PASSIFLORALES. 109. Cacteae. XIV. FICOIDALES. 122. Compositae. XVII. ASTRALES. 135. Primulaceae. XX. PRIMULALES. 145. Asclepiadeae. XXII. GENTIANALES. 151. Convolvulaceae. XXIII. POLEMONIALES. 154. Boragineae. DITTO156. Nolaneae. DITTO157. Solaneae. XXIV. SOLANALES. 181. Chenopodieae. XXVII. CHENOPODIALES. 202. Euphorbiaceae. XXXII. EUPHORBIALES. 211. Cupuliferae. XXXVI. QUERNALES. 212. Corylaceae. DITTO

Sub-class II. --Gymnosperms. 223. Coniferae. 224. Cycadeae. 

Class II. --MONOCOTYLEDONS. 2. Cannaceae. II. AMOMALES. 34. Liliaceae. XI. LILIALES. 41. Asparageae. DITTO55. Gramineae. XV. GLUMALES. 

SUB-KINGDOM II. --Cryptogamic Plants. 

1. Filices. I. FILICALES. 6. Lycopodiaceae. DITTO[page 69]

Radicles. --In all the germinating seeds observed by us, the first change isthe protrusion of the radicle, which immediately bends downwards andendeavours to penetrate the ground. In order to effect this, it is almostnecessary that the seed should be pressed down so as to offer someresistance, unless indeed the soil is extremely loose; for otherwise theseed is lifted up, instead of the radicle penetrating the surface. Butseeds often get covered by earth thrown up by burrowing quadrupeds orscratching birds, by the castings of earth-worms, by heaps of excrement, the decaying branches of trees, etc. , and will thus be pressed down; andthey must often fall into cracks when the ground is dry, or into holes. Even with seeds lying on the bare surface, the first developed root-hairs, by becoming attached to stones or other objects on the surface, are able tohold down the upper part of the radicle, whilst the tip penetrates theground. Sachs has shown* how well and closely root-hairs adapt themselvesby growth to the most irregular particles in the soil, and become firmlyattached to them. This attachment seems to be effected by the softening orliquefaction of the outer surface of the wall of the hair and itssubsequent consolidation, as will be on some future occasion more fullydescribed. This intimate union plays an important part, according to Sachs, in the absorption of water and of the inorganic matter dissolved in it. Themechanical aid afforded by the root-hairs in penetrating the ground isprobably only a secondary service. 

The tip of the radicle, as soon as it protrudes from the seed-coats, beginsto circumnutate, and the whole

* 'Physiologie Végétale, ' 1868, pp. 199, 205. [page 70]

growing part continues to do so, probably for as long as growth continues. This movement of the radicle has been described in Brassica, Aesculus, Phaseolus, Vicia, Cucurbita, Quercus and Zea. The probability of itsoccurrence was inferred by Sachs, * from radicles placed vertically upwardsbeing acted on by geotropism (which we likewise found to be the case), forif they had remained absolutely perpendicular, the attraction of gravitycould not have caused them to bend to any one side. Circumnutation wasobserved in the above specified cases, either by means of extremely finefilaments of glass affixed to the radicles in the manner previouslydescribed, or by their being allowed to grow downwards over inclined smokedglass-plates, on which they left their tracks. In the latter cases theserpentine course (see Figs. 19, 21, 27, 41) showed unequivocally that theapex had continually moved from side to side. This lateral movement wassmall in extent, being in the case of Phaseolus at most about 1 mm. From amedial line to both sides. But there was also movement in a vertical planeat right angles to the inclined glass-plates. This was shown by the tracksoften being alternately a little broader and narrower, due to the radicleshaving alternately pressed with greater and less force on the plates. Occasionally little bridges of soot were left across the tracks, showingthat the apex had at these spots been lifted up. This latter fact wasespecially apt to occur* 'Ueber das Wachsthum der Wurzeln: Arbeiten des bot. Instituts inWürzburg, ' Heft iii. 1873, p. 460. This memoir, besides its intrinsic andgreat interest, deserves to be studied as a model of careful investigation, and we shall have occasion to refer to it repeatedly. Dr. Frank hadpreviously remarked ('Beiträge zur Pflanzenphysiologie, 1868, p. 81) on thefact of radicles placed vertically upwards being acted on by geotropism, and he explained it by the supposition that their growth was not equal onall sides. 

[page 71]when the radicle instead of travelling straight down the glass made asemicircular bend; but Fig. 52 shows that this may occur when the track isrectilinear. The apex by thus rising, was in one instance able to surmounta bristle cemented across an inclined glass-plate; but slips of wood only1/40 of an inch in thickness always caused the radicles to bendrectangularly to one side, so that the apex did not rise to this smallheight in opposition to geotropism. 

In those cases in which radicles with attached filaments were placed so asto stand up almost vertically, they curved downwards through the action ofgeotropism, circumnutating at the same time, and their courses wereconsequently zigzag. Sometimes, however, they made great circular sweeps, the lines being likewise zigzag. 

Radicles closely surrounded by earth, even when this is thoroughly soakedand softened, may perhaps be quite prevented from circumnutating. Yet weshould remember that the circumnutating sheath-like cotyledons of Phalaris, the hypocotyls of Solanum, and the epicotyls of Asparagus formed roundthemselves little circular cracks or furrows in a superficial layer of dampargillaceous sand. They were also able, as well as the hypocotyls ofBrassica, to form straight furrows in damp sand, whilst circumnutating andbending towards a lateral light. In a future chapter it will be shown thatthe rocking or circumnutating movement of the flower-heads of Trifoliumsubterraneum aids them in burying themselves. It is therefore probable thatthe circumnutation of the tip of the radicle aids it slightly inpenetrating the ground; and it may be observed in several of the previouslygiven diagrams, that the movement is more strongly pronounced in radicleswhen they first[page 72]protrude from the seed than at a rather later period; but whether this isan accidental or an adaptive coincidence we do not pretend to decide. Nevertheless, when young radicles of Phaseolus multiflorus were fixedvertically close over damp sand, in the expectation that as soon as theyreached it they would form circular furrows, this did not occur, --a factwhich may be accounted for, as we believe, by the furrow being filled up assoon as formed by the rapid increase of thickness in the apex of theradicle. Whether or not a radicle, when surrounded by softened earth, isaided in forming a passage for itself by circumnutating, this movement canhardly fail to be of high importance, by guiding the radicle along a lineof least resistance, as will be seen in the next chapter when we treat ofthe sensibility of the tip to contact. If, however, a radicle in itsdownward growth breaks obliquely into any crevice, or a hole left by adecayed root, or one made by the larva of an insect, and more especially byworms, the circumnutating movement of the tip will materially aid it infollowing such open passage; and we have observed that roots commonly rundown the old burrows of worms. *

 When a radicle is placed in a horizontal or inclined position, theterminal growing part, as is well known, bends down towards the centre ofthe earth; and Sachs* has shown that whilst thus bending, the growth of thelower surface is greatly retarded, whilst that

* See, also, Prof. Hensen's statements ('Zeitschrift für Wissen, Zool. , ' B. Xxviii. P. 354, 1877) to the same effect. He goes so far as to believe thatroots are able to penetrate the ground to a great depth only by means ofthe burrows made by worms. 

* 'Arbeiten des bot. Inst. Würzburg, ' vol. I. 1873, p. 461. See also p. 397for the length of the growing part, and p. 451 on the force of geotropism. [page 73]

of the upper surface continues at the normal rate, or may be even somewhatincreased. He has further shown by attaching a thread, running over apulley, to a horizontal radicle of large size, namely that of the commonbean, that it was able to pull up a weight of only one gramme, or 15. 4grains. We may therefore conclude that geotropism does not give a radicleforce sufficient to penetrate the ground, but merely tells it (if such anexpression may be used) which course to pursue. Before we knew of Sachs'more precise observations we covered a flat surface of damp sand with thethinnest tin-foil which we could procure (. 02 to . 03 mm. , or . 00012 to. 00079 of an inch in thickness), and placed a radicle close above, in sucha position that it grew almost perpendicularly downwards. When the apexcame into contact with the polished level surface it turned at right anglesand glided over it without leaving any impression; yet the tin-foil was soflexible, that a little stick of soft wood, pointed to the same degree asthe end of the radicle and gently loaded with a weight of only a quarter ofan ounce (120 grains) plainly indented the tin-foil. 

Radicles are able to penetrate the ground by the force due to theirlongitudinal and transverse growth; the seeds themselves being held down bythe weight of the superincumbent soil. In the case of the bean the apex, protected by the root-cap, is sharp, and the growing part, from 8 to 10 mm. In length, is much more rigid, as Sachs has proved, than the partimmediately above, which has ceased to increase in length. We endeavouredto ascertain the downward pressure of the growing part, by placinggerminating beans between two small metal plates, the upper one of whichwas loaded with a known weight; and the[page 74]radicle was then allowed to grow into a narrow hole in wood, 2 or 3 tenthsof an inch in depth, and closed at the bottom. The wood was so cut that theshort space of radicle between the mouth of the hole and the bean could notbend laterally on three sides; but it was impossible to protect the fourthside, close to the bean. Consequently, as long as the radicle continued toincrease in length and remained straight, the weighted bean would be liftedup after the tip had reached the bottom of the shallow hole. Beans thusarranged, surrounded by damp sand, lifted up a quarter of a pound in 24 h. After the tip of the radicle had entered the hole. With a greater weightthe radicles themselves always became bent on the one unguarded side; butthis probably would not have occurred if they had been closely surroundedon all sides by compact earth. There was, however, a possible, but notprobable, source of error in these trials, for it was not ascertainedwhether the beans themselves go on swelling for several days after theyhave germinated, and after having been treated in the manner in which ourshad been; namely, being first left for 24 h. In water, then allowed togerminate in very damp air, afterwards placed over the hole and almostsurrounded by damp sand in a closed box. 

Fig. 55. Outline of piece of stick (reduced to one-half natural size) witha hole through which the radicle of a bean grew. Thickness of stick atnarrow end . 08 inch, at broad end . 16; depth of hole . 1 inch. We succeeded better in ascertaining the force exerted transversely by theseradicles. Two were so placed as to penetrate small holes made in littlesticks, one of which was cut into the shape here exactly copied (Fig. 55). The short end of the stick beyond the hole was purposely split, but not theopposite[page 75]end. As the wood was highly elastic, the split or fissure closedimmediately after being made. After six days the stick and bean were dugout of the damp sand, and the radicle was found to be much enlarged aboveand beneath the hole. The fissure which was at first quite closed, was nowopen to a width of 4 mm. ; as soon as the radicle was extracted, itimmediately closed to a width of 2 mm. The stick was then suspendedhorizontally by a fine wire passing through the hole lately filled by theradicle, and a little saucer was suspended beneath to receive the weights;and it required 8 lbs. 8 ozs. To open the fissure to the width of 4 mm. --that is, the width before the root was extracted. But the part of theradicle (only . 1 of an inch in length) which was embedded in the hole, probably exerted a greater transverse strain even than 8 lbs. 8 ozs. , forit had split the solid wood for a length of rather more than a quarter ofan inch (exactly . 275 inch), and this fissure is shown in Fig. 55. A secondstick was tried in the same manner with almost exactly the same result. 

Fig. 56. Wooden pincers, kept closed by a spiral brass spring, with a hole(. 14 inch in diameter and . 6 inch in depth) bored through the narrow closedpart, through which a radicle of a bean was allowed to grow. Temp. 50o -60o F. 

We then followed a better plan. Holes were bored near the narrow end of twowooden clips or pincers (Fig. 56), kept closed by brass spiral springs. Tworadicles in damp sand were allowed to grow through these holes. The[page 76]pincers rested on glass-plates to lessen the friction from the sand. Theholes were a little larger (viz.. 14 inch) and considerably deeper (viz.. 6inch) than in the trials with the sticks; so that a greater length of arather thicker radicle exerted a transverse strain. After 13 days they weretaken up. The distance of two dots (see the figure) on the longer ends ofthe pincers was now carefully measured; the radicles were then extractedfrom the holes, and the pincers of course closed. They were then suspendedhorizontally in the same manner as were the bits of sticks, and a weight of1500 grams (or 3 pounds 4 ounces) was necessary with one of the pincers toopen them to the same extent as had been effected by the transverse growthof the radicle. As soon as this radicle had slightly opened the pincers, ithad grown into a flattened form and had escaped a little beyond the hole;its diameter in one direction being 4. 2 mm. , and at rightangles 3. 5 mm. Ifthis escape and flattening could have been prevented, the radicle wouldprobably have exerted a greater strain than the 3 pounds 4 ounces. With theother pincers the radicle escaped still further out of the hole; and theweight required to open them to the same extent as had been effected by theradicle, was only 600 grams. 

With these facts before us, there seems little difficulty in understandinghow a radicle penetrates the ground. The apex is pointed and is protectedby the root-cap; the terminal growing part is rigid, and increases inlength with a force equal, as far as our observations can be trusted, tothe pressure of at least a quarter of a pound, probably with a much greaterforce when prevented from bending to any side by the surrounding earth. Whilst thus increasing in length it increases in thickness, pushing awaythe damp[page 77]earth on all sides, with a force of above 8 pounds in one case, of 3 poundsin another case. It was impossible to decide whether the actual apexexerts, relatively to its diameter, the same transverse strain as the partsa little higher up; but there seems no reason to doubt that this would bethe case. The growing part therefore does not act like a nail when hammeredinto a board, but more like a wedge of wood, which whilst slowly driveninto a crevice continually expands at the same time by the absorption ofwater; and a wedge thus acting will split even a mass of rock. 

Manner in which Hypocotyls, Epicotyls, etc. , rise up and break through theground. --After the radicle has penetrated the ground and fixed the seed, the hypocotyls of all the dicotyledonous seedlings observed by us, whichlift their cotyledons above the surface, break through the ground in theform of an arch. When the cotyledons are hypogean, that is, remain buriedin the soil, the hypocotyl is hardly developed, and the epicotyl or plumulerises in like manner as an arch through the ground. In all, or at least inmost of such cases, the downwardly bent apex remains for a time enclosedwithin the seed-coats. With Corylus avellena the cotyledons are hypogean, and the epicotyl is arched; but in the particular case described in thelast chapter its apex had been injured, and it grew laterally through thesoil like a root; and in consequence of this it had emitted two secondaryshoots, which likewise broke through the ground as arches. 

Cyclamen does not produce any distinct stem, and only a single cotyledonappears at first;* its petiole

* This is the conclusion arrived at by Dr. H. Gressner ('Bot. Zeitung, '1874, p. 837), who maintains that what has been considered by otherbotanists as the first true leaf is really the second cotyledon, which isgreatly delayed in its development. [page 78]

breaks through the ground as an arch (Fig. 57). Abronia has only a singlefully developed cotyledon, but in this case it is the hypocotyl which firstemerges and is arched. Abronia umbellata, however, presents thispeculiarity, that the enfolded blade of the one developed cotyledon (withthe enclosed endosperm) whilst still beneath the surface has its apexupturned and parallel to the descending leg of the arched hypocotyl; but itis dragged out of the ground by the continued growth of the hypocotyl, withthe apex pointing downward. With Cycas pectinata the cotyledons arehypogean, and a true leaf first breaks through the ground with its petioleforming an arch. 

Fig. 57. Cyclamen Persicum: seedling, figure enlarged: c, blade ofcotyledon, not yet expanded, with arched petiole beginning to straightenitself; h, hypocotyl developed into a corm; r, secondary radicles. 

Fig. 58. Acanthus mollis: seedling with the hypogean cotyledon on the nearside removed and the radicles cut off; a, blade of first leaf beginning toexpand, with petiole still partially arched; b, second and opposite leaf, as yet very imperfectly developed; c, hypogean cotyledon on the oppositeside. 

In the genus Acanthus the cotyledons are likewise hypogean. In A. Mollis, asingle leaf first breaks through the ground with its petiole arched, andwith the opposite leaf much less developed, short, straight, of a yellowishcolour, and with the petiole at first not half as thick as that of theother. The undeveloped leaf is protected by standing beneath its archedfellow; and it is an instruc-[page 79]tive fact that it is not arched, as it has not to force for itself apassage through the ground. In the accompanying sketch (Fig. 58) thepetiole of the first leaf has already partially straightened itself, andthe blade is beginning to unfold. The small second leaf ultimately grows toan equal size with the first, but this process is effected at verydifferent rates in different individuals: in one instance the second leafdid not appear fully above the ground until six weeks after the first leaf. As the leaves in the whole family of the Acanthaceae stand either oppositeone another or in whorls, and as these are of equal size, the greatinequality between the first two leaves is a singular fact. We can see howthis inequality of development and the arching of the petiole could havebeen gradually acquired, if they were beneficial to the seedlings byfavouring their emergence; for with A. Candelabrum, spinosus, andlatifolius there was a great variability in the inequality between the twofirst leaves and in the arching of their petioles. In one seedling of A. Candelabrum the first leaf was arched and nine times as long as the second, which latter consisted of a mere little, yellowish-white, straight, hairystyle. In other seedlings the difference in length between the two leaveswas as 3 to 2, or as 4 to 3, or as only . 76 to . 62 inch. In these lattercases the first and taller leaf was not properly arched. Lastly, in anotherseedling there was not the least difference in size between the two firstleaves, and both of them had their petioles straight; their laminae wereenfolded and pressed against each other, forming a lance or wedge, by whichmeans they had broken through the ground. Therefore in differentindividuals of this same species of Acanthus the first pair of leavesbreaks through the ground by two widely different methods; and if[page 80]either had proved decidedly advantageous or disadvantageous, one of them nodoubt would soon have prevailed. 

Asa Gray has described* the peculiar manner of germination of three widelydifferent plants, in which the hypocotyl is hardly at all developed. Thesewere therefore observed by us in relation to our present subject. 

Delphinium nudicaule. --The elongated petioles of the two cotyledons areconfluent (as are sometimes their blades at the base), and they breakthrough the ground as an arch. They thus resemble in a most deceptivemanner a hypocotyl. At first they are solid, but after a time becometubular; and the basal part beneath the ground is enlarged into a hollowchamber, within which the young leaves are developed without any prominentplumule. Externally root-hairs are formed on the confluent petioles, eithera little above, or on a level with, the plumule. The first leaf at an earlyperiod of its growth and whilst within the chamber is quite straight, butthe petiole soon becomes arched; and the swelling of this part (andprobably of the blade) splits open one side of the chamber, and the leafthen emerges. The slit was found in one case to be 3. 2 mm. In length, andit is seated on the line of confluence of the two petioles. The leaf whenit first escapes from the chamber is buried beneath the ground, and now anupper part of the petiole near the blade becomes arched in the usualmanner. The second leaf comes out of the slit either straight or somewhatarched, but afterwards the upper part of the petiole, --certainly in some, and we believe in all cases, --arches itself whilst forcing a passagethrough the soil. 

* 'Botanical Text-Book, ' 1879, p. 22. [page 81]

Megarrhiza Californica. --The cotyledons of this Gourd never free themselvesfrom the seed-coats and are hypogean. Their petioles are completelyconfluent, forming a tube which terminates downwards in a little solidpoint, consisting of a minute radicle and hypocotyl, with the likewiseminute plumule enclosed within the base of the tube. This structure waswell exhibited in an abnormal specimen, in which one of the two cotyledonsfailed to produce a petiole, whilst the other produced one consisting of anopen semicylinder ending in a sharp point, formed of the parts justdescribed. As soon as the confluent petioles protrude from the seed theybend down, as they are strongly geotropic, and penetrate the ground. Theseed itself retains its original position, either on the surface or buriedat some depth, as the case may be. If, however, the point of the confluentpetioles meets with some obstacle in the soil, as appears to have occurredwith the seedlings described and figured by Asa Gray, * the cotyledons arelifted up above the ground. The petioles are clothed with root-hairs likethose on a true radicle, and they likewise resemble radicles in becomingbrown when immersed in a solution of permanganate of potassium. Our seedswere subjected to a high temperature, and in the course of three or fourdays the petioles penetrated the soil perpendicularly to a depth of from 2to 2 ½ inches; and not until then did the true radicle begin to grow. Inone specimen which was closely observed, the petioles in 7 days after theirfirst protrusion attained a length of 2 ½ inches, and the radicle by thistime had also become well developed. The plumule, still enclosed within thetube, was now

* 'American Journal of Science, ' vol. Xiv. 1877, p. 21. [page 82]

. 3 inch in length, and was quite straight; but from having increased inthickness it had just begun to split open the lower part of the petioles onone side, along the line of their confluence. By the following morning theupper part of the plumule had arched itself into a right angle, and theconvex side or elbow had thus been forced out through the slit. Here thenthe arching of the plumule plays the same part as in the case of thepetioles of the Delphinium. As the plumule continued to grow, the tipbecame more arched, and in the course of six days it emerged through the 2½ inches of superincumbent soil, still retaining its arched form. Afterreaching the surface it straightened itself in the usual manner. In theaccompanying figure (Fig. 58, A) we have a sketch of a seedling in thisadvanced state of development; the surface of the ground being representedby the line G........... G. 

Fig. 58, A. Megarrhiza Californica: sketch of seedling, copied from AsaGray, reduced to one-half scale: c, cotyledons within seed-coats; p, thetwo confluent petioles; h and r, hypocotyl and radicle; p1, plumule;G.......... G, surface of soil. 

The germination of the seeds in their native Californian home proceeds in arather different manner, as we infer from an interesting letter from Mr. Rattan, sent to us by Prof. Asa Gray. The petioles protrude from the seedssoon after the autumnal rains, and penetrate the ground, generally in avertical direction, to a depth of from 4 to even 6 inches. They were foundin this state by Mr. Rattan during the Christmas vacation, with the plu-[page 83]mules still enclosed within the tubes; and he remarks that if the plumuleshad been at once developed and had reached the surface (as occurred withour seeds which were exposed to a high temperature), they would surely havebeen killed by the frost. As it is, they lie dormant at some depth beneaththe surface, and are thus protected from the cold; and the root-hairs onthe petioles would supply them with sufficient moisture. We shall hereaftersee that many seedlings are protected from frost, but by a widely differentprocess, namely, by being drawn beneath the surface by the contraction oftheir radicles. We may, however, believe that the extraordinary manner ofgermination of Megarrhiza has another and secondary advantage. The radiclebegins in a few weeks to enlarge into a little tuber, which then aboundswith starch and is only slightly bitter. It would therefore be very liableto be devoured by animals, were it not protected by being buried whilstyoung and tender, at a depth of some inches beneath the surface. Ultimatelyit grows to a huge size. 

Ipomoea leptophylla. --In most of the species of this genus the hypocotyl iswell developed, and breaks through the ground as an arch. But the seeds ofthe present species in germinating behave like those of Megarrhiza, excepting that the elongated petioles of the cotyledons are not confluent. After they have protruded from the seed, they are united at their lowerends with the undeveloped hypocotyl and undeveloped radicle, which togetherform a point only about . 1 inch in length. They are at first highlygeotropic, and penetrate the ground to a depth of rather above half aninch. The radicle then begins to grow. On four occasions after the petioleshad grown for a short distance vertically downwards, they[page 84]were placed in a horizontal position in damp air in the dark, and in thecourse of 4 hours they again became curved vertically downwards, havingpassed through 90o in this time. But their sensitiveness to geotropismlasts for only 2 or 3 days; and the terminal part alone, for a length ofbetween . 2 and . 4 inch, is thus sensitive. Although the petioles of ourspecimens did not penetrate the ground to a greater depth than about ½inch, yet they continued for some time to grow rapidly, and finallyattained the great length of about 3 inches. The upper part isapogeotropic, and therefore grows vertically upwards, excepting a shortportion close to the blades, which at an early period bends downwards andbecomes arched, and thus breaks through the ground. Afterwards this portionstraightens itself, and the cotyledons then free themselves from theseed-coats. Thus we here have in different parts of the same organ widelydifferent kinds of movement and of sensitiveness; for the basal part isgeotropic, the upper part apogeotropic, and a portion near the bladestemporarily and spontaneously arches itself. The plumule is not developedfor some little time; and as it rises between the bases of the parallel andclosely approximate petioles of the cotyledons, which in breaking throughthe ground have formed an almost open passage, it does not require to bearched and is consequently always straight. Whether the plumule remainsburied and dormant for a time in its native country, and is thus protectedfrom the cold of winter, we do not know. The radicle, like that of theMegarrhiza, grows into a tuber-like mass, which ultimately attains a greatsize. So it is with Ipomoea pandurata, the germination of which, as AsaGray informs us, resembles that of I. Leptophylla. 

The following case is interesting in connection with[page 85]the root-like nature of the petioles. The radicle of a seedling was cutoff, as it was completely decayed, and the two now separated cotyledonswere planted. They emitted roots from their bases, and continued green andhealthy for two months. The blades of both then withered, and on removingthe earth the bases of the petioles (instead of the radicle) were foundenlarged into little tubers. Whether these would have had the power ofproducing two independent plants in the following summer, we do not know. 

In Quercus virens, according to Dr. Engelmann, * both the cotyledons andtheir petioles are confluent. The latter grow to a length "of an inch oreven more;" and, if we understand rightly, penetrate the ground, so thatthey must be geotropic. The nutriment within the cotyledons is then quicklytransferred to the hypocotyl or radicle, which thus becomes developed intoa fusiform tuber. The fact of tubers being formed by the foregoing threewidely distinct plants, makes us believe that their protection from animalsat an early age and whilst tender, is one at least of the advantages gainedby the remarkable elongation of the petioles of the cotyledons, togetherwith their power of penetrating the ground like roots under the guidance ofgeotropism. 

The following cases may be here given, as they bear on our present subject, though not relating to seedlings. The flower-stem of the parasitic Lathraeasquamaria, which is destitute of true leaves, breaks through the ground asan arch;** so does the flower-

* 'Transact. St. Louis Acad. Science, ' vol. Iv. P. 190. 

** The passage of the flower-stem of the Lathraea through the ground cannotfail to be greatly facilitated by the extraordinary quantity of watersecreted at this period of the year by the subter-[[page 86]]ranean scale-like leaves; not that there is any reason to suppose that thesecretion is a special adaptation for this purpose: it probably followsfrom the great quantity of sap absorbed in the early spring by theparasitic roots. After a long period without any rain, the earth had becomelight-coloured and very dry, but it was dark-coloured and damp, even inparts quite wet, for a distance of at least six inches all round eachflower-stem. The water is secreted by glands (described by Cohn, 'Bericht. Bot. Sect. Der Schlesischen Gesell. , ' 1876, p. 113) which line thelongitudinal channels running through each scale-like leaf. A large plantwas dug up, washed so as to remove the earth, left for some time to drain, and then placed in the evening on a dry glass-plate, covered with abell-glass, and by next morning it had secreted a large pool of water. Theplate was wiped dry, and in the course of the succeeding 7 or 8 hoursanother little pool was secreted, and after 16 additional hours severallarge drops. A smaller plant was washed and placed in a large jar, whichwas left inclined for an hour, by which time no more water drained off. Thejar was then placed upright and closed: after 23 hours two drachms of waterwere collected from the bottom, and a little more after 25 additionalhours. The flower-stems were now cut off, for they do not secrete, and thesubterranean part of the plant was found to weigh 106. 8 grams (1611grains), and the water secreted during the 48 hours weighed 11. 9 grams (183grains), --that is, one-ninth of the whole weight of the plant, excludingthe flower-stems. We should remember that plants in a state of nature wouldprobably secrete in 48 hours much more than the above large amount, fortheir roots would continue all the time absorbing sap from the plant onwhich they were parasitic. [page 86]

stem of the parasitic and leafless Monotropa hypopitys. With Helleborusniger, the flower-stems, which rise up independently of the leaves, likewise break through the ground as arches. This is also the case with thegreatly elongated flower-stems, as well as with the petioles of Epimediumpinnatum. So it is with the petioles of Ranunculus ficaria, when they haveto break through the ground, but when they arise from the summit of thebulb above ground, they are from the first quite straight; and this is afact which deserves notice. The rachis of the bracken fern (Pterisaquilina), and of some, probably many, other ferns, likewise rises aboveground under the form of an arch. No doubt other analogous instances couldbe found by careful search. In all ordinary cases of bulbs, rhizomes, [page 87]root-stocks, etc. , buried beneath the ground, the surface is broken by acone formed by the young imbricated leaves, the combined growth of whichgives them force sufficient for the purpose. 

With germinating monocotyledonous seeds, of which, however, we did notobserve a large number, the plumules, for instance, those of Asparagus andCanna, are straight whilst breaking through the ground. With the Gramineae, the sheath-like cotyledons are likewise straight; they, however, terminatein a sharp crest, which is white and somewhat indurated; and this structureobviously facilitates their emergence from the soil: the first true leavesescape from the sheath through a slit beneath the chisel-like apex and atright angles to it. In the case of the onion (Allium cepa) we again meetwith an arch; the leaf-like cotyledon being abruptly bowed, when it breaksthrough the ground, with the apex still enclosed within the seed-coats. Thecrown of the arch, as previously described, is developed into a whiteconical protuberance, which we may safely believe to be a specialadaptation for this office. 

The fact of so many organs of different kinds--hypocotyls and epicotyls, the petioles of some cotyledons and of some first leaves, the cotyledons ofthe onion, the rachis of some ferns, and some flower-stems--being allarched whilst they break through the ground, shows how just are Dr. Haberlandt's* remarks on the importance of the arch to seedling plants. Heattributes its chief importance to the upper, young, and more tender partsof the hypocotyl

* 'Die Schutzeinrichtungen in der Entwickelung der Keimpflanze, ' 1877. Wehave learned much from this interesting essay, though our observations leadus to differ on some points from the author. [page 88]

or epicotyl, being thus saved from abrasion and pressure whilst breakingthrough the ground. But we think that some importance may be attributed tothe increased force gained by the hypocotyl, epicotyl, or other organ bybeing at first arched; for both legs of the arch increase in length, andboth have points of resistance as long as the tip remains enclosed withinthe seed-coats; and thus the crown of the arch is pushed up through theearth with twice as much force as that which a straight hypocotyl, etc. , could exert. As soon, however, as the upper end has freed itself, all thework has to be done by the basal leg. In the case of the epicotyl of thecommon bean, the basal leg (the apex having freed itself from theseed-coats) grew upwards with a force sufficient to lift a thin plate ofzinc, loaded with 12 ounces. Two more ounces were added, and the 14 ounceswere lifted up to a very little height, and then the epicotyl yielded andbent to one side. 

With respect to the primary cause of the arching process, we long thoughtin the case of many seedlings that this might be attributed to the mannerin which the hypocotyl or epicotyl was packed and curved within theseed-coats; and that the arched shape thus acquired was merely retaineduntil the parts in question reached the surface of the ground. But it isdoubtful whether this is the whole of the truth in any case. For instance, with the common bean, the epicotyl or plumule is bowed into an arch whilstbreaking through the seed-coats, as shown in Fig. 59 (p. 92). The plumulefirst protrudes as a solid knob (e in A), which after twenty-four hours'growth is seen (e in B) to be the crown of an arch. Nevertheless, withseveral beans which germinated in damp air, and had otherwise been treatedin an unnatural manner, little[page 89]plumules were developed in the axils of the petioles of both cotyledons, and these were as perfectly arched as the normal plumule; yet they had notbeen subjected to any confinement or pressure, for the seed-coats werecompletely ruptured, and they grew in the open air. This proves that theplumule has an innate or spontaneous tendency to arch itself. 

In some other cases the hypocotyl or epicotyl protrudes from the seed atfirst only slightly bowed; but the bowing afterwards increasesindependently of any constraint. The arch is thus made narrow, with the twolegs, which are sometimes much elongated, parallel and close together, andthus it becomes well fitted for breaking through the ground. 

With many kinds of plants, the radicle, whilst still enclosed within theseed and likewise after its first protrusion, lies in a straight line withthe future hypocotyl and with the longitudinal axis of the cotyledons. Thisis the case with Cucurbita ovifera: nevertheless, in whatever position theseeds were buried, the hypocotyl always came up arched in one particulardirection. Seeds were planted in friable peat at a depth of about an inchin a vertical position, with the end from which the radicle protrudesdownwards. Therefore all the parts occupied the same relative positionswhich they would ultimately hold after the seedlings had risen clear abovethe surface. Notwithstanding this fact, the hypocotyl arched itself; and asthe arch grew upwards through the peat, the buried seeds were turned eitherupside down, or were laid horizontally, being afterwards dragged above theground. Ultimately the hypocotyl straightened itself in the usual manner;and now after all these movements the several parts occupied the sameposition relatively to one another and to the centre of the earth, whichthey[page 90]had done when the seeds were first buried. But it may be argued in this andother such cases that, as the hypocotyl grows up through the soil, the seedwill almost certainly be tilted to one side; and then from the resistancewhich it must offer during its further elevation, the upper part of thehypocotyl will be doubled down and thus become arched. This view seems themore probable, because with Ranunculus ficaria only the petioles of theleaves which forced a passage through the earth were arched; and not thosewhich arose from the summits of the bulbs above the ground. Nevertheless, this explanation does not apply to the Cucurbita, for when germinatingseeds were suspended in damp air in various positions by pins passingthrough the cotyledons, fixed to the inside of the lids of jars, in whichcase the hypocotyls were not subjected to any friction or constraint, yetthe upper part became spontaneously arched. This fact, moreover, provesthat it is not the weight of the cotyledons which causes the arching. Seedsof Helianthus annuus and of two species of Ipomoea (those of 'I. Bona nox'being for the genus large and heavy) were pinned in the same manner, andthe hypocotyls became spontaneously arched; the radicles, which had beenvertically dependent, assumed in consequence a horizontal position. In thecase of Ipomoea leptophylla it is the petioles of the cotyledons whichbecome arched whilst rising through the ground; and this occurredspontaneously when the seeds were fixed to the lids of jars. 

It may, however, be suggested with some degree of probability that thearching was aboriginally caused by mechanical compulsion, owing to theconfinement of the parts in question within the seed-coats, or to frictionwhilst they were being dragged upwards. But[page 91]if this is so, we must admit from the cases just given, that a tendency inthe upper part of the several specified organs to bend downwards and thusto become arched, has now become with many plants firmly inherited. Thearching, to whatever cause it may be due, is the result of modifiedcircumnutation, through increased growth along the convex side of the part;such growth being only temporary, for the part always straightens itselfsubsequently by increased growth along the concave side, as will hereafterbe described. 

It is a curious fact that the hypocotyls of some plants, which are butlittle developed and which never raise their cotyledons above the ground, nevertheless inherit a slight tendency to arch themselves, although thismovement is not of the least use to them. We refer to a movement observedby Sachs in the hypocotyls of the bean and some other Leguminosae, andwhich is shown in the accompanying figure (Fig. 59), copied from hisEssay. * The hypocotyl and radicle at first grow perpendicularly downwards, as at A, and then bend, often in the course of 24 hours, into the positionshown at B. As we shall hereafter often have to recur to this movement, wewill, for brevity sake, call it "Sachs' curvature. " At first sight it mightbe thought that the altered position of the radicle in B was wholly due tothe outgrowth of the epicotyl (e), the petiole (p) serving as a hinge; andit is probable that this is partly the cause; but the hypocotyl and upperpart of the radicle themselves become slightly curved. 

The above movement in the bean was repeatedly seen by us; but ourobservations were made chiefly on Phaseolus multiflorus, the cotyledons ofwhich are like-

* 'Arbeiten des bot. Instit. Würzburg, ' vol. I. 1873, p. 403. [page 92]

wise hypogean. Some seedlings with well-developed radicles were firstimmersed in a solution of permanganate of potassium; and, judging from thechanges of colour (though these were not very clearly defined), thehypocotyl is about . 3 inch in length. Straight, thin, black lines of thislength were now drawn from the bases of the short petioles along thehypocotyls

Fig. 59. Vicia faba: germinating seeds, suspended in damp air: A, withradicle growing perpendicularly downwards; B, the same bean after 24 hoursand after the radicle has curved itself; r. Radicle; h, short hypocotyl; e, epicotyl appearing as a knob in A and as an arch in B; p, petiole of thecotyledon, the latter enclosed within the seed-coats. 

of 23 germinating seeds, which were pinned to the lids of jars, generallywith the hilum downwards, and with their radicles pointing to the centre ofthe earth. After an interval of from 24 to 48 hours the black lines on thehypocotyls of 16 out of the 23 seedlings became distinctly curved, but invery various degrees (namely, with radii between 20 and[page 93]80 mm. On Sachs' cyclometer) in the same relative direction as shown at Bin Fig. 59. As geotropism will obviously tend to check this curvature, seven seeds were allowed to germinate with proper precautions for theirgrowth in a klinostat, * by which means geotropism was eliminated. Theposition of the hypocotyls was observed during four successive days, andthey continued to bend towards the hilum and lower surface of the seed. Onthe fourth day they were deflected by an average angle of 63o from a lineperpendicular to the lower surface, and were therefore considerably morecurved than the hypocotyl and radicle in the bean at B (Fig. 59), though inthe same relative direction. 

It will, we presume, be admitted that all leguminous plants with hypogeancotyledons are descended from forms which once raised their cotyledonsabove the ground in the ordinary manner; and in doing so, it is certainthat their hypocotyls would have been abruptly arched, as in the case ofevery other dicotyledonous plant. This is especially clear in the case ofPhaseolus, for out of five species, the seedlings of which we observed, namely, P. Multiflorus, caracalla, vulgaris, Hernandesii and Roxburghii(inhabitants of the Old and New Worlds), the three last-named species havewell-developed hypocotyls which break through the ground as arches. Now, ifwe imagine a seedling of the common bean or of P. Multiflorus, to behave asits progenitors once did, the hypocotyl (h, Fig. 59), in whatever positionthe seed may have been buried, would become so much arched that the upperpart would be doubled down parallel to the lower part; and

* An instrument devised by Sachs, consisting essentially of a slowlyrevolving horizontal axis, on which the plant under observation issupported: see 'Würzburg Arbeiten, ' 1879, p. 209. [page 94]

this is exactly the kind of curvature which actually occurs in these twoplants, though to a much less degree. Therefore we can hardly doubt thattheir short hypocotyls have retained by inheritance a tendency to curvethemselves in the same manner as they did at a former period, when thismovement was highly important to them for breaking through the ground, though now rendered useless by the cotyledons being hypogean. Rudimentarystructures are in most cases highly variable, and we might expect thatrudimentary or obsolete actions would be equally so; and Sachs' curvaturevaries extremely in amount, and sometimes altogether fails. This is thesole instance known to us of the inheritance, though in a feeble degree, ofmovements which have become superfluous from changes which the species hasundergone. 

Rudimentary Cotyledons. --A few remarks on this subject may be hereinterpolated. It is well known that some dicotyledonous plants produce onlya single cotyledon; for instance, certain species of Ranunculus, Corydalis, Chaerophyllum; and we will here endeavour to show that the loss of one orboth cotyledons is apparently due to a store of nutriment being laid up insome other part, as in the hypocotyl or one of the two cotyledons, or oneof the secondary radicles. 

Fig. 60. Citrus aurantium: two young seedlings: c, larger cotyledon; c', smaller cotyledon; h, thickened hypocotyl; r, radicle. In A the epicotyl isstill arched, in B it has become erect. [page 95]

With the orange (Citrus aurantium) the cotyledons are hypogean, and one islarger than the other, as may be seen in A (Fig. 60). In B the inequalityis rather greater, and the stem has grown between the points of insertionof the two petioles, so that they do not stand opposite to one another; inanother case the separation amounted to one-fifth of an inch. The smallercotyledon of one seedling was extremely thin, and not half the length ofthe larger one, so that it was clearly becoming rudimentary, * In all theseseedlings the hypocotyl was enlarged or swollen. 

Fig. 61. Abronia umbellata: seedling twice natural size: c cotyledon; c', rudimentary cotyledon; h, enlarged hypocotyl, with a heel or projection(h') at the lower end; r, radicle. 

With Abronia umbellata one of the cotyledons is quite rudimentary, as maybe seen (c') in Fig. 61. In this specimen it consisted of a little greenflap, 1/84th inch in length, destitute of a petiole and covered with glandslike those on the fully developed cotyledon (c). At first it stood oppositeto the larger cotyledon; but as the petiole of the latter increased inlength and grew in the same line with the hypocotyl (h), the rudimentappeared in older seedlings as if seated some way down the hypocotyl. WithAbronia arenaria there is a similar rudiment, which in one

* In Pachira aquatica, as described by Mr. R. I. Lynch ('Journal Linn. Soc. Bot. ' vol. Xvii. 1878, p. 147), one of the hypogean cotyledons is ofimmense size; the other is small and soon falls off; the pair do not alwaysstand opposite. In another and very different water-plant, 'Trapa natans', one of the cotyledons, filled with farinaceous matter, is much larger thanthe other, which is scarcely visible, as is stated by Aug. De Candolle, 'Physiologie Veg. ' tom. Ii. P. 834, 1832. [page 96]

specimen was only 1/100th and in another 1/60th inch in length; itultimately appeared as if seated halfway down the hypocotyl. In both thesespecies the hypocotyl is so much enlarged, especially at a very early age, that it might almost be called a corm. The lower end forms a heel orprojection, the use of which will hereafter be described. 

 In Cyclamen Persicum the hypocotyl, even whilst still within the seed, isenlarged into a regular corm, * and only a single cotyledon is at firstdeveloped (see former Fig. 57). With Ranunculus ficaria two cotyledons arenever produced, and here one of the secondary radicles is developed at anearly age into a so-called bulb. ** Again, certain species of Chaerophyllumand Corydalis produce only a single cotyledon;*** in the former thehypocotyl, and in the latter the radicle is enlarged, according to Irmisch, into a bulb. 

In the several foregoing cases one of the cotyledons is delayed in itsdevelopment, or reduced in size, or rendered rudimentary, or quite aborted;but in other cases both cotyledons are represented by mere rudiments. WithOpuntia basilaris this is not the case, for both cotyledons are thick andlarge, and the hypocotyl shows at first no signs of enlargement; butafterwards, when the cotyledons have withered and disarticulatedthemselves, it becomes thickened, and from its tapering form, together withits smooth, tough, brown skin, appears, when ultimately drawn down to somedepth into the soil, like a root. On the other

* Dr. H. Gressner, 'Bot. Zeitung, ' 1874, p. 824. 

** Irmisch, 'Beiträge zur Morphologie der Pflanzen, ' 1854, pp. 11, 12;'Bot. Zeitung, ' 1874, p. 805. 

*** Delpino, 'Rivista Botanica, ' 1877, p. 21. It is evident from Vaucher'saccount ('Hist. Phys. Des Plantes d'Europe, ' tom. I. 1841, p. 149) of thegermination of the seeds of several species of Corydalis, that the bulb ortubercule begins to be formed at an extremely early age. [page 97]

hand, with several other Cacteae, the hypocotyl is from the first muchenlarged, and both cotyledons are almost or quite rudimentary. Thus withCereus Landbeckii two little triangular projections, representing thecotyledons, are narrower than the hypocotyl, which is pear-shaped, with thepoint downwards. In Rhipsalis cassytha the cotyledons are represented bymere points on the enlarged hypocotyl. In Echinocactus viridescens thehypocotyl is globular, with two little prominences on its summit. InPilocereus Houlletii the hypocotyl, much swollen in the upper part, ismerely notched on the summit; and each side of the notch evidentlyrepresents a cotyledon. Stapelia sarpedon, a member of the very distinctfamily of the Asclepiadeae, is fleshy like a cactus; and here again theupper part of the flattened hypocotyl is much thickened and bears twominute cotyledons, which, measured internally, were only . 15 inch inlength, and in breadth not equal to one-fourth of the diameter of thehypocotyl in its narrow axis; yet these minute cotyledons are probably notquite useless, for when the hypocotyl breaks through the ground in the formof an arch, they are closed or pressed against one another, and thusprotect the plumule. They afterwards open. 

From the several cases now given, which refer to widely distinct plants, wemay infer that there is some close connection between the reduced size ofone or both cotyledons and the formation, by the enlargement of thehypocotyl or of the radicle, of a so-called bulb. But it may be asked, didthe cotyledons first tend to abort, or did a bulb first begin to be formed?As all dicotyledons naturally produce two well-developed cotyledons, whilstthe thickness of the hypocotyl and of the radicle differs much in differentplants, it seems probable that these latter organs first became from[page 98]some cause thickened--in several instances apparently in correlation withthe fleshy nature of the mature plant--so as to contain a store ofnutriment sufficient for the seedling, and then that one or bothcotyledons, from being superfluous, decreased in size. It is not surprisingthat one cotyledon alone should sometimes have been thus affected, for withcertain plants, for instance the cabbage, the cotyledons are at first ofunequal size, owing apparently to the manner in which they are packedwithin the seed. It does not, however, follow from the above connection, that whenever a bulb is formed at an early age, one or both cotyledons willnecessarily become superfluous, and consequently more or less rudimentary. Finally, these cases offer a good illustration of the principle ofcompensation or balancement of growth, or, as Goethe expresses it, "inorder to spend on one side, Nature is forced to economise on the otherside. "

Circumnutation and other movements of Hypocotyls and Epicotyls, whilststill arched and buried beneath the ground, and whilst breaking throughit. --According to the position in which a seed may chance to have beenburied, the arched hypocotyl or epicotyl will begin to protrude in ahorizontal, a more or less inclined, or in a vertical plane. Except whenalready standing vertically upwards, both legs of the arch are acted onfrom the earliest period by apogeotropism. Consequently they both bendupwards until the arch becomes vertical. During the whole of this process, even before the arch has broken through the ground, it is continuallytrying to circumnutate to a slight extent; as it likewise does if ithappens at first to stand vertically up, --all which cases have beenobserved and described, more or less fully, in the last chapter. After thearch has grown to some[page 99]height upwards the basal part ceases to circumnutate, whilst the upper partcontinues to do so. 

That an arched hypocotyl or epicotyl, with the two legs fixed in theground, should be able to circumnutate, seemed to us, until we had readProf. Wiesner's observations, an inexplicable fact. He has shown* in thecase of certain seedlings, whose tips are bent downwards (or which nutate), that whilst the posterior side of the upper or dependent portion growsquickest, the anterior and opposite side of the basal portion of the sameinternode grows quickest; these two portions being separated by anindifferent zone, where the growth is equal on all sides. There may be evenmore than one indifferent zone in the same internode; and the oppositesides of the parts above and below each such zone grow quickest. Thispeculiar manner of growth is called by Wiesner "undulatory nutation. "Circumnutation depends on one side of an organ growing quickest (probablypreceded by increased turgescence), and then another side, generally almostthe opposite one, growing quickest. Now if we look at an arch like this[upside down U] and suppose the whole of one side--we will say the wholeconvex side of both legs--to increase in length, this would not cause thearch to bend to either side. But if the outer side or surface of the leftleg were to increase in length the arch would be pushed over to the right, and this would be aided by the inner side of the right leg increasing inlength. If afterwards the process were reversed, the arch would be pushedover to the opposite or left side, and so on alternately, --that is, itwould circumnutate. As an arched hypo-

* 'Die undulirende Nutation der Internodien, ' Akad. Der Wissench. (Vienna), Jan. 17th, 1878. Also published separately, see p. 32. [page 100]

cotyl, with the two legs fixed in the ground, certainly circumnutates, andas it consists of a single internode, we may conclude that it grows in themanner described by Wiesner. It may be added, that the crown of the archdoes not grow, or grows very slowly, for it does not increase much inbreadth, whilst the arch itself increases greatly in height. 

The circumnutating movements of arched hypocotyls and epicotyls can hardlyfail to aid them in breaking through the ground, if this be damp and soft;though no doubt their emergence depends mainly on the force exerted bytheir longitudinal growth. Although the arch circumnutates only to a slightextent and probably with little force, yet it is able to move the soil nearthe surface, though it may not be able to do so at a moderate depth. A potwith seeds of Solanum palinacanthum, the tall arched hypocotyls of whichhad emerged and were growing rather slowly, was covered with fineargillaceous sand kept damp, and this at first closely surrounded the basesof the arches; but soon a narrow open crack was formed round each of them, which could be accounted for only by their having pushed away the sand onall sides; for no such cracks surrounded some little sticks and pins whichhad been driven into the sand. It has already been stated that thecotyledons of Phalaris and Avena, the plumules of Asparagus and thehypocotyls of Brassica, were likewise able to displace the same kind ofsand, either whilst simply circumnutating or whilst bending towards alateral light. 

As long as an arched hypocotyl or epicotyl remains buried beneath theground, the two legs cannot separate from one another, except to a slightextent from the yielding of the soil; but as soon as the arch rises abovethe ground, or at an earlier period if[page 101]the pressure of the surrounding earth be artificially removed, the archimmediately begins to straighten itself. This no doubt is due to growthalong the whole inner surface of both legs of the arch; such growth beingchecked or prevented, as long as the two legs of the arch are firmlypressed together. When the earth is removed all round an arch and the twolegs are tied together at their bases, the growth on the under side of thecrown causes it after a time to become much flatter and broader thannaturally occurs. The straightening process consists of a modified form ofcircumnutation, for the lines described during this process (as with thehypocotyl of Brassica, and the epicotyls of Vicia and Corylus) were oftenplainly zigzag and sometimes looped. After hypocotyls or epicotyls haveemerged from the ground, they quickly become perfectly straight. No traceis left of their former abrupt curvature, excepting in the case of Alliumcepa, in which the cotyledon rarely becomes quite straight, owing to theprotuberance developed on the crown of the arch. 

The increased growth along the inner surface of the arch which renders itstraight, apparently begins in the basal leg or that which is united to theradicle; for this leg, as we often observed, is first bowed backwards fromthe other leg. This movement facilitates the withdrawal of the tip of theepicotyl or of the cotyledons, as the case may be, from within theseed-coats and from the ground. But the cotyledons often emerge from theground still tightly enclosed within the seed-coats, which apparently serveto protect them. The seed-coats are afterwards ruptured and cast off by theswelling of the closely conjoined cotyledons, and not by any movement ortheir separation from one another. 

Nevertheless, in some few cases, especially with the[page 102]Cucurbitaceae, the seed-coats are ruptured by a curious contrivance, described by M. Flahault. * A heel or peg is developed on one side of thesummit of the radicle or base of the hypocotyl; and this holds down thelower half of the seed-coats (the radicle being fixed into the ground)whilst the continued growth of the arched hypocotyl forced upwards theupper half, and tears asunder the seed-coats at one end, and the cotyledonsare then easily withdrawn. 

Fig. 62. Cucurbita ovifera: germinating seed, showing the heel or pegprojecting on one side from summit of radicle and holding down lower tip ofseed-coats, which have been partially ruptured by the growth of the archedhypocotyl. 

The accompanying figure (Fig. 62) will render this descriptionintelligible. Forty-one seeds of Cucurbita ovifera were laid on friablepeat and were covered by a layer about an inch in thickness, not muchpressed down, so that the cotyledons in being dragged up were subjected tovery little friction, yet forty of them came up naked, the seed-coats beingleft buried in the peat. This was certainly due to the action of the peg, for when it was prevented from acting, the cotyledons, as we shallpresently see, were lifted up still enclosed in their seed-coats. Theywere, however, cast off in the course of two or three days by the swellingof the cotyledons. Until this occurs light is excluded, and the cotyledonscannot decompose carbonic acid; but no one probably would have thought thatthe advantage thus gained by a little earlier cast-

* 'Bull. Soc. Bot. De France, ' tom. Xxiv. 1877, p. 201. [page 103]

ing off of the seed-coats would be sufficient to account for thedevelopment of the peg. Yet according to M. Flahault, seedlings which havebeen prevented from casting their seed-coats whilst beneath the ground, areinferior to those which have emerged with their cotyledons naked and readyto act. 

The peg is developed with extraordinary rapidity; for it could only just bedistinguished in two seedlings, having radicles . 35 inch in length, butafter an interval of only 24 hours was well developed in both. It isformed, according to Flahault, by the enlargement of the layers of thecortical parenchyma at the base of the hypocotyl. If, however, we judge bythe effects of a solution of permanganate of potassium, it is developed onthe exact line of junction between the hypocotyl and radicle; for the flatlower surface, as well as the edges, were coloured brown like the radicle;whilst the upper slightly inclined surface was left uncoloured like thehypocotyl, excepting indeed in one out of 33 immersed seedlings in which alarge part of the upper surface was coloured brown. Secondary rootssometimes spring from the lower surface of the peg, which thus seems in allrespects to partake of the nature of the radicle. The peg is alwaysdeveloped on the side which becomes concave by the arching of thehypocotyl; and it would be of no service if it were formed on any otherside. It is also always developed with the flat lower side, which, as juststated, forms a part of the radicle, at right angles to it, and in ahorizontal plane. This fact was clearly shown by burying some of the thinflat seeds in the same position as in Fig. 62, excepting that they were notlaid on their flat broad sides, but with one edge downwards. Nine seedswere thus planted, and the peg was developed in the[page 104]same position, relatively to the radicle, as in the figure; consequently itdid not rest on the flat tip of the lower half of the seed-coats, but wasinserted like a wedge between the two tips. As the arched hypocotyl grewupwards it tended to draw up the whole seed, and the peg necessarily rubbedagainst both tips, but did not hold either down. The result was, that thecotyledons of five out of the nine seeds thus placed were raised above theground still enclosed within their seed-coats. Four seeds were buried withthe end from which the radicle protrudes pointing vertically downwards, andowing to the peg being always developed in the same position, its apexalone came into contact with, and rubbed against the tip on one side; theresult was, that the cotyledons of all four emerged still within theirseed-coats. These cases show us how the peg acts in co-ordination with theposition which the flat, thin, broad seeds would almost always occupy whennaturally sown. When the tip of the lower half of the seed-coats was cutoff, Flahault found (as we did likewise) that the peg could not act, sinceit had nothing to press on, and the cotyledons were raised above the groundwith their seed-coats not cast off. Lastly, nature shows us the use of thepeg; for in the one Cucurbitaceous genus known to us, in which thecotyledons are hypogean and do not cast their seed-coats, namely, Megarrhiza, there is no vestige of a peg. This structure seems to bepresent in most of the other genera in the family, judging from Flahault'sstatements' we found it well-developed and properly acting in Trichosanthesanguina, in which we hardly expected to find it, as the cotyledons aresomewhat thick and fleshy. Few cases can be advanced of a structure betteradapted for a special purpose than the present one. [page 105]

With Mimosa pudica the radicle protrudes from a small hole in the sharpedge of the seed; and on its summit, where united with the hypocotyl, atransverse ridge is developed at an early age, which clearly aids insplitting the tough seed-coats; but it does not aid in casting them off, asthis is subsequently effected by the swelling of the cotyledons after theyhave been raised above the ground. The ridge or heel therefore acts ratherdifferently from that of Cucurbita. Its lower surface and the edges werecoloured brown by the permanganate of potassium, but not the upper surface. It is a singular fact that after the ridge has done its work and hasescaped from the seed-coats, it is developed into a frill all round thesummit of the radicle. *

At the base of the enlarged hypocotyl of Abronia umbellata, where it blendsinto the radicle, there is a projection or heel which varies in shape, butits outline is too angular in our former figure (Fig. 61). The radiclefirst protrudes from a small hole at one end of the tough, leathery, wingedfruit. At this period the upper part of the radicle is packed within thefruit parallel to the hypocotyl, and the single cotyledon is doubled backparallel to the latter. The swelling of these three parts, and especiallythe rapid development of the thick heel between the hypocotyl and radicleat the point where they are doubled, ruptures the tough fruit at the upperend and allows the arched hypocotyl to emerge; and this seems to be thefunction of the heel. A seed was cut out of the fruit and

* Our attention was called to this case by a brief statement by Nobbe inhis 'Handbuch der Samenkunde, ' 1876, p. 215, where a figure is also givenof a seedling of Martynia with a heel or ridge at the junction of theradicle and hypocotyl. This seed possesses a very hard and tough coat, andwould be likely to require aid in bursting and freeing the cotyledons. [page 106]

allowed to germinate in damp air, and now a thin flat disc was developedall round the base of the hypocotyl and grew to an extraordinary breadth, like the frill described under Mimosa, but somewhat broader. Flahault saysthat with Mirabilis, a member of the same family with Abronia, a heel orcollar is developed all round the base of the hypocotyl, but more on oneside than on the other; and that it frees the cotyledons from theirseed-coats. We observed only old seeds, and these were ruptured by theabsorption of moisture, independently of any aid from the heel and beforethe protrusion of the radicle; but it does not follow from our experiencethat fresh and tough fruits would behave in a like manner. 

In concluding this section of the present chapter it may be convenient tosummarise, under the form of an illustration, the usual movements of thehypocotyls and epicotyls of seedlings, whilst breaking through the groundand immediately afterwards. We may suppose a man to be thrown down on hishands and knees, and at the same time to one side, by a load of hay fallingon him. He would first endeavour to get his arched back upright, wrigglingat the same time in all directions to free himself a little from thesurrounding pressure; and this may represent the combined effects ofapogeotropism and circumnutation, when a seed is so buried that the archedhypocotyl or epicotyl protrudes at first in a horizontal or inclined plane. The man, still wriggling, would then raise his arched back as high as hecould; and this may represent the growth and continued circumnutation of anarched hypocotyl or epicotyl, before it has reached the surface of theground. As soon as the man felt himself at all free, he would raise theupper part of his body, whilst still on[page 107]his knees and still wriggling; and this may represent the bowing backwardsof the basal leg of the arch, which in most cases aids in the withdrawal ofthe cotyledons from the buried and ruptured seed-coats, and the subsequentstraightening of the whole hypocotyl or epicotyl--circumnutation stillcontinuing. 

Circumnutation of Hypocotyls and Epicotyls, when erect. --The hypocotyls, epicotyls, and first shoots of the many seedlings observed by us, afterthey had become straight and erect, circumnutated continuously. Thediversified figures described by them, often during two successive days, have been shown in the woodcuts in the last chapter. It should berecollected that the dots were joined by straight lines, so that thefigures are angular; but if the observations had been made every fewminutes the lines would have been more or less curvilinear, and irregularellipses or ovals, or perhaps occasionally circles, would have been formed. The direction of the longer axes of the ellipses made during the same dayor on successive days generally changed completely, so as to stand at rightangles to one another. The number of irregular ellipses or circles madewithin a given time differs much with different species. Thus with Brassicaoleracea, Cerinthe major, and Cucurbita ovifera about four such figureswere completed in 12 h. ; whereas with Solanum palinacanthum and Opuntiabasilaris, scarcely more than one. The figures likewise differ greatly insize; thus they were very small and in some degree doubtful in Stapelia, and large in Brassica, etc. The ellipses described by Lathyrus nissolia andBrassica were narrow, whilst those made by the Oak were broad. The figuresare often complicated by small loops and zigzag lines. 

As most seedling plants before the development of true leaves are of low, sometimes very low stature, [page 108]the extreme amount of movement from side to side of their circumnutatingstems was small; that of the hypocotyl of Githago segetum was about . 2 ofan inch, and that of Cucurbita ovifera about . 28. A very young shoot ofLathyrus nissolia moved about . 14, that of an American oak . 2, that of thecommon nut only . 04, and a rather tall shoot of the Asparagus . 11 of aninch. The extreme amount of movement of the sheath-like cotyledon ofPhalaris Canariensis was . 3 of an inch; but it did not move very quickly, the tip crossing on one occasion five divisions of the micrometer, that is, 1/100th of an inch, in 22 m. 5 s. A seedling Nolana prostrata travelled thesame distance in 10 m. 38 s. Seedling cabbages circumnutate much morequickly, for the tip of a cotyledon crossed 1/100th of an inch on themicrometer in 3 m. 20 s. ; and this rapid movement, accompanied by incessantoscillations, was a wonderful spectacle when beheld under the microscope. 

The absence of light, for at least a day, does not interfere in the leastwith the circumnutation of the hypocotyls, epicotyls, or young shoots ofthe various dicotyledonous seedlings observed by us; nor with that of theyoung shoots of some monocotyledons. The circumnutation was indeed muchplainer in darkness than in light, for if the light was at all lateral thestem bent towards it in a more or less zigzag course. 

Finally, the hypocotyls of many seedlings are drawn during the winter intothe ground, or even beneath it so that they disappear. This remarkableprocess, which apparently serves for their protection, has been fullydescribed by De Vries. * He shows that

* 'Bot. Zeitung, ' 1879, p. 649. See also Winkler in 'Verhandl. Des Bot. Vereins der P. Brandenburg, ' Jahrg. Xvi. P. 16, as quoted by Haberlandt, 'Schutzeinrichungen der Keimpflanze, ' 1877, p. 52. [page 109]

it is effected by the contraction of the parenchyma-cells of the root. Butthe hypocotyl itself in some cases contracts greatly, and although at firstsmooth becomes covered with zigzag ridges, as we observed with Githagosegetum. How much of the drawing down and burying of the hypocotyl ofOpuntia basilaris was due to the contraction of this part and how much tothat of the radicle, we did not observe. 

Circumnutation of Cotyledons. --With all the dicotyledonous seedlingsdescribed in the last chapter, the cotyledons were in constant movement, chiefly in a vertical plane, and commonly once up and once down in thecourse of the 24 hours. But there were many exceptions to such simplicityof movement; thus the cotyledons of Ipomoea caerulea moved 13 times eitherupwards or downwards in the course of 16 h.. 18 m. Those of Oxalis roseamoved in the same manner 7 times in the course of 24 h. ; and those ofCassia tora described 5 irregular ellipses in 9 h. The cotyledons of someindividuals of Mimosa pudica and of Lotus Jacobaeus moved only once up anddown in 24 h. , whilst those of others performed within the same period anadditional small oscillation. Thus with different species, and withdifferent individuals of the same species, there were many gradations froma single diurnal movement to oscillations as complex as those of theIpomoea and Cassia. The opposite cotyledons on the same seedling move to acertain extent independently of one another. This was conspicuous withthose of Oxalis sensitiva, in which one cotyledon might be seen during thedaytime rising up until it stood vertically, whilst the opposite one wassinking down. 

Although the movements of cotyledons were generally in nearly the samevertical plane, yet their upward and downward courses never exactly coin-[page 110]cided; so that ellipses, more or less narrow, were described, and thecotyledons may safely be said to have circumnutated. Nor could this fact beaccounted for by the mere increase in length of the cotyledons throughgrowth, for this by itself would not induce any lateral movement. Thatthere was lateral movement in some instances, as with the cotyledons of thecabbage, was evident; for these, besides moving up and down, changed theircourse from right to left 12 times in 14 h. 15 m. With Solanum lycopersicumthe cotyledons, after falling in the forenoon, zigzagged from side to sidebetween 12 and 4 P. M. , and then commenced rising. The cotyledons of Lupinusluteus are so thick (about . 08 of an inch) and fleshy, * that they seemedlittle likely to move, and were therefore observed with especial interest;they certainly moved largely up and down, and as the line traced was zigzagthere was some lateral movement. The nine cotyledons of a seedling Pinuspinaster plainly circumnutated; and the figures described approached morenearly to irregular circles than to irregular ovals or ellipses. Thesheath-like cotyledons of the Gramineae circumnutate, that is, move to allsides, as plainly as do the hypocotyls or epicotyls of any dicotyledonousplants. Lastly, the very young fronds of a Fern and of a Selaginellacircumnutated. 

In a large majority of the cases which were carefully observed, thecotyledons sink a little downwards in the forenoon, and rise a little inthe afternoon or evening. They thus stand rather more highly inclinedduring the night than during the mid-day, at which

* The cotyledons, though bright green, resemble to a certain extenthypogean ones; see the interesting discussion by Haberlandt ('DieSchutzeinrichtungen, ' etc. , 1877, p. 95), on the gradations in theLeguminosae between subaërial and subterranean cotyledons. [page 111]

time they are expanded almost horizontally. The circumnutating movement isthus at least partially periodic, no doubt in connection, as we shallhereafter see, with the daily alternations of light and darkness. Thecotyledons of several plants move up so much at night as to stand nearly orquite vertically; and in this latter case they come into close contact withone another. On the other hand, the cotyledons of a few plants sink almostor quite vertically down at night; and in this latter case they clasp theupper part of the hypocotyl. In the same genus Oxalis the cotyledons ofcertain species stand vertically up, and those of other species verticallydown, at night. In all such cases the cotyledons may be said to sleep, forthey act in the same manner as do the leaves of many sleeping plants. Thisis a movement for a special purpose, and will therefore be considered in afuture chapter devoted to this subject. 

In order to gain some rude notion of the proportional number of cases inwhich the cotyledons of dicotyledonous plants (hypogean ones being ofcourse excluded) changed their position in a conspicuous manner at night, one or more species in several genera were cursorily observed, besidesthose described in the last chapter. Altogether 153 genera, included in asmany families as could be procured, were thus observed by us. Thecotyledons were looked at in the middle of the day and again at night; andthose were noted as sleeping which stood either vertically or at an angleof at least 60o above or beneath the horizon. Of such genera there were 26;and in 21 of them the cotyledons of some of the species rose, and in only 6sank at night; and some of these latter cases are rather doubtful fromcauses to be explained in the chapter on the sleep of cotyledons. When[page 112]cotyledons which at noon were nearly horizontal, stood at night at morethan 20o and less than 60o above the horizon, they were recorded as"plainly raised;" and of such genera there were 38. We did not meet withany distinct instances of cotyledons periodically sinking only a fewdegrees at night, although no doubt such occur. We have now accounted for64 genera out of the 153, and there remain 89 in which the cotyledons didnot change their position at night by as much as 20o--that is, in aconspicuous manner which could easily be detected by the unaided eye and bymemory; but it must not be inferred from this statement that thesecotyledons did not move at all, for in several cases a rise of a fewdegrees was recorded, when they were carefully observed. The number 89might have been a little increased, for the cotyledons remained almosthorizontal at night in some species in a few genera, for instance, Trifolium and Geranium, which are included amongst the sleepers, suchgenera might therefore have been added to the 89. Again, one species ofOxalis generally raised its cotyledons at night more than 20o and less than60o above the horizon; so that this genus might have been included undertwo heads. But as several species in the same genus were not oftenobserved, such double entries have been avoided. 

In a future chapter it will be shown that the leaves of many plants whichdo not sleep, rise a few degrees in the evening and during the early partof the night; and it will be convenient to defer until then theconsideration of the periodicity of the movements of cotyledons. 

On the Pulvini or Joints of Cotyledons. --With several of the seedlingsdescribed in this and the last chapter, the summit of the petiole isdeveloped into a pulvinus, [page 113]cushion, or joint (as this organ has been variously called), like that withwhich many leaves are provided. It consists of a mass of small cellsusually of a pale colour from the absence of chlorophyll, and with itsoutline more or less convex, as shown in the annexed figure. In the case ofOxalis sensitiva two-thirds of the petiole, and in that of Mimosa pudica, apparently the whole of the short sub-petioles of the leaflets have beenconverted into pulvini. With pulvinated leaves (i. E. Those provided with apulvinus) their periodical movements depend, according to Pfeffer, * on thecells of the pulvinus alternately expanding more quickly on one side thanon the other; whereas the similar movements of leaves not provided withpulvini, depend on their growth being alternately more rapid on one sidethan on the other. ** As long as a leaf provided with a pulvinus is youngand continues to grow, its movement depends on both these causescombined;*** and if the view now held by many botanists be sound, namely, that growth is always preceded by the expansion of the growing cells, thenthe difference between the movements induced by the aid of pulvini and

Fig. 63. Oxalis rosea: longitudinal section of a pulvinus on the summit ofthe petiole of a cotyledon, drawn with the camera lucida, magnified 75times: p, p, petiole; f, fibro-vascular bundle: b, b, commencement of bladeof cotyledon. 

* 'Die Periodische Bewegungen der Blattorgane, ' 1875. 

** Batalin, 'Flora, ' Oct. 1st, 1873

*** Pfeffer, ibid. P. 5. [page 114]

without such aid, is reduced to the expansion of the cells not beingfollowed by growth in the first case, and being so followed in the secondcase. 

Dots were made with Indian ink along the midrib of both pulvinatedcotyledons of a rather old seedling of Oxalis Valdiviana; their distanceswere repeatedly measured with an eye-piece micrometer during 8 3/4 days, and they did not exhibit the least trace of increase. It is thereforealmost certain that the pulvinus itself was not then growing. Nevertheless, during this whole time and for ten days afterwards, these cotyledons rosevertically every night. In the case of some seedlings raised from seedspurchased under the name of Oxalis floribunda, the cotyledons continued fora long time to move vertically down at night, and the movement apparentlydepended exclusively on the pulvini, for their petioles were of nearly thesame length in young, and in old seedlings which had produced true leaves. With some species of Cassia, on the other hand, it was obvious without anymeasurement that the pulvinated cotyledons continued to increase greatly inlength during some weeks; so that here the expansion of the cells of thepulvini and the growth of the petiole were probably combined in causingtheir prolonged periodic movements. It was equally evident that thecotyledons of many plants, not provided with pulvini, increased rapidly inlength; and their periodic movements no doubt were exclusively due togrowth. 

In accordance with the view that the periodic movements of all cotyledonsdepend primarily on the expansion of the cells, whether or not followed bygrowth, we can understand the fact that there is but little difference inthe kind or form of movement in the two sets of cases. This may be seen bycom-[page 115]paring the diagrams given in the last chapter. Thus the movements of thecotyledons of Brassica oleracea and of Ipomoea caerulea, which are notprovided with pulvini, are as complex as those of Oxalis and Cassia whichare thus provided. The pulvinated cotyledons of some individuals of Mimosapudica and Lotus Jacobaeus made only a single oscillation, whilst those ofother individuals moved twice up and down in the course of 24 hours; so itwas occasionally with the cotyledons of Cucurbita ovifera, which aredestitute of a pulvinus. The movements of pulvinated cotyledons aregenerally larger in extent than those without a pulvinus; nevertheless someof the latter moved through an angle of 90o. There is, however, oneimportant difference in the two sets of cases; the nocturnal movements ofcotyledons without pulvini, for instance, those in the Cruciferae, Cucurbitaceae, Githago, and Beta, never last even for a week, to anyconspicuous degree. Pulvinated cotyledons, on the other hand, continue torise at night for a much longer period, even for more than a month, as weshall now show. But the period no doubt depends largely on the temperatureto which the seedlings are exposed and their consequent rate ofdevelopment. 

[Oxalis Valdiviana. --Some cotyledons which had lately opened and werehorizontal on March 6th at noon, stood at night vertically up; on the 13ththe first true leaf was formed, and was embraced at night by thecotyledons; on April 9th, after an interval of 35 days, six leaves weredeveloped, and yet the cotyledons rose almost vertically at night. Thecotyledons of another seedling, which when first observed had alreadyproduced a leaf, stood vertically at night and continued to do so for 11additional days. After 16 days from the first observation two leaves weredeveloped, and the cotyledons were still greatly raised at night. After 21days the cotyledons during the day were deflected beneath the horizon, butat night were raised 45o[page 116]above it. After 24 days from the first observation (begun after a true leafhad been developed) the cotyledons ceased to rise at night. 

Oxalis (Biophytum) sensitiva. --The cotyledons of several seedlings, 45 daysafter their first expansion, stood nearly vertical at night, and closelyembraced either one or two true leaves which by this time had been formed. These seedlings had been kept in a very warm house, and their developmenthad been rapid. 

Oxalis corniculata. --The cotyledons do not stand vertical at night, butgenerally rise to an angle of about 45o above the horizon. They continuedthus to act for 23 days after their first expansion, by which time twoleaves had been formed; even after 29 days they still rose moderately abovetheir horizontal or downwardly deflected diurnal position. 

Mimosa pudica. --The cotyledons were expanded for the first time on Nov. 2nd, and stood vertical at night. On the 15th the first leaf was formed, and at night the cotyledons were vertical. On the 28th they behaved in thesame manner. On Dec. 15th, that is after 44 days, the cotyledons were stillconsiderably raised at night; but those of another seedling, only one dayolder, were raised very little. 

Mimosa albida. --A seedling was observed during only 12 days, by which timea leaf had been formed, and the cotyledons were then quite vertical atnight. 

Trifolium subterraneum. --A seedling, 8 days old, had its cotyledonshorizontal at 10. 30 A. M. And vertical at 9. 15 P. M. After an interval of twomonths, by which time the first and second true leaves had been developed, the cotyledons still performed the same movement. They had now increasedgreatly in size, and had become oval; and their petioles were actually . 8of an inch in length!

Trifolium strictum. --After 17 days the cotyledons still rose at night, butwere not afterwards observed. 

Lotus Jacoboeus. --The cotyledons of some seedlings having well-developedleaves rose to an angle of about 45o at night; and even after 3 or 4 whorlsof leaves had been formed, the cotyledons rose at night considerably abovetheir diurnal horizontal position. 

Cassia mimosoides. --The cotyledons of this Indian species, 14 days aftertheir first expansion, and when a leaf had been formed, stood during theday horizontal, and at night vertical. 

Cassia sp? (a large S. Brazilian tree raised from seeds sent us[page 117]by F. Müller). --The cotyledons, after 16 days from their first expansion, had increased greatly in size with two leaves just formed. They stoodhorizontally during the day and vertically at night, but were notafterwards observed. 

Cassia neglecta (likewise a S. Brazilian species). --A seedling, 34 daysafter the first expansion of its cotyledons, was between 3 and 4 inches inheight, with 3 well-developed leaves; and the cotyledons, which during theday were nearly horizontal, at night stood vertical, closely embracing theyoung stem. The cotyledons of another seedling of the same age, 5 inches inheight, with 4 well-developed leaves, behaved at night in exactly the samemanner. ]

It is known* that there is no difference in structure between the upper andlower halves of the pulvini of leaves, sufficient to account for theirupward or downward movements. In this respect cotyledons offer an unusuallygood opportunity for comparing the structure of the two halves; for thecotyledons of Oxalis Valdiviana rise vertically at night, whilst those ofO. Rosea sink vertically; yet when sections of their pulvini were made, noclear difference could be detected between the corresponding halves of thisorgan in the two species which move so differently. With O. Rosea, however, there were rather more cells in the lower than in the upper half, but thiswas likewise the case in one specimen of O. Valdiviana. The cotyledons ofboth species (3 ½ mm. In length) were examined in the morning whilstextended horizontally, and the upper surface of the pulvinus of O. Roseawas then wrinkled transversely, showing that it was in a state ofcompression, and this might have been expected, as the cotyledons sink atnight; with O. Valdiviana it was the lower surface which was wrinkled, andits cotyledons rise at night. 

Trifolium is a natural genus, and the leaves of all

* Pfeffer, 'Die Period. Bewegungen, ' 1875, p. 157. [page 118]

the species seen by us are pulvinated; so it is with the cotyledons of T. Subterraneum and strictum, which stand vertically at night; whereas thoseof T. Resupinatum exhibit not a trace of a pulvinus, nor of any nocturnalmovement. This was ascertained by measuring the distance between the tipsof the cotyledons of four seedlings at mid-day and at night. In thisspecies, however, as in the others, the first-formed leaf, which is simpleor not trifoliate, rises up and sleeps like the terminal leaflet on amature plant. 

In another natural genus, Oxalis, the cotyledons of O. Valdiviana, rosea, floribunda, articulata, and sensitiva are pulvinated, and all move at nightinto an upward or downward vertical position. In these several species thepulvinus is seated close to the blade of the cotyledon, as is the usualrule with most plants. Oxalis corniculata (var. Atro-purpurea) differs inseveral respects; the cotyledons rise at night to a very variable amount, rarely more than 45o; and in one lot of seedlings (purchased under the nameof O. Tropaeoloides, but certainly belonging to the above variety) theyrose only from 5o to 15o above the horizon. The pulvinus is developedimperfectly and to an extremely variable degree, so that apparently it istending towards abortion. No such case has hitherto, we believe, beendescribed. It is coloured green from its cells containing chlorophyll; andit is seated nearly in the middle of the petiole, instead of at the upperend as in all the other species. The nocturnal movement is effected partlyby its aid, and partly by the growth of the upper part of the petiole as inthe case of plants destitute of a pulvinus. From these several reasons andfrom our having partially traced the development of the pulvinus from anearly age, the case seems worth describing in some detail. [page 119]

[When the cotyledons of O. Corniculata were dissected out of a seed fromwhich they would soon have naturally emerged, no trace of a pulvinus couldbe detected; and all the cells forming the short petiole, 7 in number in alongitudinal row, were of nearly equal size. In seedlings one or two daysold, the pulvinus was so indistinct that we thought at first that it didnot exist; but in the middle of the petiole an ill-defined transverse zoneof cells could be seen, which were much shorter than those both above andbelow, although of the same breadth with them. They presented theappearance of having been just formed by the transverse division of longercells; and there can be little doubt that this had occurred, for the cellsin the petiole which had

Fig. 64. Oxalis corniculata: A and B the almost rudimentary pulvini of thecotyledons of two rather old seedlings, viewed as transparent objects. Magnified 50 times. 

been dissected out of the seed averaged in length 7 divisions of themicrometer (each division equalling . 003 mm. ), and were a little longerthan those forming a well-developed pulvinus, which varied between 4 and 6of these same divisions. After a few additional days the ill-defined zoneof cells becomes distinct, and although it does not extend across the wholewidth of the petiole, and although the cells are of a green colour fromcontaining chlorophyll, yet they certainly constitute a pulvinus, which aswe shall presently see, acts as one. These small cells were arranged inlongitudinal rows, and varied from 4 to 7 in number; and the cellsthemselves varied in length in different parts of the[page 120]same pulvinus and in different individuals. In the accompanying figures, Aand B (Fig. 64), we have views of the epidermis* in the middle part of thepetioles of two seedlings, in which the pulvinus was for this species welldeveloped. They offer a striking contrast with the pulvinus of O. Rosea(see former Fig. 63), or of O. Valdiviana. With the seedlings, falselycalled O. Tropaeoloides, the cotyledons of which rise very little at night, the small cells were still fewer in number and in parts formed a singletransverse row, and in other parts short longitudinal rows of only two orthree. Nevertheless they sufficed to attract the eye, when the wholepetiole was viewed as a transparent object beneath the microscope. In theseseedlings there could hardly be a doubt that the pulvinus was becomingrudimentary and tending to disappear; and this accounts for its greatvariability in structure and function. 

In the following Table some measurements of the cells in fairlywell-developed pulvini of O. Corniculata are given:--

Seedling 1 day old, with cotyledon 2. 3 mm. In length. Divisions of Micrometer. **Average length of cells ofpulvinus.................................................. 6 to 7Length of longest cell below thepulvinus..................................... 13Length of longest cell above thepulvinus...................................... 20

Seedling 5 days old, cotyledon 3. 1 mm. In length, with the pulvinus quitedistinct. Average length of cells ofpulvinus.................................................. 6Length of longest cell below thepulvinus..................................... 22Length of longest cell above thepulvinus...................................... 40

Seedling 8 days old, cotyledon 5 mm. In length, with a true leaf formed butnot yet expanded. Average length of cells ofpulvinus.................................................. 9Length of longest cell below thepulvinus..................................... 44Length of longest cell above thepulvinus...................................... 70

Seedling 13 days old, cotyledon 4. 5 mm. In length, with a small true leaffully developed. Average length of cells ofpulvinus.................................................. 7Length of longest cell below thepulvinus..................................... 30Length of longest cell above thepulvinus...................................... 60

______________________________________

* Longitudinal sections show that the forms of the epidermic cells may betaken as a fair representation of those constituting the pulvinus. 

** Each division equalled . 003 mm. [page 121]

We here see that the cells of the pulvinus increase but little in lengthwith advancing age, in comparison with those of the petiole both above andbelow it; but they continue to grow in width, and keep equal in thisrespect with the other cells of the petiole. The rate of growth, however, varies in all parts of the cotyledons, as may be observed in themeasurements of the 8-days' old seedling. 

The cotyledons of seedlings only a day old rise at night considerably, sometimes as much as afterwards; but there was much variation in thisrespect. As the pulvinus is so indistinct at first, the movement probablydoes not then depend on the expansion of its cells, but on periodicallyunequal growth in the petiole. By the comparison of seedlings of differentknown ages, it was evident that the chief seat of growth of the petiole wasin the upper part between the pulvinus and the blade; and this agrees withthe fact (shown in the measurements above given) that the cells grow to agreater length in the upper than in the lower part. With a seedling 11 daysold, the nocturnal rise was found to depend largely on the action of thepulvinus, for the petiole at night was curved upwards at this point; andduring the day, whilst the petiole was horizontal, the lower surface of thepulvinus was wrinkled with the upper surface tense. Although the cotyledonsat an advanced age do not rise at night to a higher inclination than whilstyoung, yet they have to pass through a larger angle (in one instanceamounting to 63o) to gain their nocturnal position, as they are generallydeflected beneath the horizon during the day. Even with the 11-days' oldseedling the movement did not depend exclusively on the pulvinus, for theblade where joined to the petiole was curved upwards, and this must beattributed to unequal growth. Therefore the periodic movements of thecotyledons of 'O. Corniculata' depend on two distinct but conjoint actions, namely, the expansion of the cells of the pulvinus and on the growth of theupper part of the petiole, including the base of the blade. 

Lotus Jacoboeus. --The seedlings of this plant present a case parallel tothat of Oxalis corniculata in some respects, and in others unique, as faras we have seen. The cotyledons during the first 4 or 5 days of their lifedo not exhibit any plain nocturnal movement; but afterwards they standvertically or almost vertically up at night. There is, however, some degreeof variability in this respect, apparently dependent on the season and onthe degree to which they have been illuminated during[page 122]the day. With older seedlings, having cotyledons 4 mm. In length, whichrise considerably at night, there is a well-developed pulvinus close to theblade, colourless, and rather narrower than the rest of the petiole, fromwhich it is abruptly separated. It is formed of a mass of small cells of anaverage length of . 021 mm. ; whereas the cells in the lower part of thepetiole are about . 06 mm. , and those in the blade from . 034 to . 04 mm. Inlength. The epidermic cells in the lower part of the petiole projectconically, and thus differ in shape from those over the pulvinus. 

Turning now to very young seedlings, the cotyledons of which do not rise atnight and are only from 2 to 2 ½ mm. In length, their petioles do notexhibit any defined zone of small cells, destitute of chlorophyll anddiffering in shape exteriorly from the lower ones. Nevertheless, the cellsat the place where a pulvinus will afterwards be developed are smaller(being on an average . 015 mm. In length) than those in the lower parts ofthe same petiole, which gradually become larger in proceeding downwards, the largest being . 030 mm. In length. At this early age the cells of theblade are about . 027 mm. In length. We thus see that the pulvinus is formedby the cells in the uppermost part of the petiole, continuing for only ashort time to increase in length, then being arrested in their growth, accompanied by the loss of their chlorophyll grains; whilst the cells inthe lower part of the petiole continue for a long time to increase inlength, those of the epidermis becoming more conical. The singular fact ofthe cotyledons of this plant not sleeping at first is therefore due to thepulvinus not being developed at an early age. ]

We learn from these two cases of Lotus and Oxalis, that the development ofa pulvinus follows from the growth of the cells over a small defined spaceof the petiole being almost arrested at an early age. With Lotus Jacobaeusthe cells at first increase a little in length; in Oxalis corniculata theydecrease a little, owing to self-division. A mass of such small cellsforming a pulvinus, might therefore be either acquired or lost without anyspecial difficulty, by different species in the same natural genus: and weknow that[page 123]with seedlings of Trifolium, Lotus, and Oxalis some of the species have awell-developed pulvinus, and others have none, or one in a rudimentarycondition. As the movements caused by the alternate turgescence of thecells in the two halves of a pulvinus, must be largely determined by theextensibility and subsequent contraction of their walls, we can perhapsunderstand why a large number of small cells will be more efficient than asmall number of large cells occupying the same space. As a pulvinus isformed by the arrestment of the growth of its cells, movements dependent ontheir action may be long-continued without any increase in length of thepart thus provided; and such long-continued movements seem to be one chiefend gained by the development of a pulvinus. Long-continued movement wouldbe impossible in any part, without an inordinate increase in its length, ifthe turgescence of the cells was always followed by growth. 

Disturbance of the Periodic Movements of Cotyledons by Light. --Thehypocotyls and cotyledons of most seedling plants are, as is well known, extremely heliotropic; but cotyledons, besides being heliotropic, areaffected paratonically (to use Sachs' expression) by light; that is, theirdaily periodic movements are greatly and quickly disturbed by changes inits intensity or by its absence. It is not that they cease to circumnutatein darkness, for in all the many cases observed by us they continued to doso; but the normal order of their movements in relation to the alternationsof day and night is much disturbed or quite annulled. This holds good withspecies the cotyledons of which rise or sink so much at night that they maybe said to sleep, as well as with others which rise only a little. Butdifferent species are affected in very different degrees by changes in thelight. [page 124]

[For instance, the cotyledons of Beta vulgaris, Solanum lycopersicum, Cerinthe major, and Lupinus luteus, when placed in darkness, moved downduring the afternoon and early night, instead of rising as they would havedone if they had been exposed to the light. All the individuals of theSolanum did not behave in the same manner, for the cotyledons of onecircumnutated about the same spot between 2. 30 and 10 P. M. The cotyledonsof a seedling of Oxalis corniculata, which was feebly illuminated fromabove, moved downwards during the first morning in the normal manner, buton the second morning it moved upwards. The cotyledons of Lotus Jacoboeuswere not affected by 4 h. Of complete darkness, but when placed under adouble skylight and thus feebly illuminated, they quite lost theirperiodical movements on the third morning. On the other hand, thecotyledons of Cucurbita ovifera moved in the normal manner during a wholeday in darkness. 

Seedlings of Githago segetum were feebly illuminated from above in themorning before their cotyledons had expanded, and they remained closed forthe next 40 h. Other seedlings were placed in the dark after theircotyledons had opened in the morning and these did not begin to close untilabout 4 h. Had elapsed. The cotyledons of Oxalis rosea sank verticallydownwards after being left for 1 h. 20 m. In darkness; but those of someother species of Oxalis were not affected by several hours of darkness. Thecotyledons of several species of Cassia are eminently susceptible tochanges in the degree of light to which they are exposed: thus seedlings ofan unnamed S. Brazilian species (a large and beautiful tree) were broughtout of the hot-house and placed on a table in the middle of a room with twonorth-east and one north-west window, so that they were fairly wellilluminated, though of course less so than in the hot-house, the day beingmoderately bright; and after 36 m. The cotyledons which had been horizontalrose up vertically and closed together as when asleep; after thus remainingon the table for 1 h. 13 m. They began to open. The cotyledons of youngseedlings of another Brazilian species and of C. Neglecta, treated in thesame manner, behaved similarly, excepting that they did not rise up quiteso much: they again became horizontal after about an hour. 

Here is a more interesting case: seedlings of Cassia tora in two pots, which had stood for some time on the table in the room just described, hadtheir cotyledons horizontal. One pot was now exposed for 2 h. To dullsunshine, and the cotyledons[page 125]remained horizontal; it was then brought back to the table, and after 50 m. The cotyledons had risen 68o above the horizon. The other pot was placedduring the same 2 h. Behind a screen in the room, where the light was veryobscure, and the cotyledons rose 63o above the horizon; the pot was thenreplaced on the table, and after 50 m. The cotyledons had fallen 33o. Thesetwo pots with seedlings of the same age stood close together, and wereexposed to exactly the same amount of light, yet the cotyledons in the onepot were rising, whilst those in the other pot were at the same timesinking. This fact illustrates in a striking manner that their movementsare not governed by the actual amount, but by a change in the intensity ordegree of the light. A similar experiment was tried with two sets ofseedlings, both exposed to a dull light, but different in degree, and theresult was the same. The movements of the cotyledons of this Cassia are, however, determined (as in many other cases) largely by habit orinheritance, independently of light; for seedlings which had beenmoderately illuminated during the day, were kept all night and on thefollowing morning in complete darkness; yet the cotyledons were partiallyopen in the morning and remained open in the dark for about 6 h. Thecotyledons in another pot, similarly treated on another occasion, were openat 7 A. M. And remained open in the dark for 4 h. 30 m. , after which timethey began to close. Yet these same seedlings, when brought in the middleof the day from a moderately bright into only a moderately dull lightraised, as we have seen, their cotyledons high above the horizon. 

Sensitiveness of Cotyledons to contact. --This subject does not possess muchinterest, as it is not known that sensitiveness of this kind is of anyservice to seedling plants. We have observed cases in only four genera, though we have vainly observed the cotyledons of many others. The genuscassia seems to be pre-eminent in this respect: thus, the cotyledons of C. Tora, when extended horizontally, were both lightly tapped with a very thintwig for 3 m. And in the course of a few minutes they formed together anangle of 90o, so that each had risen 45o. A single cotyledon of anotherseedling was tapped in a like manner for 1 m. , and it rose 27o in 9 m. ; andafter eight additional minutes it had risen 10o more; the oppositecotyledon, which was not tapped, hardly moved at all. The cotyledons in allthese cases became horizontal again in less than half an hour. The pulvinusis the most sensitive part, for on slightly pricking three cotyledons witha[page 126]pin in this part, they rose up vertically; but the blade was found also tobe sensitive, care having been taken that the pulvinus was not touched. Drops of water placed quietly on these cotyledons produced no effect, butan extremely fine stream of water, ejected from a syringe, caused them tomove upwards. When a pot of seedlings was rapidly hit with a stick and thusjarred, the cotyledons rose slightly. When a minute drop of nitric acid wasplaced on both pulvini of a seedling, the cotyledons rose so quickly thatthey could easily be seen to move, and almost immediately afterwards theybegan to fall; but the pulvini had been killed and became brown. 

The cotyledons of an unnamed species of Cassia (a large tree from S. Brazil) rose 31o in the course of 26 m. After the pulvini and the bladeshad both been rubbed during 1 m. With a twig; but when the blade alone wassimilarly rubbed the cotyledons rose only 8o. The remarkably long andnarrow cotyledons, of a third unnamed species from S. Brazil, did not movewhen their blades were rubbed on six occasions with a pointed stick for 30s. Or for 1 m. ; but when the pulvinus was rubbed and slightly pricked witha pin, the cotyledons rose in the course of a few minutes through an angleof 60o. Several cotyledons of C. Neglecta (likewise from S. Brazil) rose infrom 5 m. To 15 m. To various angles between 16o and 34o, after beingrubbed during 1 m. With a twig. Their sensitiveness is retained to asomewhat advanced age, for the cotyledons of a little plant of C. Neglecta, 34 days old and bearing three true leaves, rose when lightly pinchedbetween the finger and thumb. Some seedlings were exposed for 30 m. To awind (temp. 50o F. ) sufficiently strong to keep the cotyledons vibrating, but this to our surprise did not cause any movement. The cotyledons of fourseedlings of the Indian C. Glauca were either rubbed with a thin twig for 2m. Or were lightly pinched: one rose 34o; a second only 6o; a third 13o; anda fourth 17o. A cotyledon of C. Florida similarly treated rose 9o; one ofC. Corymbosa rose 7 1/2o, and one of the very distinct C. Mimosoides only6o. Those of C. Pubescens did not appear to be in the least sensitive; norwere those of C. Nodosa, but these latter are rather thick and fleshy, anddo not rise at night or go to sleep. 

Smithia sensitiva. --This plant belongs to a distinct sub-order of theLeguminosae from Cassia. Both cotyledons of an oldish seedling, with thefirst true leaf partially unfolded, were rubbed for 1 m. With a fine twig, and in 5 m. Each rose 32o; they[page 127]remained in this position for 15 m. , but when looked at again 40 m. Afterthe rubbing, each had fallen 14o. Both cotyledons of another and youngerseedling were lightly rubbed in the same manner for 1 m. , and after aninterval of 32 m. Each had risen 30o. They were hardly at all sensitive toa fine jet of water. The cotyledons of S. Pfundii, an African water plant, are thick and fleshy; they are not sensitive and do not go to sleep. 

Mimosa pudica and albida. --The blades of several cotyledons of both theseplants were rubbed or slightly scratched with a needle during 1 m. Or 2 m. ;but they did not move in the least. When, however, the pulvini of sixcotyledons of M. Pudica were thus scratched, two of them were slightlyraised. In these two cases perhaps the pulvinus was accidentally pricked, for on pricking the pulvinus of another cotyledon it rose a little. It thusappears that the cotyledons of Mimosa are less sensitive than those of thepreviously mentioned plants. *

Oxalis sensitiva. --The blades and pulvini of two cotyledons, standinghorizontally, were rubbed or rather tickled for 30 s. With a fine splitbristle, and in 10 m. Each had risen 48o; when looked at again in 35 m. After being rubbed they had risen 4o more; after 30 additional minutes theywere again horizontal. On hitting a pot rapidly with a stick for 1 m. , thecotyledons of two seedlings were considerably raised in the course of 11 m. A pot was carried a little distance on a tray and thus jolted; and thecotyledons of four seedlings were all raised in 10 m. ; after 17 m. One hadrisen 56o, a second 45o, a third almost 90o, and a fourth 90o. After anadditional interval of 40 m. Three of them had re-expanded to aconsiderable extent. These observations were made before we were aware atwhat an extraordinarily rapid rate the cotyledons circumnutate, and aretherefore liable to error. Nevertheless it is extremely improbable that thecotyledons in the eight cases given, should all have been rising at thetime when they were irritated. The cotyledons of Oxalis Valdiviana androsea were rubbed and did not exhibit any sensitiveness. ]

Finally, there seems to exist some relation between

* The sole notice which we have met with on the sensitiveness ofcotyledons, relates to Mimosa; for Aug. P. De Candolle says ('Phys. Vég. , '1832, tom. Ii. P. 865), "les cotyledons du M. Pudica tendent à se raprocherpar leurs faces supérieures lorsqu'on les irrite. "[page 128]

the habit of cotyledons rising vertically at night or going to sleep, andtheir sensitiveness, especially that of their pulvini, to a touch; for allthe above-named plants sleep at night. On the other hand, there are manyplants the cotyledons of which sleep, and are not in the least sensitive. As the cotyledons of several species of Cassia are easily affected both byslightly diminished light and by contact, we thought that these two kindsof sensitiveness might be connected; but this is not necessarily the case, for the cotyledons of Oxalis sensitiva did not rise when kept on oneoccasion for 1 ½ h. , and on a second occasion for nearly 4 h. , in a darkcloset. Some other cotyledons, as those of Githago segetum, are muchaffected by a feeble light, but do not move when scratched by a needle. That with the same plant there is some relation between the sensitivenessof its cotyledons and leaves seems highly probable, for the above describedSmithia and Oxalis have been called sensitiva, owing to their leaves beingsensitive; and though the leaves of the several species of Cassia are notsensitive to a touch, yet if a branch be shaken or syringed with water, they partially assume their nocturnal dependent position. But the relationbetween the sensitiveness to contact of the cotyledons and of the leaves ofthe same plant is not very close, as may be inferred from the cotyledons ofMimosa pudica being only slightly sensitive, whilst the leaves are wellknown to be so in the highest degree. Again, the leaves of Neptuniaoleracea are very sensitive to a touch, whilst the cotyledons do not appearto be so in any degree. [page 129]

CHAPTER III. 

SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS. 

Manner in which radicles bend when they encounter an obstacle in the soil--Vicia faba, tips of radicles highly sensitive to contact and otherirritants--Effects of too high a temperature--Power of discriminatingbetween objects attached on opposite sides--Tips of secondary radiclessensitive--Pisum, tips of radicles sensitive--Effects of such sensitivenessin overcoming geotropism--Secondary radicles--Phaseolus, tips of radicleshardly sensitive to contact, but highly sensitive to caustic and to theremoval of a slice--Tropaeolum--Gossypium--Cucurbita--Raphanus--Aesculus, tip not sensitive to slight contact, highly sensitive to caustic--Quercus, tip highly sensitive to contact--Power of discrimination--Zea, tip highlysensitive, secondary radicles--Sensitiveness of radicles to moist air--Summary of chapter. 

IN order to see how the radicles of seedlings would pass over stones, roots, and other obstacles, which they must incessantly encounter in thesoil, germinating beans (Vicia faba) were so placed that the tips of theradicles came into contact, almost rectangularly or at a high angle, withunderlying plates of glass. In other cases the beans were turned aboutwhilst their radicles were growing, so that they descended nearlyvertically on their own smooth, almost flat, broad upper surfaces. Thedelicate root-cap, when it first touched any directly opposing surface, wasa little flattened transversely; the flattening soon became oblique, and ina few hours quite disappeared, the apex now pointing at right angles, or atnearly right angles, to its former course. The radicle then seemed to glidein its new direction over the surface which had opposed[page 130]it, pressing on it with very little force. How far such abrupt changes inits former course are aided by the circumnutation of the tip must be leftdoubtful. Thin slips of wood were cemented on more or less steeply inclinedglass-plates, at right angles to the radicles which were gliding down them. Straight lines had been painted along the growing terminal part of some ofthese radicles, before they met the opposing slip of wood; and the linesbecame sensibly curved in 2 h. After the apex had come into contact withthe slips. In one case of a radicle, which was growing rather slowly, theroot-cap, after encountering a rough slip of wood at right angles, was atfirst slightly flattened transversely: after an interval of 2 h. 30 m. Theflattening became oblique; and after an additional 3 hours the flatteninghad wholly disappeared, and the apex now pointed at right angles to itsformer course. It then continued to grow in its new direction alongside theslip of wood, until it came to the end of it, round which it bentrectangularly. Soon afterwards when coming to the edge of the plate ofglass, it was again bent at a large angle, and descended perpendicularlyinto the damp sand. 

When, as in the above cases, radicles encountered an obstacle at rightangles to their course, the terminal growing part became curved for alength of between . 3 and . 4 of an inch (8-10 mm. ), measured from the apex. This was well shown by the black lines which had been previously painted onthem. The first and most obvious explanation of the curvature is, that itresults merely from the mechanical resistance to the growth of the radiclein its original direction. Nevertheless, this explanation did not seem tous satisfactory. The radicles did not present the appearance of having beensubjected to a sufficient pressure to account for[page 131]their curvature; and Sachs has shown* that the growing part is more rigidthan the part immediately above which has ceased to grow, so that thelatter might have been expected to yield and become curved as soon as theapex encountered an unyielding object; whereas it was the stiff growingpart which became curved. Moreover, an object which yields with thegreatest ease will deflect a radicle: thus, as we have seen, when the apexof the radicle of the bean encountered the polished surface of extremelythin tin-foil laid on soft sand, no impression was left on it, yet theradicle became deflected at right angles. A second explanation occurred tous, namely, that even the gentlest pressure might check the growth of theapex, and in this case growth could continue only on one side, and thus theradicle would assume a rectangular form; but this view leaves whollyunexplained the curvature of the upper part, extending for a length of 8-10mm. 

We were therefore led to suspect that the apex was sensitive to contact, and that an effect was transmitted from it to the upper part of theradicle, which was thus excited to bend away from the touching object. As alittle loop of fine thread hung on a tendril or on the petiole of aleaf-climbing plant, causes it to bend, we thought that any small hardobject affixed to the tip of a radicle, freely suspended and growing indamp air, might cause it to bend, if it were sensitive, and yet would notoffer any mechanical resistance to its growth. Full details will be givenof the experiments which were tried, as the result proved remarkable. Thefact of the apex of a radicle being sensitive to contact has never beenobserved, though, as we shall

* 'Arbeiten Bot. Inst. Würzburg, ' Heft iii. 1873, p. 398. 

[page 132]hereafter see, Sachs discovered that the radicle a little above the apex issensitive, and bends like a tendril towards the touching object. But whenone side of the apex is pressed by any object, the growing part bends awayfrom the object; and this seems a beautiful adaptation for avoidingobstacles in the soil, and, as we shall see, for following the lines ofleast resistance. Many organs, when touched, bend in one fixed direction, such as the stamens of Berberis, the lobes of Dionaea, etc. ; and manyorgans, such as tendrils, whether modified leaves or flower-peduncles, andsome few stems, bend towards a touching object; but no case, we believe, isknown of an organ bending away from a touching object. 

Sensitiveness of the Apex of the Radicle of Vicia faba. --Common beans, after being soaked in water for 24 h. , were pinned with the hilum downwards(in the manner followed by Sachs), inside the cork lids of glass-vessels, which were half filled with water; the sides and the cork were wellmoistened, and light was excluded. As soon as the beans had protrudedradicles, some to a length of less than a tenth of an inch, and others to alength of several tenths, little squares or oblongs of card were affixed tothe short sloping sides of their conical tips. The squares thereforeadhered obliquely with reference to the longitudinal axis of the radicle;and this is a very necessary precaution, for if the bits of cardaccidentally became displaced, or were drawn by the viscid matter employedso as to adhere parallel to the side of the radicle, although only a littleway above the conical apex, the radicle did not bend in the peculiar mannerwhich we are here considering. Squares of about the 1/20th of an inch (i. E. About 1 ½ mm. ), or oblong bits of nearly the same size, were found to[page 133]be the most convenient and effective. We employed at first ordinary thincard, such as visiting cards, or bits of very thin glass, and various otherobjects; but afterwards sand-paper was chiefly employed, for it was almostas stiff as thin card, and the roughened surface favoured its adhesion. Atfirst we generally used very thick gum-water; and this of course, under thecircumstances, never dried in the least; on the contrary, it sometimesseemed to absorb vapour, so that the bits of card became separated by alayer of fluid from the tip. When there was no such absorption and the cardwas not displaced, it acted well and caused the radicle to bend to theopposite side. I should state that thick gum-water by itself induces noaction. In most cases the bits of card were touched with an extremely smallquantity of a solution of shellac in spirits of wine, which had been leftto evaporate until it was thick; it then set hard in a few seconds, andfixed the bits of card well. When small drops of the shellac were placed onthe tips without any card, they set into hard little beads, and these actedlike any other hard object, causing the radicles to bend to the oppositeside. Even extremely minute beads of the shellac occasionally acted in aslight degree, as will hereafter be described. But that it was the cardswhich chiefly acted in our many trials, was proved by coating one side ofthe tip with a little bit of goldbeaters' skin (which by itself hardlyacts), and then fixing a bit of card to the skin with shellac which nevercame into contact with the radicle: nevertheless the radicle bent away fromthe attached card in the ordinary manner. 

Some preliminary trials were made, presently to be described, by which theproper temperature was determined, and then the following experiments weremade. It should be premised that the beans were[page 134]always fixed to the cork-lids, for the convenience of manipulation, withthe edge from which the radicle and plumule protrudes, outwards; and itmust be remembered that owing to what we have called Sachs' curvature, theradicles, instead of growing perpendicularly downwards, often bendsomewhat, even as much

Fig. 65. Vicia faba: A, radicle beginning to bend from the attached littlesquare of card; B, bent at a rectangle; C, bent into a circle or loop, withthe tip beginning to bend downwards through the action of geotropism. 

as about 45o inwards, or under the suspended bean. Therefore when a squareof card was fixed to the apex in front, the bowing induced by it coincidedwith Sachs' curvature, and could be distinguished from it only by beingmore strongly pronounced or by occurring more quickly. To avoid this sourceof doubt, the squares[page 135]were fixed either behind, causing a curvature in direct opposition to thatof Sachs', or more commonly to the right or left sides. For the sake ofbrevity, we will speak of the bits of card, etc. , as fixed in front, orbehind, or laterally. As the chief curvature of the radicle is at a littledistance from the apex, and as the extreme terminal and basal portions arenearly straight, it is possible to estimate in a rough manner the amount ofcurvature by an angle; and when it is said that the radicle becamedeflected at any angle from the perpendicular, this implies that the apexwas turned upwards by so many degrees from the downward direction which itwould naturally have followed, and to the side opposite to that to whichthe card was affixed. That the reader may have a clear idea of the kind ofmovement excited by the bits of attached card, we append here accuratesketches of three germinating beans thus treated, and selected out ofseveral specimens to show the gradations in the degrees of curvature. Wewill now give in detail a series of experiments, and afterwards a summaryof the results. 

[In the first 12 trials, little squares or oblongs of sanded card, 1. 8 mm. In length, and 1. 5 or only 0. 9 mm. In breadth (i. E. . 071 of an inch inlength and . 059 or . 035 of an inch in breadth) were fixed with shellac tothe tips of the radicles. In the subsequent trials the little squares wereonly occasionally measured, but were of about the same size. 

(1. ) A young radicle, 4 mm. In length, had a card fixed behind: after 9 h. Deflected in the plane in which the bean is flattened, 50o from theperpendicular and from the card, and in opposition to Sachs' curvature: nochange next morning, 23 h. From the time of attachment. 

(2. ) Radicle 5. 5 mm. In length, card fixed behind: after 9 h. Deflected inthe plane of the bean 20o from the perpendicular and from the card, and inopposition to Sachs' curvature: after 23 h. No change. [page 136]

(3. ) Radicle 11 mm. In length, card fixed behind: after 9 h. Deflected inthe plane of the bean 40o from the perpendicular and from the card, and inopposition to Sachs' curvature. The tip of the radicle more curved than theupper part, but in the same plane. After 23 h. The extreme tip was slightlybent towards the card; the general curvature of the radicle remaining thesame. 

(4. ) Radicle 9 mm. Long, card fixed behind and a little laterally: after 9h. Deflected in the plane of the bean only about 7o or 8o from theperpendicular and from the card, in opposition to Sachs' curvature. Therewas in addition a slight lateral curvature directed partly from the card. After 23 h. No change. 

(5. ) Radicle 8 mm. Long, card affixed almost laterally: after 9 h. Deflected 30o from the perpendicular, in the plane of the bean and inopposition to Sachs' curvature; also deflected in a plane at right anglesto the above one, 20o from the perpendicular: after 23 h. No change. 

(6. ) Radicle 9 mm. Long, card affixed in front: after 9 h. Deflected in theplane of the bean about 40o from the vertical, away from the card and inthe direction of Sachs' curvature. Here therefore we have no evidence ofthe card being the cause of the deflection, except that a radicle nevermoves spontaneously, as far as we have seen, as much as 40o in the courseof 9 h. After 23 h. No change. 

(7. ) Radicle 7 mm. Long, card affixed to the back: after 9 h. The terminalpart of the radicle deflected in the plane of the bean 20o from thevertical, away from the card and in opposition to Sachs' curvature. After22 h. 30 m. This part of the radicle had become straight. 

(8. ) Radicle 12 mm. Long, card affixed almost laterally: after 9 h. Deflected laterally in a plane at right angles to that of the bean between40o and 50o from the vertical and from the card. In the plane of the beanitself the deflection amounted to 8o or 9o from the vertical and from thecard, in opposition to Sachs' curvature. After 22 h. 30 m. The extreme tiphad become slightly curved towards the card. 

(9. ) Card fixed laterally: after 11 h. 30 m. No effect, the radicle beingstill almost vertical. 

(10. ) Card fixed almost laterally: after 11 h. 30 m. Deflected 90o from thevertical and from the card, in a plane intermediate between that of thebean itself and one at right[page 137]angles to it. Radicle consequently partially deflected from Sachs'curvature. 

(11. ) Tip of radicle protected with goldbeaters' skin, with a square ofcard of the usual dimensions affixed with shellac: after 11 h. Greatlydeflected in the plane of the bean, in the direction of Sachs' curvature, but to a much greater degree and in less time than ever occursspontaneously. 

(12. ) Tip of radicle protected as in last case: after 11 h. No effect, butafter 24 h. 40 m. Radicle clearly deflected from the card. This slow actionwas probably due to a portion of the goldbeaters' skin having curled roundand lightly touched the opposite side of the tip and thus irritated it. 

(13. ) A radicle of considerable length had a small square of card fixedwith shellac to its apex laterally: after only 7 h. 15 m. A length of . 4 ofan inch from the apex, measured along the middle, was considerably curvedfrom the side bearing the card. 

(14. ) Case like the last in all respects, except that a length of only . 25of an inch of the radicle was thus deflected. 

(15. ) A small square of card fixed with shellac to the apex of a youngradicle; after 9 h. 15 m. Deflected through 90o from the perpendicular andfrom the card. After 24 h. Deflection much decreased, and after anadditional day, reduced to 23o from the perpendicular. 

(16. ) Square of card fixed with shellac behind the apex of a radicle, whichfrom its position having been changed during growth had become verycrooked; but the terminal portion was straight, and this became deflectedto about 45o from the perpendicular and from the card, in opposition toSachs' curvature. 

(17. ) Square of card affixed with shellac: after 8 h. Radicle curved atright angles from the perpendicular and from the card. After 15 additionalhours curvature much decreased. 

(18. ) Square of card affixed with shellac: after 8 h. No effect; after 23h. 3 m. From time of affixing, radicle much curved from the square. (19. )Square of card affixed with shellac: after 24 h. No effect, but the radiclehad not grown well and seemed sickly. 

(20. ) Square of card affixed with shellac: after 24 h. No effect. 

(21, 22. ) Squares of card affixed with shellac: after 24 h. Radicles ofboth curved at about 45o from the perpendicular and from the cards. 

(23. ) Square of card fixed with shellac to young radicle: after[page 138]9 h. Very slightly curved from the card; after 24 h. Tip curved towardscard. Refixed new square laterally, after 9 h. Distinctly curved from thecard, and after 24 h. Curved at right angles from the perpendicular andfrom the card. 

(24. ) A rather large oblong piece of card fixed with shellac to apex: after24 h. No effect, but the card was found not to be touching the apex. Asmall square was now refixed with shellac; after 16 h. Slight deflectionfrom the perpendicular and from the card. After an additional day theradicle became almost straight. 

(25. ) Square of card fixed laterally to apex of young radicle; after 9 h. Deflection from the perpendicular considerable; after 24 h. Deflectionreduced. Refixed a fresh square with shellac: after 24 h. Deflection about40o from the perpendicular and from the card. 

(26. ) A very small square of card fixed with shellac to apex of youngradicle: after 9 h. The deflection from the perpendicular and from the cardamounted to nearly a right angle; after 24 h. Deflection much reduced;after an additional 24 h. Radicle almost straight. 

(27. ) Square of card fixed with shellac to apex of young radicle: after 9h. Deflection from the card and from the perpendicular a right angle; nextmorning quite straight. Refixed a square laterally with shellac; after 9 h. A little deflection, which after 24 h. Increased to nearly 20o from theperpendicular and from the card. 

(28. ) Square of card fixed with shellac; after 9 h. Some deflection; nextmorning the card dropped off; refixed it with shellac; it again becameloose and was refixed; and now on the third trial the radicle was deflectedafter 14 h. At right angles from the card. 

(29. ) A small square of card was first fixed with thick gum-water to theapex. It produced a slight effect but soon fell off. A similar square wasnow affixed laterally with shellac: after 9 h. The radicle was deflectednearly 45o from the perpendicular and from the card. After 36 additionalhours angle of deflection reduced to about 30o. 

(30. ) A very small piece, less than 1/20th of an inch square, of thintin-foil fixed with shellac to the apex of a young radicle; after 24 h. Noeffect. Tin-foil removed, and a small square of sanded card fixed withshellac; after 9 h. Deflection at nearly right angles from theperpendicular and from the card. Next[page 139]morning deflection reduced to about 40o from the perpendicular. 

(31. ) A splinter of thin glass gummed to apex, after 9 h. No effect, but itwas then found not to be touching the apex of the radicle. Next morning asquare of card was fixed with shellac to it, and after 9 h. Radicle greatlydeflected from the card. After two additional days the deflection haddecreased and was only 35o from the perpendicular. 

(32. ) Small square of sanded card, attached with thick gum-water laterallyto the apex of a long straight radicle: after 9 h. Greatly deflected fromthe perpendicular and from the card. Curvature extended for a length of . 22of an inch from the apex. After 3 additional hours terminal portiondeflected at right angles from the perpendicular. Next morning the curvedportion was . 36 in length. 

(33. ) Square of card gummed to apex: after 15 h. Deflected at nearly 90ofrom the perpendicular and from the card. 

(34. ) Small oblong of sanded card gummed to apex: after 15 h. Deflected 90ofrom the perpendicular and from the card: in the course of the threefollowing days the terminal portion became much contorted and ultimatelycoiled into a helix. 

(35. ) Square of card gummed to apex: after 9 h. Deflected from card: after24 h. From time of attachment greatly deflected obliquely and partly inopposition to Sachs' curvature. 

(36. ) Small piece of card, rather less than 1/20th of an inch square, gummed to apex: in 9 h. Considerably deflected from card and in oppositionto Sachs' curvature; after 24 h. Greatly deflected in the same direction. After an additional day the extreme tip was curved towards the card. 

(37. ) Square of card, gummed to apex in front, caused after 8 h. 30 m. Hardly any effect; refixed fresh square laterally, after 15 h. Deflectedalmost 90o from the perpendicular and from the card. After 2 additionaldays deflection much reduced. 

(38. ) Square of card gummed to apex: after 9 h. Much deflection, whichafter 24 h. From time of fixing increased to nearly 90o. After anadditional day terminal portion was curled into a loop, and on thefollowing day into a helix. 

(39. ) Small oblong piece of card gummed to apex, nearly in front, but alittle to one side; in 9 h. Slightly deflected in the direction of Sachs'curvature, but rather obliquely, and to side opposite to card. Next daymore curved in the same direction, and after 2 additional days coiled intoa ring. [page 140]

(40. ) Square of card gummed to apex: after 9 h. Slightly curved from card;next morning radicle straight, and apex had grown beyond the card. Refixedanother square laterally with shellac; in 9 h. Deflected laterally, butalso in the direction of Sachs' curvature. After 2 additional days'curvature considerably increased in the same direction. 

(41. ) Little square of tin-foil fixed with gum to one side of apex of ayoung and short radicle: after 15 h. No effect, but tin-foil had becomedisplaced. A little square of card was now gummed to one side of apex, which after 8 h. 40 m. Was slightly deflected; in 24 h. From the time ofattachment deflected at 90o from the perpendicular and from the card; after9 additional hours became hooked, with the apex pointing to the zenith. In3 days from the time of attachment the terminal portion of the radicleformed a ring or circle. 

(42. ) A little square of thick letter-paper gummed to the apex of aradicle, which after 9 h. Was deflected from it. In 24 h. From time whenthe paper was affixed the deflection much increased, and after 2 additionaldays it amounted to 50o from the perpendicular and from the paper. 

(43. ) A narrow chip of a quill was fixed with shellac to the apex of aradicle. After 9 h. No effect; after 24 h. Moderate deflection, but now thequill had ceased to touch the apex. Removed quill and gummed a littlesquare of card to apex, which after 8 h. Caused slight deflection. On thefourth day from the first attachment of any object, the extreme tip wascurved towards the card. 

(44. ) A rather long and narrow splinter of extremely thin glass, fixed withshellac to apex, it caused in 9 h. Slight deflection, which disappeared in24 h. ; the splinter was then found not touching the apex. It was twicerefixed, with nearly similar results, that is, it caused slight deflection, which soon disappeared. On the fourth day from the time of first attachmentthe tip was bent towards the splinter. ]

From these experiments it is clear that the apex of the radicle of the beanis sensitive to contact, and that it causes the upper part to bend awayfrom the touching object. But before giving a summary of the results, itwill be convenient briefly to give a few other observations. Bits of verythin glass and little squares[page 141]of common card were affixed with thick gum-water to the tips of theradicles of seven beans, as a preliminary trial. Six of these were plainlyacted on, and in two cases the radicles became coiled up into completeloops. One radicle was curved into a semi-circle in so short a period as 6h. 10 m. The seventh radicle which was not affected was apparently sickly, as it became brown on the following day; so that it formed no realexception. Some of these trials were made in the early spring during coldweather in a sitting-room, and others in a greenhouse, but the temperaturewas not recorded. These six striking cases almost convinced us that theapex was sensitive, but of course we determined to make many more trials. As we had noticed that the radicles grew much more quickly when subjectedto considerable heat, and as we imagined that heat would increase theirsensitiveness, vessels with germinating beans suspended in damp air wereplaced on a chimney-piece, where they were subjected during the greaterpart of the day to a temperature of between 69o and 72o F. ; some, however, were placed in the hot-house where the temperature was rather higher. Abovetwo dozen beans were thus tried; and when a square of glass or card did notact, it was removed, and a fresh one affixed, this being often done thriceto the same radicle. Therefore between five and six dozen trials werealtogether made. But there was moderately distinct deflection from theperpendicular and from the attached object in only one radicle out of thislarge number of cases. In five other cases there was very slight anddoubtful deflection. We were astonished at this result, and concluded thatwe had made some inexplicable mistake in the first six experiments. Butbefore finally relinquishing the subject, we resolved to make one[page 142]other trial for it occurred to us that sensitiveness is easily affected byexternal conditions, and that radicles growing naturally in the earth inthe early spring would not be subjected to a temperature nearly so high as70o F. We therefore allowed the radicles of 12 beans to grow at atemperature of between 55o and 60o F. The result was that in every one ofthese cases (included in the above-described experiments) the radicle wasdeflected in the course of a few hours from the attached object. All theabove recorded successful trials, and some others presently to be given, were made in a sitting-room at the temperatures just specified. Ittherefore appears that a temperature of about, or rather above, 70o F. Destroys the sensitiveness of the radicles, either directly, or indirectlythrough abnormally accelerated growth; and this curious fact probablyexplains why Sachs, who expressly states that his beans were kept at a hightemperature, failed to detect the sensitiveness of the apex of the radicle. 

But other causes interfere with this sensibility. Eighteen radicles weretried with little squares of sanded card, some affixed with shellac andsome with gum-water, during the few last days of 1878, and few first daysof the next year. They were kept in a room at the proper temperature duringthe day, but were probably too cold at night, as there was a hard frost atthe time. The radicles looked healthy but grew very slowly. The result wasthat only 6 out of the 18 were deflected from the attached cards, and thisonly to a slight degree and at a very slow rate. These radicles thereforepresented a striking contrast with the 44 above described. On March 6th and7th, when the temperature of the room varied between 53o and 59o F. , elevengerminating beans were tried in the[page 143]same manner, and now every one of the radicles became curved away from thecards, though one was only slightly deflected. Some horticulturists believethat certain kinds of seeds will not germinate properly in the middle ofthe winter, although kept at a right temperature. If there really is anyproper period for the germination of the bean, the feeble degree ofsensibility of the above radicles may have resulted from the trial havingbeen made in the middle of the winter, and not simply from the nights beingtoo cold. Lastly, the radicles of four beans, which from some innate causegerminated later than all the others of the same lot, and which grew slowlythough appearing healthy, were similarly tried, and even after 24 h. Theywere hardly at all deflected from the attached cards. We may thereforeinfer that any cause which renders the growth of the radicles either sloweror more rapid than the normal rate, lessens or annuls the sensibility oftheir tips to contact. It deserves particular attention that when theattached objects failed to act, there was no bending of any kind, exceptingSachs' curvature. The force of our evidence would have been greatlyweakened if occasionally, though rarely, the radicles had become curved inany direction independently of the attached objects. In the foregoingnumbered paragraphs, however, it may be observed that the extreme tipsometimes becomes, after a considerable interval of time, abruptly curvedtowards the bit of card; but this is a totally distinct phenomenon, as willpresently be explained. 

Summary of the Results of the foregoing Experiments on the Radicles ofVicia faba. --Altogether little squares (about 1/20th of an inch), generallyof sanded paper as stiff as thin card (between . 15 and . 20 mm. Inthickness), sometimes of ordinary card, or little frag-[page 144]ments of very thin glass etc. , were affixed at different times to one sideof the conical tips of 55 radicles. The 11 last-mentioned cases, but notthe preliminary ones, are here included. The squares, etc. , were mostcommonly affixed with shellac, but in 19 cases with thick gum-water. Whenthe latter was used, the squares were sometimes found, as previouslystated, to be separated from the apex by a layer of thick fluid, so thatthere was no contact, and consequently no bending of the radicle; and suchfew cases were not recorded. But in every instance in which shellac wasemployed, unless the square fell off very soon, the result was recorded. Inseveral instances when the squares became displaced, so as to standparallel to the radicle, or were separated by fluid from the apex, or soonfell off, fresh squares were attached, and these cases (described under thenumbered paragraphs) are here included. Out of 55 radicles experimented onunder the proper temperature, 52 became bent, generally to a considerableextent from the perpendicular, and away from the side to which the objectwas attached. Of the three failures, one can be accounted for, as theradicle became sickly on the following day; and a second was observed onlyduring 11 h. 30 m. As in several cases the terminal growing part of theradicle continued for some time to bend from the attached object, it formeditself into a hook, with the apex pointing to the zenith, or even into aring, and occasionally into a spire or helix. It is remarkable that theselatter cases occurred more frequently when objects were attached with thickgum-water, which never became dry, than when shellac was employed. Thecurvature was often well-marked in from 7 h. To 11 h. ; and in one instancea semicircle was formed in 6 h. 10 m, from the time[page 145]of attachment. But in order to see the phenomenon as well displayed as inthe above described cases, it is indispensable that the bits of card, etc. , should be made to adhere closely to one side of the conical apex; thathealthy radicles should be selected and kept at not too high or too low atemperature, and apparently that the trials should not be made in themiddle of the winter. 

In ten instances, radicles which had curved away from a square of card orother object attached to their tips, straightened themselves to a certainextent, or even completely, in the course of from one to two days from thetime of attachment. This was more especially apt to occur when thecurvature was slight. But in one instance (No. 27) a radicle which in 9 h. Had been deflected about 90o from the perpendicular, became quite straightin 24 h. From the period of attachment. With No. 26, the radicle was almoststraight in 48 h. We at first attributed the straightening process to theradicles becoming accustomed to a slight stimulus, in the same manner as atendril or sensitive petiole becomes accustomed to a very light loop ofthread, and unbends itself though the loop remains still suspended; butSachs states* that radicles of the bean placed horizontally in damp airafter curving downwards through geotropism, straighten themselves a littleby growth along their lower or concave sides. Why this should occur is notclear: but perhaps it likewise occurred in the above ten cases. There isanother occasional movement which must not be passed over: the tip of theradicle, for a length of from 2 to 3 mm. , was found in six instances, * 'Arbeiten Bot. Instit. , Würzburg, ' Heft iii. P. 456. [page 146]

after an interval of about 24 or more hours, bent towards the bit of stillattached card, --that is, in a direction exactly opposite to the previouslyinduced curvature of the whole growing part for a length of from 7 to 8 mm. This occurred chiefly when the first curvature was small, and when anobject had been affixed more than once to the apex of the same radicle. Theattachment of a bit of card by shellac to one side of the tender apex maysometimes mechanically prevent its growth; or the application of thickgum-water more than once to the same side may injure it; and then checkedgrowth on this side with continued growth on the opposite and unaffectedside would account for the reversed curvature of the apex. 

Various trials were made for ascertaining, as far as we could, the natureand degree of irritation to which the apex must be subjected, in order thatthe terminal growing part should bend away, as if to avoid the cause ofirritation. We have seen in the numbered experiments, that a little squareof rather thick letter-paper gummed to the apex induced, though slowly, considerable deflection. Judging from several cases in which variousobjects had been affixed with gum, and had soon become separated from theapex by a layer of fluid, as well as from some trials in which drops ofthick gum-water alone had been applied, this fluid never causes bending. Wehave also seen in the numbered experiments that narrow splinters of quilland of very thin glass, affixed with shellac, caused only a slight degreeof deflection, and this may perhaps have been due to the shellac itself. Little squares of goldbeaters' skin, which is excessively thin, weredamped, and thus made to adhere to one side of the tips of two radicles;one of these, after 24 h. , produced no effect; nor did the[page 147]other in 8 h. , within which time squares of card usually act; but after 24h. There was slight deflection. 

An oval bead, or rather cake, of dried shellac, 1. 01 mm. In length and 0. 63in breadth, caused a radicle to become deflected at nearly right angles inthe course of only 6 h. ; but after 23 h. It had nearly straightened itself. A very small quantity of dissolved shellac was spread over a bit of card, and the tips of 9 radicles were touched laterally with it; only two of thembecame slightly deflected to the side opposite to that bearing the speck ofdried shellac, and they afterwards straightened themselves. These speckswere removed, and both together weighed less than 1/100th of a grain; sothat a weight of rather less than 1/200th of a grain (0. 32 mg. ) sufficed toexcite movement in two out of the nine radicles. Here then we haveapparently reached nearly the minimum weight which will act. 

A moderately thick bristle (which on measurement was found ratherflattened, being 0. 33 mm. In one diameter, and 0. 20 mm. In the other) wascut into lengths of about 1/20th of an inch. These after being touched withthick gum-water, were placed on the tips of eleven radicles. Three of themwere affected; one being deflected in 8 h. 15 m. To an angle of about 90ofrom the perpendicular; a second to the same amount when looked at after 9h. ; but after 24 h. From the time of first attachment the deflection haddecreased to only 19o; the third was only slightly deflected after 9 h. , and the bit of bristle was then found not touching the apex; it wasreplaced, and after 15 additional hours the deflection amounted to 26o fromthe perpendicular. The remaining eight radicles were not at all acted on bythe bits of bristle, so that we here appear to have nearly reached theminimum[page 148]of size of an object which will act on the radicle of the bean. But it isremarkable that when the bits of bristle did act, that they should haveacted so quickly and efficiently. 

As the apex of a radicle in penetrating the ground must be pressed on allsides, we wished to learn whether it could distinguish between harder ormore resisting, and softer substances. A square of the sanded paper, almostas stiff as card, and a square of extremely thin paper (too thin forwriting on), of exactly the same size (about 1/20th of an inch), were fixedwith shellac on opposite sides of the apices of 12 suspended radicles. Thesanded card was between 0. 15 and 0. 20 mm. (or between 0. 0059 and 0. 0079 ofan inch), and the thin paper only 0. 045 mm. (or 0. 00176 of an inch) inthickness. In 8 out of the 12 cases there could be no doubt that theradicle was deflected from the side to which the card-like paper wasattached, and towards the opposite side, bearing the very thin paper. Thisoccurred in some instances in 9 h. , but in others not until 24 h. Hadelapsed. Moreover, some of the four failures can hardly be considered asreally failures: thus, in one of them, in which the radicle remained quitestraight, the square of thin paper was found, when both were removed fromthe apex, to have been so thickly coated with shellac that it was almost asstiff as the card: in the second case, the radicle was bent upwards into asemicircle, but the deflection was not directly from the side bearing thecard, and this was explained by the two squares having become cementedlaterally together, forming a sort of stiff gable, from which the radiclewas deflected: in the third case, the square of card had been fixed bymistake in front, and though there was deflection from it, this might havebeen due to Sachs' curvature:[page 149]in the fourth case alone no reason could be assigned why the radicle hadnot been at all deflected. These experiments suffice to prove that the apexof the radicle possesses the extraordinary power of discriminating betweenthin card and very thin paper, and is deflected from the side pressed bythe more resisting or harder substance. 

Some trials were next made by irritating the tips without any object beingleft in contact with them. Nine radicles, suspended over water, had theirtips rubbed, each six times with a needle, with sufficient force to shakethe whole bean; the temperature was favourable, viz. About 63o F. In 7 outof these cases no effect whatever was produced; in the eighth case theradicle became slightly deflected from, and in the ninth case slightlydeflected towards, the rubbed side; but these two latter opposed curvatureswere probably accidental, as radicles do not always grow perfectly straightdownwards. The tips of two other radicles were rubbed in the same mannerfor 15 seconds with a little round twig, two others for 30 seconds, and twoothers for 1 minute, but without any effect being produced. We maytherefore conclude from these 15 trials that the radicles are not sensitiveto temporary contact, but are acted on only by prolonged, though veryslight, pressure. 

We then tried the effects of cutting off a very thin slice parallel to oneof the sloping sides of the apex, as we thought that the wound would causeprolonged irritation, which might induce bending towards the opposite side, as in the case of an attached object. Two preliminary trials were made:firstly, slices were cut from the radicles of 6 beans suspended in dampair, with a pair of scissors, which, though sharp, probably causedconsiderable crushing, and no curva-[page 150]ture followed. Secondly, thin slices were cut with a razor obliquely offthe tips of three radicles similarly suspended; and after 44 h. Two werefound plainly bent from the sliced surface; and the third, the whole apexof which had been cut off obliquely by accident, was curled upwards overthe bean, but it was not clearly ascertained whether the curvature had beenat first directed from the cut surface. These results led us to pursue theexperiment, and 18 radicles, which had grown vertically downwards in dampair, had one side of their conical tips sliced off with a razor. The tipswere allowed just to enter the water in the jars, and they were exposed toa temperature 14o - 16o C. (57o - 61o F. ). The observations were made atdifferent times. Three were examined 12 h. After being sliced, and were allslightly curved from the cut surface; and the curvature increasedconsiderably after an additional 12 h. Eight were examined after 19 h. ;four after 22 h. 30 m. ; and three after 25 h. The final result was that outof the 18 radicles thus tried, 13 were plainly bent from the cut surfaceafter the above intervals of time; and one other became so after anadditional interval of 13 h. 30 m. So that only 4 out of the 18 radicleswere not acted on. To these 18 cases the 3 previously mentioned ones shouldbe added. It may, therefore, be concluded that a thin slice removed by arazor from one side of the conical apex of the radicle causes irritation, like that from an attached object, and induces curvature from the injuredsurface. 

Lastly, dry caustic (nitrate of silver) was employed to irritate one sideof the apex. If one side of the apex or of the whole terminal growing partof a radicle, is by any means killed or badly injured, the other sidecontinues to grow; and this causes the part[page 151]to bend over towards the injured side. * But in the following experiments weendeavoured, generally with success, to irritate the tips on one side, without badly injuring them. This was effected by first drying the tip asfar as possible with blotting-paper, though it still remained somewhatdamp, and then touching it once with quite dry caustic. Seventeen radicleswere thus treated, and were suspended in moist air over water at atemperature of 58o F. They were examined after an interval of 21 h. Or 24h. The tips of two were found blackened equally all round, so that they couldtell nothing and were rejected, 15 being left. Of these, 10 were curvedfrom the side which had been touched, where there was a minute brown orblackish mark. Five of these radicles, three of which were already slightlydeflected, were allowed to enter the water in the jar, and were re-examinedafter an additional interval of 27 h. (i. E. In 48 h. After the applicationof the caustic), and now four of them had become hooked, being bent fromthe discoloured side, with their points directed to the zenith; the fifthremained unaffected and straight. Thus 11 radicles out of the 15 were actedon. But the curvature of the four just described was so plain, that theyalone would have sufficed to show that the radicles of the bean bend awayfrom that side of the apex which has been slightly irritated by caustic. 

The Power of an Irritant on the apex of the Radicle

* Ciesielski found this to be the case ('Untersuchungen über dieAbwartskrümmung der Wurzel, ' 1871, p. 28) after burning with heatedplatinum one side of a radicle. So did we when we painted longitudinallyhalf of the whole length of 7 radicles, suspended over water, with a thicklayer of grease, which is very injurious or even fatal to growing parts;for after 48 hours five of these radicles were curved towards the greasedside, two remaining straight. [page 152]

of the Bean, compared with that of Geotropism. --We know that when a littlesquare of card or other object is fixed to one side of the tip of avertically dependent radicle, the growing part bends from it often into asemicircle, in opposition to geotropism, which force is conquered by theeffect of the irritation from the attached object. Radicles were thereforeextended horizontally in damp air, kept at the proper low temperature forfull sensitiveness, and squares of card were affixed with shellac on thelower sides of their tips, so that if the squares acted, the terminalgrowing part would curve upwards. Firstly, eight beans were so placed thattheir short, young, horizontally extended radicles would be simultaneouslyacted on both by geotropism and by Sachs' curvature, if the latter cameinto play; and they all eight became bowed downwards to the centre of theearth in 20 h. , excepting one which was only slightly acted on. Two of themwere a little bowed downwards in only 5 h. ! Therefore the cards, affixed tothe lower sides of their tips, seemed to produce no effect; and geotropismeasily conquered the effects of the irritation thus caused. Secondly, 5oldish radicles, 1 ½ inch in length, and therefore less sensitive than theabove-mentioned young ones, were similarly placed and similarly treated. From what has been seen on many other occasions, it may be safely inferredthat if they had been suspended vertically they would have bent away fromthe cards; and if they had been extended horizontally, without cardsattached to them, they would have quickly bent vertically downwards throughgeotropism; but the result was that two of these radicles were stillhorizontal after 23 h. ; two were curved only slightly, and the fifth asmuch as 40o beneath the horizon. Thirdly, 5 beans were fastened[page 153]with their flat surfaces parallel to the cork-lid, so that Sachs' curvaturewould not tend to make the horizontally extended radicles turn eitherupwards or downwards, and little squares of card were affixed as before, tothe lower sides of their tips. The result was that all five radicles werebent down, or towards the centre of the earth, after only 8 h. 20 m. At thesame time and within the same jars, 3 radicles of the same age, withsquares affixed to one side, were suspended vertically; and after 8 h. 20m. They were considerably deflected from the cards, and therefore curvedupwards in opposition to geotropism. In these latter cases the irritationfrom the squares had over-powered geotropism; whilst in the former cases, in which the radicles were extended horizontally, geotropism hadoverpowered the irritation. Thus within the same jars, some of the radicleswere curving upwards and others downwards at the same time--these oppositemovements depending on whether the radicles, when the squares were firstattached to them, projected vertically down, or were extended horizontally. This difference in their behaviour seems at first inexplicable, but can, webelieve, be simply explained by the difference between the initial power ofthe two forces under the above circumstances, combined with the well-knownprinciple of the after-effects of a stimulus. When a young and sensitiveradicle is extended horizontally, with a square attached to the lower sideof the tip, geotropism acts on it at right angles, and, as we have seen, isthen evidently more efficient than the irritation from the square; and thepower of geotropism will be strengthened at each successive period by itsprevious action--that is, by its after-effects. On the other hand, when asquare is affixed to a vertically dependent radicle, and the apex begins to[page 154]curve upwards, this movement will be opposed by geotropism acting only at avery oblique angle, and the irritation from the card will be strengthenedby its previous action. We may therefore conclude that the initial power ofan irritant on the apex of the radicle of the bean, is less than that ofgeotropism when acting at right angles, but greater than that of geotropismwhen acting obliquely on it. 

Sensitiveness of the tips of the Secondary Radicles of the Bean tocontact. --All the previous observations relate to the main or primaryradicle. Some beans suspended to cork-lids, with their radicles dippinginto water, had developed secondary or lateral radicles, which wereafterwards kept in very damp air, at the proper low temperature for fullsensitiveness. They projected, as usual, almost horizontally, with only aslight downward curvature, and retained this position during several days. Sachs has shown* that these secondary roots are acted on in a peculiarmanner by geotropism, so that if displaced they reassume their formersub-horizontal position, and do not bend vertically downwards like theprimary radicle. Minute squares of the stiff sanded paper were affixed bymeans of shellac (but in some instances with thick gum-water) to the tipsof 39 secondary radicles of different ages, generally the uppermost ones. Most of the squares were fixed to the lower sides of the apex, so that ifthey acted the radicle would bend upwards; but some were fixed laterally, and a few on the upper side. Owing to the extreme tenuity of theseradicles, it was very difficult to attach the square to the actual apex. Whether owing to this or some other circumstance, only nine of the squaresinduced any* 'Arbeiten Bot. Inst. , Würzburg, ' Heft iv. 1874, p. 605-617. [page 155]

curvature. The curvature amounted in some cases to about 45o above thehorizon, in others to 90o, and then the tip pointed to the zenith. In oneinstance a distinct upward curvature was observed in 8 h. 15 m. , butusually not until 24 h. Had elapsed. Although only 9 out of 39 radicleswere affected, yet the curvature was so distinct in several of them, thatthere could be no doubt that the tip is sensitive to slight contact, andthat the growing part bends away from the touching object. It is possiblethat some secondary radicles are more sensitive than others; for Sachs hasproved* the interesting fact that each individual secondary radiclepossesses its own peculiar constitution. 

Sensitiveness to contact of the Primary Radicle, a little above the apex, in the Bean (Vicia faba) and Pea (Pisum sativum). --The sensitiveness of theapex of the radicle in the previously described cases, and the consequentcurvature of the upper part from the touching object or other source ofirritation, is the more remarkable, because Sachs** has shown that pressureat the distance of a few millimeters above the apex causes the radicle tobend, like a tendril, towards the touching object. By fixing pins so thatthey pressed against the radicles of beans suspended vertically in dampair, we saw this kind of curvature; but rubbing the part with a twig orneedle for a few minutes produced no effect. Haberlandt remarks, *** thatthese radicles in breaking through the seed-coats often rub and pressagainst the ruptured edges, and consequently bend round them. As littlesquares of the card-like paper affixed with shellac to the tips were highlyefficient in causing the radicles to bend away from them, similar pieces(of about 1/20th

* 'Arbeiten Bot. Instit. , Würzburg, ' Heft, iv. 1874, p. 620. 

** Ibid. Heft iii. 1873, p. 437. 

*** 'Die Schutzeinrichtungen der Keimpflanze, ' 1877, p. 25. [page 156]

inch square, or rather less) were attached in the same manner to one sideof the radicle at a distance of 3 or 4 mm. Above the apex. In our firsttrial on 15 radicles no effect was produced. In a second trial on the samenumber, three became abruptly curved (but only one strongly) towards thecard within 24 h. From these cases we may infer that the pressure from abit of card affixed with shellac to one side above the apex, is hardly asufficient irritant; but that it occasionally causes the radicle to bendlike a tendril towards this side. 

We next tried the effect of rubbing several radicles at a distance of 4 mm. From the apex for a few seconds with lunar caustic (nitrate of silver); andalthough the radicles had been wiped dry and the stick of caustic was dry, yet the part rubbed was much injured and a slight permanent depression wasleft. In such cases the opposite side continues to grow, and the radiclenecessarily becomes bent towards the injured side. But when a point 4 mm. From the apex was momentarily touched with dry caustic, it was only faintlydiscoloured, and no permanent injury was caused. This was shown by severalradicles thus treated straightening themselves after one or two days; yetat first they became curved towards the touched side, as if they had beenthere subjected to slight continued pressure. These cases deserve notice, because when one side of the apex was just touched with caustic, theradicle, as we have seen, curved itself in an opposite direction, that is, away from the touched side. 

The radicle of the common pea at a point a little above the apex is rathermore sensitive to continued pressure than that of the bean, and bendstowards the pressed side. * We experimented on a variety (York-

* Sachs, 'Arbeiten Bot. Institut. , Würzburg, ' Heft iii. P. 438. [page 157]

shire Hero) which has a much wrinkled tough skin, too large for theincluded cotyledons; so that out of 30 peas which had been soaked for 24 h. And allowed to germinate on damp sand, the radicles of three were unable toescape, and were crumpled up in a strange manner within the skin; fourother radicles were abruptly bent round the edges of the ruptured skinagainst which they had pressed. Such abnormalities would probably never, orvery rarely, occur with forms developed in a state of nature and subjectedto natural selection. One of the four radicles just mentioned in doublingbackwards came into contact with the pin by which the pea was fixed to thecork-lid; and now it bent at right angles round the pin, in a directionquite different from that of the first curvature due to contact with theruptured skin; and it thus afforded a good illustration of the tendril-likesensitiveness of the radicle a little above the apex. 

Little squares of the card-like paper were next affixed to radicles of thepea at 4 mm. Above the apex, in the same manner as with the bean. Twenty-eight radicles suspended vertically over water were thus treated ondifferent occasions, and 13 of them became curved towards the cards. Thegreatest degree of curvature amounted to 62o from the perpendicular; but solarge an angle was only once formed. On one occasion a slight curvature wasperceptible after 5 h. 45 m. , and it was generally well-marked after 14 h. There can therefore be no doubt that with the pea, irritation from a bit ofcard attached to one side of the radicle above the apex suffices to inducecurvature. 

Squares of card were attached to one side of the tips of 11 radicles withinthe same jars in which the above trials were made, and five of them becameplainly, and one slightly, curved away from this side. Other[page 158]analogous cases will be immediately described. The fact is here mentionedbecause it was a striking spectacle, showing the difference in thesensitiveness of the radicle in different parts, to behold in the same jarone set of radicles curved away from the squares on their tips, and anotherset curved towards the squares attached a little higher up. Moreover, thekind of curvature in the two cases is different. The squares attached abovethe apex cause the radicle to bend abruptly, the part above and beneathremaining nearly straight; so that here there is little or no transmittedeffect. On the other hand, the squares attached to the apex affect theradicle for a length of from about 4 to even 8 mm. , inducing in most casesa symmetrical curvature; so that here some influence is transmitted fromthe apex for this distance along the radicle. 

Pisum sativum (var. Yorkshire Hero): Sensitiveness of the apex of theRadicle. --Little squares of the same card-like paper were affixed (April24th) with shellac to one side of the apex of 10 vertically suspendedradicles: the temperature of the water in the bottom of the jars was 60o -61o F. Most of these radicles were acted on in 8 h. 30 m. ; and eight ofthem became in the course of 24 h. Conspicuously, and the remaining twoslightly, deflected from the perpendicular and from the side bearing theattached squares. Thus all were acted on; but it will suffice to describetwo conspicuous cases. In one the terminal portion of the radicle was bentat right angles (A, Fig. 66) after 24h. ; and in the other (B) it had bythis time become hooked, with the apex pointing to the zenith. The two bitsof card here used were . 07 inch in length and . 04 inch in breadth. Twoother radicles, which after 8 h. 30 m. Were moderately deflected, becamestraight again after 24h. Another[page 159]trial was made in the same manner with 15 radicles; but from circumstances, not worth explaining, they were only once and briefly examined after theshort

Fig. 66. Pisum sativum: deflection produced within 24 hours in the growthof vertically dependent radicles, by little squares of card affixed withshellac to one side of apex: A, bent at right angles; B, hooked. 

interval of 5 h. 30 m. ; and we merely record in our notes "almost all bentslightly from the perpendicular, and away from the squares; the deflectionamounting in one or two instances to nearly a rectangle. " These two sets ofcases, especially the first one, prove that the apex of the radicle issensitive to slight contact and that the upper part bends from the touchingobject. Nevertheless, on June 1st and 4th, 8 other radicles were tried inthe same manner at a temperature of 58o - 60o F. , and after 24 h. Only 1was decidedly bent from the card, 4 slightly, 2 doubtfully, and 1 not inthe least. The amount of curvature was unaccountably small; but all theradicles which were at all bent, were bent away from the cards. 

We now tried the effects of widely different temperatures on thesensitiveness of these radicles with squares[page 160]of card attached to their tips. Firstly, 13 peas, most of them having veryshort and young radicles, were placed in an ice-box, in which thetemperature rose during three days from 44o to 47o F. They grew slowly, but10 out of the 13 became in the course of the three days very slightlycurved from the squares; the other 3 were not affected; so that thistemperature was too low for any high degree of sensitiveness or for muchmovement. Jars with 13 other radicles were next placed on a chimney-piece, where they were subjected to a temperature of between 68o and 72o F. , andafter 24 h. , 4 were conspicuously curved from the cards, 2 slightly, and 7not at all; so that this temperature was rather too high. Lastly 12radicles were subjected to a temperature varying between 72o and 85o F. , and none of them were in the least affected by the squares. The aboveseveral trials, especially the first recorded one, indicate that the mostfavourable temperature for the sensitiveness of the radicle of the pea isabout 60o F. 

The tips of 6 vertically dependent radicles were touched once with drycaustic, in the manner described under Vicia faba. After 24 h. Four of themwere bent from the side bearing a minute black mark; and the curvatureincreased in one case after 38 h. , and in another case after 48 h. , untilthe terminal part projected almost horizontally. The two remaining radicleswere not affected. 

With radicles of the bean, when extended horizontally in damp air, geotropism always conquered the effects of the irritation caused by squaresof card attached to the lower sides of their tips. A similar experiment wastried on 13 radicles of the pea; the squares being attached with shellac, and the temperature between 58o - 60o F. The result was somewhat different;for[page 161]these radicles are either less strongly acted on by geotropism, or, what ismore probable, are more sensitive to contact. After a time geotropismalways prevailed, but its action was often delayed; and in three instancesthere was a most curious struggle between geotropism and the irritationcaused by the cards. Four of the 13 radicles were a little curved downwardswithin 6 or 8 h. , always reckoning from the time when the squares werefirst attached, and after 23 h. Three of them pointed vertically downwards, and the fourth at an angle of 45o beneath the horizon. These four radiclestherefore did not seem

Fig. 67. Pisum sativum: a radicle extended horizontally in damp air with alittle square of card affixed to the lower side of its tip, causing it tobend upwards in opposition to geotropism. The deflection of the radicleafter 21 hours is shown at A, and of the same radicle after 45 hours at B, now forming a loop. 

to have been at all affected by the attached squares. Four others were notacted on by geotropism within the first 6 or 8 h. , but after 23 h. Weremuch bowed down. Two others remained almost horizontal for 23 h. , butafterwards were acted on. So that in these latter six cases the action ofgeotropism was much delayed. The eleventh radicle was slightly curved downafter 8 h. , but when looked at again after 23 h. The terminal portion wascurved upwards; if it had[page 162]been longer observed, the tip no doubt would have been found again curveddown, and it would have formed a loop as in the following case. The twelfthradicle after 6 h. Was slightly curved downwards; but when looked at againafter 21 h. , this curvature had disappeared and the apex pointed upwards;after 30 h. The radicle formed a hook, as shown at A (Fig. 67); which hookafter 45 h. Was converted into a loop (B). The thirteenth radicle after 6h. Was slightly curved downwards, but within 21 h. Had curved considerablyup, and then down again at an angle of 45o beneath the horizon, afterwardsbecoming perpendicular. In these three last cases geotropism and theirritation caused by the attached squares alternately prevailed in a highlyremarkable manner; geotropism being ultimately victorious. 

Similar experiments were not always quite so successful as in the abovecases. Thus 6 radicles, horizontally extended with attached squares, weretried on June 8th at a proper temperature, and after 7 h. 30 m. None werein the least curved upwards and none were distinctly geotropic; whereas of6 radicles without any attached squares, which served as standards ofcomparison or controls, 3 became slightly and 3 almost rectangularlygeotropic within the 7 h. 30 m. ; but after 23 h. The two lots were equallygeotropic. On July 10th another trial was made with 6 horizontally extendedradicles, with squares attached in the same manner beneath their tips; andafter 7 h. 30 m. , 4 were slightly geotropic, 1 remained horizontal, and 1was curved upwards in opposition to gravity or geotropism. This latterradicle after 48 h. Formed a loop, like that at B (Fig. 67). 

An analogous trial was now made, but instead of attaching squares of cardto the lower sides of the[page 163]tips, these were touched with dry caustic. The details of the experimentwill be given in the chapter on Geotropism, and it will suffice here to saythat 10 peas, with radicles extended horizontally and not cauterised, werelaid on and under damp friable peat; these, which served as standards orcontrols, as well as 10 others which had been touched on the upper sidewith the caustic, all became strongly geotropic in 24 h. Nine radicles, similarly placed, had their tips touched on the lower side with thecaustic; and after 24 h. , 3 were slightly geotropic, 2 remained horizontal, and 4 were bowed upwards in opposition to gravity and to geotropism. Thisupward curvature was distinctly visible in 8 h. 45m. After the lower sidesof the tips had been cauterised. 

Little squares of card were affixed with shellac on two occasions to thetips of 22 young and short secondary radicles, which had been emitted fromthe primary radicle whilst growing in water, but were now suspended in dampair. Besides the difficulty of attaching the squares to such finely pointedobjects as were these radicles, the temperature was too high, --varying onthe first occasion from 72o to 77o F. , and on the second being almoststeadily 78o F. ; and this probably lessened the sensitiveness of the tips. The result was that after an interval of 8 h. 30 m. , 6 of the 22 radicleswere bowed upwards (one of them greatly) in opposition to gravity, and 2laterally; the remaining 14 were not affected. Considering the unfavourablecircumstances, and bearing in mind the case of the bean, the evidenceappears sufficient to show that the tips of the secondary radicles of thepea are sensitive to slight contact. 

Phaseolus multiflorus: Sensitiveness of the apex of the Radicle. --Fifty-nine radicles were tried with squares[page 164]of various sizes of the same card-like paper, also with bits of thin glassand rough cinders, affixed with shellac to one side of the apex. Ratherlarge drops of the dissolved shellac were also placed on them and allowedto set into hard beads. The specimens were subjected to varioustemperatures between 60o and 72o F. , more commonly at about the latter. Butout of this considerable number of trials only 5 radicles were plainlybent, and 8 others slightly or even doubtfully, from the attached objects;the remaining 46 not being at all affected. It is therefore clear that thetips of the radicles of this Phaseolus are much less sensitive to contactthan are those of the bean or pea. We thought that they might be sensitiveto harder pressure, but after several trials we could not devise any methodfor pressing harder on one side of the apex than on the other, without atthe same time offering mechanical resistance to its growth. We thereforetried other irritants. 

The tips of 13 radicles, dried with blotting-paper, were thrice touched orjust rubbed on one side with dry nitrate of silver. They were rubbedthrice, because we supposed from the foregoing trials, that the tips werenot highly sensitive. After 24 h. The tips were found greatly blackened; 6were blackened equally all round, so that no curvature to any one sidecould be expected; 6 were much blackened on one side for a length of about1/10th of an inch, and this length became curved at right angles towardsthe blackened surface, the curvature afterwards increasing in severalinstances until little hooks were formed. It was manifest that theblackened side was so much injured that it could not grow, whilst theopposite side continued to grow. One alone out of these 13 radicles becamecurved from the blackened side, the[page 165]curvature extending for some little distance above the apex. 

After the experience thus gained, the tips of six almost dry radicles wereonce touched with the dry caustic on one side; and after an interval of 10m. Were allowed to enter water, which was kept at a temperature of 65o -67o F. The result was that after an interval of 8 h. A minute blackishspeck could just be distinguished on one side of the apex of five of theseradicles, all of which became curved towards the opposite side--in twocases at about an angle of 45o--in two other cases at nearly a rectangle--and in the fifth case at above a rectangle, so that the apex was a littlehooked; in this latter case the black mark was rather larger than in theothers. After 24 h. From the application of the caustic, the curvature ofthree of these radicles (including the hooked one) had diminished; in thefourth it remained the same, and in the fifth it had increased, the tipbeing now hooked. It has been said that after 8 h. Black specks could beseen on one side of the apex of five of the six radicles; on the sixth thespeck, which was extremely minute, was on the actual apex and thereforecentral; and this radicle alone did not become curved. It was thereforeagain touched on one side with caustic, and after 15 h. 30 m. Was foundcurved from the perpendicular and from the blackened side at an angle of34o, which increased in nine additional hours to 54o. 

It is therefore certain that the apex of the radicle of this Phaseolus isextremely sensitive to caustic, more so than that of the bean, though thelatter is far more sensitive to pressure. In the experiments just given, the curvature from the slightly cauterised side of the tip, extended alongthe radicle for a length of nearly 10 mm. ; whereas in the first set[page 166]of experiments, when the tips of several were greatly blackened and injuredon one side, so that their growth was arrested, a length of less than 3 mm. Became curved towards the much blackened side, owing to the continuedgrowth of the opposite side. This difference in the results is interesting, for it shows that too strong an irritant does not induce any transmittedeffect, and does not cause the adjoining, upper and growing part of theradicle to bend. We have analogous cases with Drosera, for a strongsolution of carbonate of ammonia when absorbed by the glands, or too greatheat suddenly applied to them, or crushing them, does not cause the basalpart of the tentacles to bend, whilst a weak solution of the carbonate, ora moderate heat, or slight pressure always induced such bending. Similarresults were observed with Dionaea and Pinguicula. 

The effect of cutting off with a razor a thin slice from one side of theconical apex of 14 young and short radicles was next tried. Six of themafter being operated on were suspended in damp air; the tips of the othereight, similarly suspended, were allowed to enter water at a temperature ofabout 65o F. It was recorded in each case which side of the apex had beensliced off, and when they were afterwards examined the direction of thecurvature was noted, before the record was consulted. Of the six radiclesin damp air, three had their tips curved after an interval of 10 h. 15 m. Directly away from the sliced surface, whilst the other three were notaffected and remained straight; nevertheless, one of them after 13additional hours became slightly curved from the sliced surface. Of theeight radicles with their tips immersed in water, seven were plainly curvedaway from the sliced surfaces after 10 h. 15 m. ; and with[page 167]respect to the eighth which remained quite straight, too thick a slice hadbeen accidentally removed, so that it hardly formed a real exception to thegeneral result. When the seven radicles were looked at again, after aninterval of 23 h. From the time of slicing, two had become distorted; fourwere deflected at an angle of about 70o from the perpendicular and from thecut surface; and one was deflected at nearly 90o, so that it projectedalmost horizontally, but with the extreme tip now beginning to benddownwards through the action of geotropism. It is therefore manifest that athin slice cut off one side of the conical apex, causes the upper growingpart of the radicle of this Phaseolus to bend, through the transmittedeffects of the irritation, away from the sliced surface. 

Tropaeolum majus: Sensitiveness of the apex of the Radicle to contact. --Little squares of card were attached with shellac to one side of the tipsof 19 radicles, some of which were subjected to 78o F. , and others to amuch lower temperature. Only 3 became plainly curved from the squares, 5slightly, 4 doubtfully, and 7 not at all. These seeds were, as we believed, old, so we procured a fresh lot, and now the results were widely different. Twenty-three were tried in the same manner; five of the squares produced noeffect, but three of these cases were no real exceptions, for in two ofthem the squares had slipped and were parallel to the apex, and in thethird the shellac was in excess and had spread equally all round the apex. One radicle was deflected only slightly from the perpendicular and from thecard; whilst seventeen were plainly deflected. The angles in several ofthese latter cases varied between 40o and 65o from the perpendicular; andin two of them it amounted after 15 h. Or 16 h. To about 90o. In oneinstance a loop[page 168]was nearly completed in 16 h. There can, therefore, be no doubt that theapex is highly sensitive to slight contact, and that the upper part of theradicle bends away from the touching object. 

Gossypium herbaceum: Sensitiveness of the apex of the Radicle. --Radicleswere experimented on in the same manner as before, but they provedill-fitted for our purpose, as they soon became unhealthy when suspended indamp air. Of 38 radicles thus suspended, at temperatures varying from 66oto 69o F. , with squares of card attached to their tips, 9 were plainly and7 slightly or even doubtfully deflected from the squares and from theperpendicular; 22 not being affected. We thought that perhaps the abovetemperature was not high enough, so 19 radicles with attached squares, likewise suspended in damp air, were subjected to a temperature of from 74oto 79o F. , but not one of them was acted on, and they soon becameunhealthy. Lastly, 19 radicles were suspended in water at a temperaturefrom 70o to 75o F. , with bits of glass or squares of the card attached totheir tips by means of Canada-balsam or asphalte, which adhered ratherbetter than shellac beneath the water. The radicles did not keep healthyfor long. The result was that 6 were plainly and 2 doubtfully deflectedfrom the attached objects and the perpendicular; 11 not being affected. Theevidence consequently is hardly conclusive, though from the two sets ofcases tried under a moderate temperature, it is probable that the radiclesare sensitive to contact; and would be more so under favourable conditions. 

Fifteen radicles which had germinated in friable peat were suspendedvertically over water. Seven of them served as controls, and they remainedquite straight during 24 h. The tips of the other eight radicles[page 169]were just touched with dry caustic on one side. After only 5 h. 10 m. Fiveof them were slightly curved from the perpendicular and from the sidebearing the little blackish marks. After 8 h. 40 m. , 4 out of these 5 weredeflected at angles between 15o and 65o from the perpendicular. On theother hand, one which had been slightly curved after 5 h. 10 m. , now becamestraight. After 24 h. The curvature in two cases had considerablyincreased; also in four other cases, but these latter radicles had nowbecome so contorted, some being turned upwards, that it could no longer beascertained whether they were still curved from the cauterised side. Thecontrol specimens exhibited no such irregular growth, and the two setspresented a striking contrast. Out of the 8 radicles which had been touchedwith caustic, two alone were not affected, and the marks left on their tipsby the caustic were extremely minute. These marks in all cases were oval orelongated; they were measured in three instances, and found to be of nearlythe same size, viz. 2/3 of a mm. In length. Bearing this fact in mind, itshould be observed that the length of the curved part of the radicle, whichhad become deflected from the cauterised side in the course of 8 h. 40 m. Was found to be in three cases 6, 7, and 9 mm. 

Cucurbita ovifera: Sensitiveness of the apex of the Radicle. --The tipsproved ill-fitted for the attachment of cards, as they are extremely fineand flexible. Moreover, owing to the hypocotyls being soon developed andbecoming arched, the whole radicle is quickly displaced and confusion isthus caused. A large number of trials were made, but without any definiteresult, excepting on two occasions, when out of 23 radicles 10 weredeflected from the attached squares[page 170]of card, and 13 were not acted on. Rather large squares, though difficultto affix, seemed more efficient than very small ones. 

We were much more successful with caustic; but in our first trial, 15radicles were too much cauterised, and only two became curved from theblackened side; the others being either killed on one side, or blackenedequally all round. In our next trial the dried tips of 11 radicles weretouched momentarily with dry caustic, and after a few minutes were immersedin water. The elongated marks thus caused were never black, only brown, andabout ½ mm. In length, or even less. In 4 h. 30 m. After the cauterisation, 6 of them were plainly curved from the side with the brown mark, 4slightly, and 1 not at all. The latter proved unhealthy, and never grew;and the marks on 2 of the 4 slightly curved radicles were excessivelyminute, one being distinguishable only with the aid of a lens. Of 10control specimens tried in the same jars at the same time, not one was inthe least curved. In 8 h. 40 m. After the cauterisation, 5 of the radiclesout of the 10 (the one unhealthy one being omitted) were deflected at about90o, and 3 at about 45o from the perpendicular and from the side bearingthe brown mark. After 24 h. All 10 radicles had increased immensely inlength; in 5 of them the curvature was nearly the same, in 2 it hadincreased, and in 3 it had decreased. The contrast presented by the 10controls, after both the 8 h. 40 m. And the 24 h. Intervals, was verygreat; for they had continued to grow vertically downwards, excepting twowhich, from some unknown cause, had become somewhat tortuous. 

In the chapter on Geotropism we shall see that 10 radicles of this plantwere extended horizontally on and beneath damp friable peat, under whichconditions[page 171]they grow better and more naturally than in damp air; and their tips wereslightly cauterised on the lower side, brown marks about ½ mm. In lengthbeing thus caused. Uncauterised specimens similarly placed became much bentdownwards through geotropism in the course of 5 or 6 hours. After 8 h. Only3 of the cauterised ones were bowed downwards, and this in a slight degree;4 remained horizontal; and 3 were curved upwards in opposition togeotropism and from the side bearing the brown mark. Ten other specimenshad their tips cauterised at the same time and in the same degree, on theupper side; and this, if it produced any effect, would tend to increase thepower of geotropism; and all these radicles were strongly bowed downwardsafter 8 h. From the several foregoing facts, there can be no doubt that thecauterisation of the tip of the radicle of this Cucurbita on one side, ifdone lightly enough, causes the whole growing part to bend to the oppositeside. Raphanus sativus: Sensitiveness of the apex of the Radicle. --We hereencountered many difficulties in our trials, both with squares of card andwith caustic; for when seeds were pinned to a cork-lid, many of theradicles, to which nothing had been done, grew irregularly, often curvingupwards, as if attracted by the damp surface above; and when they wereimmersed in water they likewise often grew irregularly. We did nottherefore dare to trust our experiments with attached squares of card;nevertheless some of them seemed to indicate that the tips were sensitiveto contact. Our trials with caustic generally failed from the difficulty ofnot injuring too greatly the extremely fine tips. Out of 7 radicles thustried, one became bowed after 22 h. At an angle of 60o, a second at 40o, [page 172]and a third very slightly from the perpendicular and from the cauterisedside. 

Aesculus hippocastanum: Sensitiveness of the apex of the Radicle. --Bits ofglass and squares of card were affixed with shellac or gum-water to thetips of 12 radicles of the horse-chestnut; and when these objects fell off, they were refixed; but not in a single instance was any curvature thuscaused. These massive radicles, one of which was above 2 inches in lengthand . 3 inch in diameter at its base, seemed insensible to so slight astimulus as any small attached object. Nevertheless, when the apexencountered an obstacle in its downward course, the growing part became souniformly and symmetrically curved, that its appearance indicated not meremechanical bending, but increased growth along the whole convex side, dueto the irritation of the apex. 

That this is the correct view may be inferred from the effects of the morepowerful stimulus of caustic. The bending from the cauterised side occurredmuch slower than in the previously described species, and it will perhapsbe worth while to give our trials in detail. 

[The seeds germinated in sawdust, and one side of the tips of the radicleswere slightly rubbed once with dry nitrate of silver; and after a fewminutes were allowed to dip into water. They were subjected to a rathervarying temperature, generally between 52o and 58o F. A few cases have notbeen thought worth recording, in which the whole tip was blackened, or inwhich the seedling soon became unhealthy. 

(1. ) The radicle was slightly deflected from the cauterised side in one day(i. E. 24 h. ); in three days it stood at 60o from the perpendicular; in fourdays at 90o; on the fifth day it was curved up about 40o above the horizon;so that it had passed through an angle of 130o in the five days, and thiswas the greatest amount of curvature observed. 

(2. ) In two days radicle slightly deflected; after seven days[page 173]deflected 69o from the perpendicular and from the cauterised side; aftereight days the angle amounted to nearly 90o. 

(3. ) After one day slight deflection, but the cauterised mark was so faintthat the same side was again touched with caustic. In four days from thefirst touch deflection amounted to 78o, which in an additional dayincreased to 90o. 

(4. ) After two days slight deflection, which during the next three dayscertainly increased but never became great; the radicle did not grow welland died on the eighth day. 

(5. ) After two days very slight deflection; but this on the fourth dayamounted to 56o from the perpendicular and from the cauterised side. 

(6. ) After three days doubtfully, but after four days certainly deflectedfrom the cauterised side. On the fifth day deflection amounted to 45o fromthe perpendicular, and this on the seventh day increased to about 90o. 

(7. ) After two days slightly deflected; on the third day the deflectionamounted to 25o from the perpendicular, and this did not afterwardsincrease. 

(8. ) After one day deflection distinct; on the third day it amounted to44o, and on the fourth day to 72o from the perpendicular and the cauterisedside. 

(9. ) After two days deflection slight, yet distinct; on the third day thetip was again touched on the same side with caustic and thus killed. 

(10. ) After one day slight deflection, which after six days increased to50o from the perpendicular and the cauterised side. 

(11. ) After one day decided deflection, which after six days increased to62o from the perpendicular and from the cauterised side. 

(12. ) After one day slight deflection, which on the second day amounted to35o, on the fourth day to 50o, and the sixth day to 63o from theperpendicular and the cauterised side. 

(13. ) Whole tip blackened, but more on one side than the other; on thefourth day slightly, and on the sixth day greatly deflected from the moreblackened side; the deflection on the ninth day amounted to 90o from theperpendicular. 

(14. ) Whole tip blackened in the same manner as in the last case: on thesecond day decided deflection from the more blackened side, which increasedon the seventh day to nearly 90o; on the following day the radicle appearedunhealthy. 

(15. ) Here we had the anomalous case of a radicle bending[page 174]slightly towards the cauterised side on the first day, and continuing to doso for the next three days, when the deflection amounted to about 90o fromthe perpendicular. The cause appeared to lie in the tendril-likesensitiveness of the upper part of the radicle, against which the point ofa large triangular flap of the seed-coats pressed with considerable force;and this irritation apparently conquered that from the cauterised apex. ]

These several cases show beyond doubt that the irritation of one side ofthe apex, excites the upper part of the radicle to bend slowly towards theopposite side. This fact was well exhibited in one lot of five seeds pinnedto the cork-lid of a jar; for when after 6 days the lid was turned upsidedown and viewed from directly above, the little black marks made by thecaustic were now all distinctly visible on the upper sides of the tips ofthe laterally bowed radicles. A thin slice was shaved off with a razor fromone side of the tips of 22 radicles, in the manner described under thecommon bean; but this kind of irritation did not prove very effective. Only7 out of the 22 radicles became moderately deflected in from 3 to 5 daysfrom the sliced surface, and several of the others grew irregularly. Theevidence, therefore, is far from conclusive. 

Quercus robur: Sensitiveness of the apex of the Radicle. --The tips of theradicles of the common oak are fully as sensitive to slight contact as arethose of any plant examined by us. They remained healthy in damp air for 10days, but grew slowly. Squares of the card-like paper were fixed withshellac to the tips of 15 radicles, and ten of these became conspicuouslybowed from the perpendicular and from the squares; two slightly, and threenot at all. But two of the latter were not real exceptions, as they were atfirst very short, and hardly grew afterwards. Some of the more[page 175]remarkable cases are worth describing. The radicles were examined on eachsuccessive morning, at nearly the same hour, that is, after intervals of 24h. 

[No. 1. This radicle suffered from a series of accidents, and acted in ananomalous manner, for the apex appeared at first insensible and afterwardssensitive to contact. The first square was attached on Oct 19th; on the21st the radicle was not at all curved, and the square was accidentallyknocked off; it was refixed on the 22nd, and the radicle became slightlycurved from the square, but the curvature disappeared on the 23rd, when thesquare was removed and refixed. No curvature ensued, and the square wasagain accidentally knocked off, and refixed. On the morning of the 27th itwas washed off by having reached the water in the bottom of the jar. Thesquare was refixed, and on the 29th, that is, ten days after the firstsquare had been attached, and two days after the attachment of the lastsquare, the radicle had grown to the great length of 3. 2 inches, and nowthe terminal growing part had become bent away from the square into a hook(see Fig. 68). 

Fig. 68. Quercus robur: radicle with square of card attached to one side ofapex, causing it to become hooked. Drawing one-half natural scale. 

No. 2. Square attached on the 19th; on the 20th radicle slightly deflectedfrom it and from the perpendicular; on the 21st deflected at nearly rightangles; it remained during the next two days in this position, but on the25th the upward curvature was lessened through the action of geotropism, and still more so on the 26th. 

No. 3. Square attached on the 19th; on the 21st a trace of curvature fromthe square, which amounted on the 22nd to about 40o, and on the 23rd to 53ofrom the perpendicular. 

No. 4. Square attached on the 21st; on the 22nd trace of curvature from thesquare; on the 23rd completely hooked with the point turned up to thezenith. Three days afterwards (i. E. 26th) the curvature had whollydisappeared and the apex pointed perpendicularly downwards. 

No. 5. Square attached on the 21st; on the 22nd decided[page 176]though slight curvature from the square; on the 23rd the tip had curved upabove the horizon, and on the 24th was hooked with the apex pointing almostto the zenith, as in Fig. 68. 

No. 6. Square attached on the 21st; on the 22nd slightly curved from thesquare; 23rd more curved; 25th considerably curved; 27th all curvaturelost, and the radicle was now directed perpendicularly downwards. 

No. 7. Square attached on the 21st; on the 22nd a trace of curvature fromthe square, which increased next day, and on the 24th amounted to a rightangle. 

It is, therefore, manifest that the apex of the radicle of the oak ishighly sensitive to contact, and retains its sensitiveness during severaldays. The movement thus induced was, however, slower than in any of theprevious cases, with the exception of that of Aesculus. As with the bean, the terminal growing part, after bending, sometimes straightened itselfthrough the action of geotropism, although the object still remainedattached to the tip. 

The same remarkable experiment was next tried, as in the case of the bean;namely, little squares of exactly the same size of the card-like sandedpaper and of very thin paper (the thicknesses of which have been givenunder Vicia faba) were attached with shellac on opposite sides (asaccurately as could be done) of the tips of 13 radicles, suspended in dampair, at a temperature of 65o - 66o F. The result was striking, for 9 out ofthese 13 radicles became plainly, and 1 very slightly, curved from thethick paper towards the side bearing the thin paper. In two of these casesthe apex became completely hooked after two days; in four cases thedeflection from the perpendicular and from the side bearing the thickpaper, amounted in from two to four days to angles of 90o, 72o, 60o, and49o, but in two other cases to only 18o and 15o. It should, however, bestated that in the[page 177]case in which the deflection was 49o, the two squares had accidentally comeinto contact on one side of the apex, and thus formed a lateral gable; andthe deflection was directed in part from this gable and in part from thethick paper. In three cases alone the radicles were not affected by thedifference in thickness of the squares of paper attached to their tips, andconsequently did not bend away from the side bearing the stiffer paper. 

Zea mays: Sensitiveness of the apex of the Radicle to contact. --A largenumber of trials were made on this plant, as it was the only monocotyledonon which we experimented. An abstract of the results will suffice. In thefirst place, 22 germinating seeds were pinned to cork-lids without anyobject being attached to their radicles, some being exposed to atemperature of 65o - 66o F. , and others to between 74o and 79o; and none ofthem became curved, though some were a little inclined to one side. A fewwere selected, which from having germinated on sand were crooked, but whensuspended in damp air the terminal part grew straight downwards. This facthaving been ascertained, little squares of the card-like paper were affixedwith shellac, on several occasions, to the tips of 68 radicles. Of thesethe terminal growing part of 39 became within 24 h. Conspicuously curvedaway from the attached squares and from the perpendicular; 13 out of the 39forming hooks with their points directed towards the zenith, and 8 formingloops. Moreover, 7 other radicles out of the 68, were slightly and twodoubtfully deflected from the cards. There remain 20 which were notaffected; but 10 of these ought not to be counted; for one was diseased, two had their tips quite surrounded by shellac, and the squares on 7 hadslipped so as to stand parallel to the apex, instead of obliquely[page 178]on it. There were therefore only 10 out of the 68 which certainly were notacted on. Some of the radicles which were experimented on were young andshort, most of them of moderate length, and two or three exceeded threeinches in length. The curvature in the above cases occurred within 24 h. , but it was often conspicuous within a much shorter period. For instance, the terminal growing part of one radicle was bent upwards into a rectanglein 8 h. 15 m. , and of another in 9 h. On one occasion a hook was formed in9 h. Six of the radicles in a jar containing nine seeds, which stood on asand-bath, raised to a temperature varying from 76o to 82o F. , becamehooked, and a seventh formed a complete loop, when first looked at after 15hours. 

The accompanying figures of four germinating seeds (Fig. 69) show, firstly, a radicle (A) the apex of which has become so much bent away from theattached square as to form a hook. Secondly (B), a hook converted throughthe continued irritation of the card, aided perhaps by geotropism, into analmost complete circle or loop. The tip in the act of forming a loopgenerally rubs against the upper part of the radicle, and pushes off theattached square; the loop then contracts or closes, but never disappears;and the apex afterwards grows vertically downwards, being no longerirritated by any attached object. This frequently occurred, and isrepresented at C. The jar above mentioned with the six hooked radicles andanother jar were kept for two additional days, for the sake of observinghow the hooks would be modified. Most of them became converted into simpleloops, like that figured at C; but in one case the apex did not rub againstthe upper part of the radicle and thus remove the card; and it consequentlymade, owing[page 179]to the continued irritation from the card, two complete loops, that is, ahelix of two spires; which afterwards became pressed closely together. Thengeotropism prevailed and caused the apex to grow perpendicularly downwards. In another case, shown at (D), the apex

Fig. 69. Zea mays: radicles excited to bend away from the little squares ofcard attached to one side of their tips. 

in making a second turn or spire, passed through the first loop, which wasat first widely open, and in doing so knocked off the card; it then grewperpendicularly downwards, and thus tied itself into a knot, which soonbecame tight!

Secondary Radicles of Zea. --A short time after the first radicle hasappeared, others protrude from the[page 180]seed, but not laterally from the primary one. Ten of these secondaryradicles, which were directed obliquely downwards, were experimented onwith very small squares of card attached with shellac to the lower sides oftheir tips. If therefore the squares acted, the radicles would bend upwardsin opposition to gravity. The jar stood (protected from light) on asand-bath, which varied between 76o and 82o F. After only 5 h. One appearedto be a little deflected from the square, and after 20 h. Formed a loop. Four others were considerably curved from the squares after 20 h. , andthree of them became hooked, with their tips pointing to the zenith, --oneafter 29 h. And the two others after 44 h. By this latter time a sixthradicle had become bent at a right angle from the side bearing the square. Thus altogether six out of the ten secondary radicles were acted on, fournot being affected. There can, therefore, be no doubt that the tips ofthese secondary radicles are sensitive to slight contact, and that whenthus excited they cause the upper part to bend from the touching object;but generally, as it appears, not in so short a time as in the case of thefirst-formed radicle. 

SENSITIVENESS OF THE TIP OF THE RADICLE TO MOIST AIR. 

Sachs made the interesting discovery, a few years ago, that the radicles ofmany seedling plants bend towards an adjoining damp surface. * We shall hereendeavour to show that this peculiar form of sensitiveness resides in theirtips. The movement is directly the reverse of that excited by the irritantshitherto considered, which cause the growing part of the

* 'Arbeiten des Bot. Institut. , in Würzburg, ' vol. I. 1872, p. 209. [page 181]

radicle to bend away from the source of irritation. In our experiments wefollowed Sachs' plan, and sieves with seeds germinating in damp sawdustwere suspended so that the bottom was generally inclined at 40o with thehorizon. If the radicles had been acted on solely by geotropism, they wouldhave grown out of the bottom of the sieve perpendicularly downwards; but asthey were attracted by the adjoining damp surface they bent towards it andwere deflected 50o from the perpendicular. For the sake of ascertainingwhether the tip or the whole growing part of the radicle was sensitive tothe moist air, a length of from 1 to 2 mm. Was coated in a certain numberof cases with a mixture of olive-oil and lamp-black. This mixture was madein order to give consistence to the oil, so that a thick layer could beapplied, which would exclude, at least to a large extent, the moist air, and would be easily visible. A greater number of experiments than thosewhich were actually tried would have been necessary, had not it beenclearly established that the tip of the radicle is the part which issensitive to various other irritants. 

[Phaseolus multiflorus. --Twenty-nine radicles, to which nothing had beendone, growing out of a sieve, were observed at the same time with thosewhich had their tips greased, and for an equal length of time. Of the 29, 24 curved themselves so as to come into close contact with the bottom ofthe sieve. The place of chief curvature was generally at a distance of 5 or6 mm. From the apex. Eight radicles had their tips greased for a length of2 mm. , and two others for a length of 1 ½ mm. ; they were kept at atemperature of 15o - 16o C. After intervals of from 19 h. To 24 h. All werestill vertically or almost vertically dependent, for some of them had movedtowards the adjoining damp surface by about 10o. They had therefore notbeen acted on, or only slightly acted on, by the damper air on one side, although the whole upper part was freely exposed. After 48 h. Three ofthese radicles became[page 182]considerably curved towards the sieve; and the absence of curvature in someof the others might perhaps be accounted for by their not having grown verywell. But it should be observed that during the first 19 h. To 24 h. Allgrew well; two of them having increased 2 and 3 mm. In length in 11 h. ;five others increased 5 to 8 mm. In 19 h. ; and two, which had been at first4 and 6 mm. In length, increased in 24 h. To 15 and 20 mm. 

The tips of 10 radicles, which likewise grew well, were coated with thegrease for a length of only 1 mm. , and now the result was somewhatdifferent; for of these 4 curved themselves to the sieve in from 21 h. To24h. , whilst 6 did not do so. Five of the latter were observed for anadditional day, and now all excepting one became curved to the sieve. 

The tips of 5 radicles were cauterised with nitrate of silver, and about 1mm. In length was thus destroyed. They were observed for periods varyingbetween 11 h. And 24h. , and were found to have grown well. One of them hadcurved until it came into contact with the sieve; another was curvingtowards it; whilst the remaining three were still vertically dependent. Of7 not cauterised radicles observed at the same time, all had come intocontact with the sieve. 

The tips of 11 radicles were protected by moistened gold-beaters' skin, which adheres closely, for a length varying from 1 ½ to 2 ½ mm. After 22 h. To 24 h. , 6 of these radicles were clearly bent towards or had come intocontact with the sieve; 2 were slightly curved in this direction, and 3 notat all. All had grown well. Of 14 control specimens observed at the sametime, all excepting one had closely approached the sieve. It appears fromthese cases that a cap of goldbeaters' skin checks, though only to a slightdegree, the bending of the radicles to an adjoining damp surface. Whetheran extremely thin sheet of this substance when moistened allows moisturefrom the air to pass through it, we do not know. One case indicated thatthe caps were sometimes more efficient than appears from the above results;for a radicle, which after 23 h. Had only slightly approached the sieve, had its cap (1 ½ mm. In length) removed, and during the next 15 ½ h. Itcurved itself abruptly towards the source of moisture, the chief seat ofcurvature being at a distance of 2 to 3 mm. From the apex. 

Vicia faba. --The tips of 13 radicles were coated with the grease for alength of 2 mm. ; and it should be remembered that with these radicles theseat of chief curvature is about[page 183]4 or 5 mm. From the apex. Four of them were examined after 22h. , threeafter 26 h. , and six after 36 h. , and none had been attracted towards thedamp lower surface of the sieve. In another trial 7 radicles were similarlytreated, and 5 of them still pointed perpendicularly downwards after 11 h. , whilst 2 were a little curved towards the sieve; by an accident they werenot subsequently observed. In both these trials the radicles grew well; 7of them, which were at first from 4 to 11 mm. In length, were after 11 h. Between 7 and 16 mm. ; 3 which were at first from 6 to 8 mm. After 26 h. Were 11. 5 to 18 mm. In length; and lastly, 4 radicles which were at first 5to 8 mm. After 46 h. Were 18 to 23 mm. In length. The control or ungreasedradicles were not invariably attracted towards the bottom of the sieve. Buton one occasion 12 out of 13, which were observed for periods between 22 h. And 36 h. , were thus attracted. On two other occasions taken together, 38out of 40 were similarly attracted. On another occasion only 7 out of 14behaved in this manner, but after two more days the proportion of thecurved increased to 17 out of 23. On a last occasion only 11 out of 20 werethus attracted. If we add up these numbers, we find that 78 out of 96 ofthe control specimens curved themselves towards the bottom of the sieve. Ofthe specimens with greased tips, 2 alone out of the 20 (but 7 of these werenot observed for a sufficiently long time) thus curved themselves. We can, therefore, hardly doubt that the tip for a length of 2 mm. Is the partwhich is sensitive to a moist atmosphere, and causes the upper part to bendtowards its source. 

The tips of 15 radicles were cauterised with nitrate of silver, and theygrew as well as those above described with greased tips. After an intervalof 24 h. , 9 of them were not at all curved towards the bottom of the sieve;2 were curved towards it at angles of 20o and 12o from their formervertical position, and 4 had come into close contact with it. Thus thedestruction of the tip for a length of about 1 mm. Prevented the curvatureof the greater number of these radicles to the adjoining damp surface. Of24 control specimens, 23 were bent to the sieve, and on a second occasion15 out of 16 were similarly curved in a greater or less degree. Thesecontrol trials are included in those given in the foregoing paragraph. 

Avena sativa. --The tips of 13 radicles, which projected between 2 and 4 mm. From the bottom of the sieve, many of[page 184]them not quite perpendicularly downwards, were coated with the black greasefor a length of from 1 to 1 ½ mm. The sieves were inclined at 30o with thehorizon. The greater number of these radicles were examined after 22 h. , and a few after 25 h. , and within these intervals they had grown so quicklyas to have nearly doubled their lengths. With the ungreased radicles thechief seat of curvature is at a distance of not less than between 3. 5 and5. 5 mm. , and not more than between 7 and 10 mm. From the apex. Out of the13 radicles with greased tips, 4 had not moved at all towards the sieve; 6were deflected towards it and from the perpendicular by angles varyingbetween 10o and 35o; and 3 had come into close contact with it. It appears, therefore, at first sight that greasing the tips of these radicles hadchecked but little their bending to the adjoining damp surface. But theinspection of the sieves on two occasions produced a widely differentimpression on the mind; for it was impossible to behold the radicles withthe black greased tips projecting from the bottom, and all those withungreased tips, at least 40 to 50 in number, clinging closely to it, andfeel any doubt that the greasing had produced a great effect. On closeexamination only a single ungreased radicle could be found which had notbecome curved towards the sieve. It is probable that if the tips had beenprotected by grease for a length of 2 mm. Instead of from 1 to 1 ½ mm. , they would not have been affected by the moist air and none would havebecome curved. 

Triticum vulgare. --Analogous trials were made on 8 radicles of the commonwheat; and greasing their tips produced much less effect than in the caseof the oats. After 22 h. , 5 of them had come into contact with the bottomof the sieve; 2 had moved towards it 10o and 15o, and one alone remainedperpendicular. Not one of the very numerous ungreased radicles failed tocome into close contact with the sieve. These trials were made on Nov. 28th, when the temperature was only 4. 8o C. At 10 A. M. We should hardlyhave thought this case worth notice, had it not been for the followingcircumstance. In the beginning of October, when the temperature wasconsiderably higher, viz. , 12o to 13o C. , we found that only a few of theungreased radicles became bent towards the sieve; and this indicates thatsensitiveness to moisture in the air is increased by a low temperature, aswe have seen with the radicles of Vicia faba relatively to objects attachedto their tips. But in the present instance it is possible that a differencein the dryness[page 185]of the air may have caused the difference in the results at the twoperiods. ]

Finally, the facts just given with respect to Phaseolus multiflorus, Viciafaba, and Avena sativa show, as it seems to us, that a layer of greasespread for a length of 1 ½ to 2 mm. Over the tip of the radicle, or thedestruction of the tip by caustic, greatly lessens or quite annuls in theupper and exposed part the power of bending towards a neighbouring sourceof moisture. We should bear in mind that the part which bends most, lies atsome little distance above the greased or cauterised tip; and that therapid growth of this part, proves that it has not been injured by the tipshaving been thus treated. In those cases in which the radicles with greasedtips became curved, it is possible that the layer of grease was notsufficiently thick wholly to exclude moisture, or that a sufficient lengthwas not thus protected, or, in the case of the caustic, not destroyed. Whenradicles with greased tips are left to grow for several days in damp air, the grease is drawn out into the finest reticulated threads and dots, withnarrow portions of the surface left clean. Such portions would, it isprobable, be able to absorb moisture, and thus we can account for severalof the radicles with greased tips having become curved towards the sieveafter an interval of one or two days. On the whole, we may infer thatsensitiveness to a difference in the amount of moisture in the air on thetwo sides of a radicle resides in the tip, which transmits some influenceto the upper part, causing it to bend towards the source of moisture. Consequently, the movement is the reverse of that caused by objectsattached to one side of the tip, or by a thin slice being cut off, or bybeing slightly cauterised. In a future chapter it will be shown thatsensitiveness to the attraction of[page 186]gravity likewise resides in the tip; so that it is the tip which excitesthe adjoining parts of a horizontally extended radicle to bend towards thecentre of the earth. 

SECONDARY RADICLES BECOMING VERTICALLY GEOTROPIC BY THE DESTRUCTION ORINJURY OF THE TERMINAL PART OF THE PRIMARY RADICLE. 

Sachs has shown that the lateral or secondary radicles of the bean, andprobably of other plants, are acted on by geotropism in so peculiar amanner, that they grow out horizontally or a little inclined downwards; andhe has further shown* the interesting fact, that if the end of the primaryradicle be cut off, one of the nearest secondary radicles changes itsnature and grows perpendicularly downwards, thus replacing the primaryradicle. We repeated this experiment, and planted beans with amputatedradicles in friable peat, and saw the result described by Sachs; butgenerally two or three of the secondary radicles grew perpendicularlydownwards. We also modified the experiment, by pinching young radicles alittle way above their tips, between the arms of a U-shaped piece of thickleaden wire. The part pinched was thus flattened, and was afterwardsprevented from growing thicker. Five radicles had their ends cut off, andserved as controls or standards. Eight were pinched; of these 2 werepinched too severely and their ends died and dropped off; 2 were notpinched enough and were not sensibly affected; the remaining 4 were pinchedsufficiently to check the growth of the terminal part, but did not appearotherwise injured. When the U-shaped wires were removed, after an

* 'Arbeiten Bot. Institut. , Würzburg, ' Heft iv. 1874, p. 622. [page 187]

interval of 15 days, the part beneath the wire was found to be very thinand easily broken, whilst the part above was thickened. Now in these fourcases, one or more of the secondary radicles, arising from the thickenedpart just above the wire, had grown perpendicularly downwards. In the bestcase the primary radicle (the part below the wire being 1 ½ inch in length)was somewhat distorted, and was not half as long as three adjoiningsecondary radicles, which had grown vertically, or almost vertically, downwards. Some of these secondary radicles adhered together or had becomeconfluent. We learn from these four cases that it is not necessary, inorder that a secondary radicle should assume the nature of a primary one, that the latter should be actually amputated; it is sufficient that theflow of sap into it should be checked, and consequently should be directedinto the adjoining secondary radicles; for this seems to be the mostobvious result of the primary radicle being pinched between the arms of aU-shaped wire. 

This change in the nature of secondary radicles is clearly analogous, asSachs has remarked, to that which occurs with the shoots of trees, when theleading one is destroyed and is afterwards replaced by one or more of thelateral shoots; for these now grow upright instead of sub-horizontally. Butin this latter case the lateral shoots are rendered apogeotropic, whereaswith radicles the lateral ones are rendered geotropic. We are naturally ledto suspect that the same cause acts with shoots as with roots, namely, anincreased flow of sap into the lateral ones. We made some trials with Abiescommunis and pectinata, by pinching with wire the leading and all thelateral shoots excepting one. But we believe that they were too old whenexperimented on; and some were pinched too severely, and[page 188]some not enough. Only one case succeeded, namely, with the spruce-fir. Theleading shoot was not killed, but its growth was checked; at its base therewere three lateral shoots in a whorl, two of which were pinched, one beingthus killed; the third was left untouched. These lateral shoots, whenoperated on (July 14th) stood at an angle of 8o above the horizon; by Sept. 8th the unpinched one had risen 35o; by Oct. 4th it had risen 46o, and byJan. 26th 48o, and it had now become a little curved inwards. Part of thisrise of 48o may be attributed to ordinary growth, for the pinched shootrose 12o within the same period. It thus follows that the unpinched shootstood, on Jan. 26th, 56o above the horizon, or 34o from the vertical; andit was thus obviously almost ready to replace the slowly growing, pinched, leading shoot. Nevertheless, we feel some doubt about this experiment, forwe have since observed with spruce-firs growing rather unhealthily, thatthe lateral shoots near the summit sometimes become highly inclined, whilstthe leading shoot remains apparently sound. 

A widely different agency not rarely causes shoots which naturally wouldhave brown out horizontally to grow up vertically. The lateral branches ofthe Silver Fir (A. Pectinata) are often affected by a fungus, Aecidiumelatinum, which causes the branch to enlarge into an oval knob formed ofhard wood, in one of which we counted 24 rings of growth. According to DeBary*, when the mycelium penetrates a bud beginning to elongate, the shootdeveloped from it grows vertically upwards. Such upright shoots after-

* See his valuable article in 'Bot. Zeitung, ' 1867, p. 257, on thesemonstrous growths, which are called in German "Hexenbesen, " or"witch-brooms. "[page 189]

wards produce lateral and horizontal branches; and they then present acurious appearance, as if a young fir-tree had grown out of a ball of claysurrounding the branch. These upright shoots have manifestly changed theirnature and become apogeotropic; for if they had not been affected by theAecidium, they would have grown out horizontally like all the other twigson the same branches. This change can hardly be due to an increased flow ofsap into the part; but the presence of the mycelium will have greatlydisturbed its natural constitution. 

According to Mr. Meehan, * the stems of three species of Euphorbia and ofPortulaca oleracea are "normally prostrate or procumbent;" but when theyare attacked by an Aecidium, they "assume an erect habit. " Dr. Stahlinforms us that he knows of several analogous cases; and these seem to beclosely related to that of the Abies. The rhizomes of Sparganium ramosumgrow out horizontally in the soil to a considerable length, or arediageotropic; but F. Elfving found that when they were cultivated in watertheir tips turned upwards, and they became apogeotropic. The same resultfollowed when the stem of the plant was bent until it cracked or was merelymuch bowed. **

No explanation has hitherto been attempted of such cases as the foregoing, --namely, of secondary radicles growing vertically downwards, and of lateralshoots growing vertically upwards, after the amputation of

* 'Proc. Acad. Nat. Sc. Philadelphia, ' June 16th, 1874, and July 23rd, 1875. ** See F. Elfving's interesting paper in 'Arbeiten Bot. Institut. , inWürzburg, ' vol. Ii. 1880, p. 489. Carl Kraus (Triesdorf) had previouslyobserved ('Flora, ' 1878, p. 324) that the underground shoots of Triticumrepens bend vertically up when the parts above ground are removed, and whenthe rhizomes are kept partly immersed in water. [page 190]

the primary radicle or of the leading shoot. The following considerationsgive us, as we believe, the clue. Firstly, any cause which disturbs theconstitution* is apt to induce reversion; such as the crossing of twodistinct races, or a change of conditions, as when domestic animals becomeferal. But the case which most concerns us, is the frequent appearance ofpeloric flowers on the summit of a stem, or in the centre of theinflorescence, --parts which, it is believed, receive the most sap; for whenan irregular flower becomes perfectly regular or peloric, this may beattributed, at least partly, to reversion to a primitive and normal type. Even the position of a seed at the end of the capsule sometimes gives tothe seedling developed from it a tendency to revert. Secondly, reversionsoften occur by means of buds, independently of reproduction by seed; sothat a bud may revert to the character of a former state manybud-generations ago. In the case of animals, reversions may occur in theindividual with advancing age. Thirdly and lastly, radicles when they firstprotrude from the seed are always geotropic, and plumules or shoots almostalways apogeotropic. If then any cause, such as an increased flow of sap orthe presence of mycelium, disturbs the constitution of a lateral shoot orof a secondary radicle, it is apt to revert to its primordial state; and itbecomes either apogeotropic or geotropic, as the case may be, andconsequently grows either vertically upwards or downwards. It is indeedpos-

* The facts on which the following conclusions are founded are given in'The Variation of Animals and Plants under Domestication, ' 2nd edit. 1875. On the causes leading to reversion see chap. Xii. Vol. Ii. And p. 59, chap. Xiv. On peloric flowers, chap. Xiii. P. 32; and see p. 337 on theirposition on the plant. With respect to seeds, p. 340. On reversion by meansof buds, p. 438, chap. Xi. Vol. I. [page 191]

sible, or even probable, that this tendency to reversion may have beenincreased, as it is manifestly of service to the plant. 

SUMMARY OF CHAPTER. 

A part or organ may be called sensitive, when its irritation excitesmovement in an adjoining part. Now it has been shown in this chapter, thatthe tip of the radicle of the bean is in this sense sensitive to thecontact of any small object attached to one side by shellac or gum-water;also to a slight touch with dry caustic, and to a thin slice cut off oneside. The radicles of the pea were tried with attached objects and caustic, both of which acted. With Phaseolus multiflorus the tip was hardlysensitive to small squares of attached card, but was sensitive to causticand to slicing. The radicles of Tropaeolum were highly sensitive tocontact; and so, as far as we could judge, were those of Gossypiumherbaceum, and they were certainly sensitive to caustic. The tips of theradicles of Cucurbita ovifera were likewise highly sensitive to caustic, though only moderately so to contact. Raphanus sativus offered a somewhatdoubtful case. With Aesculus the tips were quite indifferent to bodiesattached to them, though sensitive to caustic. Those of Quercus robur andZea mays were highly sensitive to contact, as were the radicles of thelatter to caustic. In several of these cases the difference insensitiveness of the tip to contact and to caustic was, as we believe, merely apparent; for with Gossypium, Raphanus, and Cucurbita, the tip wasso fine and flexible that it was very difficult to attach any object to oneof its sides. With the radicles of Aesculus, the tips were not at allsensitive to small bodies attached to them; but it does not follow fromthis[page 192]fact that they would not have been sensitive to somewhat greater continuedpressure, if this could have been applied. 

The peculiar form of sensitiveness which we are here considering, isconfined to the tip of the radicle for a length of from 1 mm. To 1. 5 mm. When this part is irritated by contact with any object, by caustic, or by athin slice being cut off, the upper adjoining part of the radicle, for alength of from 6 or 7 to even 12 mm. , is excited to bend away from the sidewhich has been irritated. Some influence must therefore be transmitted fromthe tip along the radicle for this length. The curvature thus caused isgenerally symmetrical. The part which bends most apparently coincides withthat of the most rapid growth. The tip and the basal part grow very slowlyand they bend very little. 

Considering the widely separated position in the vegetable series of theseveral above-named genera, we may conclude that the tips of the radiclesof all, or almost all, plants are similarly sensitive, and transmit aninfluence causing the upper part to bend. With respect to the tips of thesecondary radicles, those of Vicia faba, Pisum sativum, and Zea mays werealone observed, and they were found similarly sensitive. 

In order that these movements should be properly displayed, it appearsnecessary that the radicles should grow at their normal rate. If subjectedto a high temperature and made to grow rapidly, the tips seem either tolose their sensitiveness, or the upper part to lose the power of bending. So it appears to be if they grow very slowly from not being vigorous, orfrom being kept at too low a temperature; also when they are forced togerminate in the middle of the winter. [page 193]

The curvature of the radicle sometimes occurs within from 6 to 8 hoursafter the tip has been irritated, and almost always within 24 h. , exceptingin the case of the massive radicles of Aesculus. The curvature oftenamounts to a rectangle, --that is, the terminal part bends upwards until thetip, which is but little curved, projects almost horizontally. Occasionallythe tip, from the continued irritation of the attached object, continues tobend up until it forms a hook with the point directed towards the zenith, or a loop, or even a spire. After a time the radicle apparently becomesaccustomed to the irritation, as occurs in the case of tendrils, for itagain grows downwards, although the bit of card or other object may remainattached to the tip. It is evident that a small object attached to the freepoint of a vertically suspended radicle can offer no mechanical resistanceto its growth as a whole, for the object is carried downwards as theradicle elongates, or upwards as the radicle curves upwards. Nor can thegrowth of the tip itself be mechanically checked by an object attached toit by gum-water, which remains all the time perfectly soft. The weight ofthe object, though quite insignificant, is opposed to the upward curvature. We may therefore conclude that it is the irritation due to contact whichexcites the movement. The contact, however, must be prolonged, for the tipsof 15 radicles were rubbed for a short time, and this did not cause them tobend. Here then we have a case of specialised sensibility, like that of theglands of Drosera; for these are exquisitely sensitive to the slightestpressure if prolonged, but not to two or three rough touches. 

When the tip of a radicle is lightly touched on one side with dry nitrateof silver, the injury caused is[page 194]very slight, and the adjoining upper part bends away from the cauterisedpoint, with more certainty in most cases than from an object attached onone side. Here it obviously is not the mere touch, but the effect producedby the caustic, which induces the tip to transmit some influence to theadjoining part, causing it to bend away. If one side of the tip is badlyinjured or killed by the caustic, it ceases to grow, whilst the oppositeside continues growing; and the result is that the tip itself bends towardsthe injured side and often becomes completely hooked; and it is remarkablethat in this case the adjoining upper part does not bend. The stimulus istoo powerful or the shock too great for the proper influence to betransmitted from the tip. We have strictly analogous cases with Drosera, Dionaea and Pinguicula, with which plants a too powerful stimulus does notexcite the tentacles to become incurved, or the lobes to close, or themargin to be folded inwards. 

With respect to the degree of sensitiveness of the apex to contact underfavourable conditions, we have seen that with Vicia faba a little square ofwriting-paper affixed with shellac sufficed to cause movement; as did onone occasion a square of merely damped goldbeaters' skin, but it acted veryslowly. Short bits of moderately thick bristle (of which measurements havebeen given) affixed with gum-water acted in only three out of eleventrials, and beads of dried shellac under 1/200th of a grain in weight actedonly twice in nine cases; so that here we have nearly reached the minimumof necessary irritation. The apex, therefore, is much less sensitive topressure than the glands of Drosera, for these are affected by far thinnerobjects than bits of bristle, and by a very much less weight than 1/200thof a grain. [page 195]But the most interesting evidence of the delicate sensitiveness of the tipof the radicle, was afforded by its power of discriminating betweenequal-sized squares of card-like and very thin paper, when these wereattached on opposite sides, as was observed with the radicles of the beanand oak. 

When radicles of the bean are extended horizontally with squares of cardattached to the lower sides of their tips, the irritation thus caused wasalways conquered by geotropism, which then acts under the most favourableconditions at right angles to the radicle. But when objects were attachedto the radicles of any of the above-named genera, suspended vertically, theirritation conquered geotropism, which latter power at first actedobliquely on the radicle; so that the immediate irritation from theattached object, aided by its after-effects, prevailed and caused theradicle to bend upwards, until sometimes the point was directed to thezenith. We must, however, assume that the after-effects of the irritationof the tip by an attached object come into play, only after movement hasbeen excited. The tips of the radicles of the pea seem to be more sensitiveto contact than those of the bean, for when they were extended horizontallywith squares of card adhering to their lower sides, a most curious struggleoccasionally arose, sometimes one and sometimes the other force prevailing, but ultimately geotropism was always victorious; nevertheless, in twoinstances the terminal part became so much curved upwards that loops weresubsequently formed. With the pea, therefore, the irritation from anattached object, and from geotropism when acting at right angles to theradicle, are nearly balanced forces. Closely similar results were observedwith the horizontally extended radicles of Cucurbita ovifera, [page 196]when their tips were slightly cauterised on the lower side. 

Finally, the several co-ordinated movements by which radicles are enabledto perform their proper functions are admirably perfect. In whateverdirection the primary radicle first protrudes from the seed, geotropismguides it perpendicularly downwards; and the capacity to be acted on by theattraction of gravity resides in the tip. But Sachs has proved* that thesecondary radicles, or those emitted by the primary one, are acted on bygeotropism in such a manner that they tend to bend only obliquelydownwards. If they had been acted on like the primary radicle, all theradicles would have penetrated the ground in a close bundle. We have seenthat if the end of the primary radicle is cut off or injured, the adjoiningsecondary radicles become geotropic and grow vertically downwards. Thispower must often be of great service to the plant, when the primary radiclehas been destroyed by the larvae of insects, burrowing animals, or anyother accident. The tertiary radicles, or those emitted by the secondaryones, are not influenced, at least in the case of the bean, by geotropism;so they grow out freely in all directions. From this manner of growth ofthe various kinds of radicles, they are distributed, together with theirabsorbent hairs, throughout the surrounding soil, as Sachs has remarked, inthe most advantageous manner; for the whole soil is thus closely searched. 

 Geotropism, as was shown in the last chapter, excites the primary radicleto bend downwards with very little force, quite insufficient to penetratethe ground. Such penetration is effected by the pointed

* 'Arbeiten Bot. Institut, Würzburg, ' Heft iv. 1874, pp. 605-631. [page 197]

apex (protected by the root-cap) being pressed down by the longitudinalexpansion or growth of the terminal rigid portion, aided by its transverseexpansion, both of which forces act powerfully. It is, however, indispensable that the seeds should be at first held down in some manner. When they lie on the bare surface they are held down by the attachment ofthe root-hairs to any adjoining objects; and this apparently is effected bythe conversion of their outer surfaces into a cement. But many seeds getcovered up by various accidents, or they fall into crevices or holes. Withsome seeds their own weight suffices. The circumnutating movement of theterminal growing part both of the primary and secondary radicles is sofeeble that it can aid them very little in penetrating the ground, excepting when the superficial layer is very soft and damp. But it must aidthem materially when they happen to break obliquely into cracks, or intoburrows made by earth-worms or larvae. This movement, moreover, combinedwith the sensitiveness of the tip to contact, can hardly fail to be of thehighest importance; for as the tip is always endeavouring to bend to allsides it will press on all sides, and will thus be able to discriminatebetween the harder and softer adjoining surfaces, in the same manner as itdiscriminated between the attached squares of card-like and thin paper. Consequently it will tend to bend from the harder soil, and will thusfollow the lines of least resistance. So it will be if it meets with astone or the root of another plant in the soil, as must incessantly occur. If the tip were not sensitive, and if it did not excite the upper part ofthe root to bend away, whenever it encountered at right angles someobstacle in the ground, it would be liable[page 198]to be doubled up into a contorted mass. But we have seen with radiclesgrowing down inclined plates of glass, that as soon as the tip merelytouched a slip of wood cemented across the plate, the whole terminalgrowing part curved away, so that the tip soon stood at right angles to itsformer direction; and thus it would be with an obstacle encountered in theground, as far as the pressure of the surrounding soil would permit. We canalso understand why thick and strong radicles, like those of Aesculus, should be endowed with less sensitiveness than more delicate ones; for theformer would be able by the force of their growth to overcome any slightobstacle. 

After a radicle, which has been deflected by some stone or root from itsnatural downward course, reaches the edge of the obstacle, geotropism willdirect it to grow again straight downward; but we know that geotropism actswith very little force, and here another excellent adaptation, as Sachs hasremarked, * comes into play. For the upper part of the radicle, a littleabove the apex, is, as we have seen, likewise sensitive; and thissensitiveness causes the radicle to bend like a tendril towards thetouching object, so that as it rubs over the edge of an obstacle, it willbend downwards; and the curvature thus induced is abrupt, in which respectit differs from that caused by the irritation of one side of the tip. Thisdownward bending coincides with that due to geotropism, and both will causethe root to resume its original course. 

As radicles perceive an excess of moisture in the air on one side and bendtowards this side, we may infer that they will act in the same manner withrespect to moisture in the earth. The sensitiveness to moisture

* 'Arbeiten Bot. Inst. , Würzburg, ' Heft iii. P. 456. [page 199]

resides in the tip, which determines the bending of the upper part. Thiscapacity perhaps partly accounts for the extent to which drain-pipes oftenbecome choked with roots. 

Considering the several facts given in this chapter, we see that the coursefollowed by a root through the soil is governed by extraordinarily complexand diversified agencies, --by geotropism acting in a different manner onthe primary, secondary, and tertiary radicles, --by sensitiveness tocontact, different in kind in the apex and in the part immediately abovethe apex, and apparently by sensitiveness to the varying dampness ofdifferent parts of the soil. These several stimuli to movement are all morepowerful than geotropism, when this acts obliquely on a radicle, which hasbeen deflected from its perpendicular downward course. The roots, moreover, of most plants are excited by light to bend either to or from it; but asroots are not naturally exposed to the light it is doubtful whether thissensitiveness, which is perhaps only the indirect result of the radiclesbeing highly sensitive to other stimuli, is of any service to the plant. The direction which the apex takes at each successive period of the growthof a root, ultimately determines its whole course; it is therefore highlyimportant that the apex should pursue from the first the most advantageousdirection; and we can thus understand why sensitiveness to geotropism, tocontact and to moisture, all reside in the tip, and why the tip determinesthe upper growing part to bend either from or to the exciting cause. Aradicle may be compared with a burrowing animal such as a mole, whichwishes to penetrate perpendicularly down into the ground. By continuallymoving his head from side to side, or circumnutating, he will feel anystone[page 200]or other obstacle, as well as any difference in the hardness of the soil, and he will turn from that side; if the earth is damper on one than on theother side he will turn thitherward as a better hunting-ground. Nevertheless, after each interruption, guided by the sense of gravity, hewill be able to recover his downward course and to burrow to a greaterdepth. [page 201]

CHAPTER IV. 

THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS. 

Circumnutation of stems: concluding remarks on--Circumnutation of stolons:aid thus afforded in winding amongst the stems of surrounding plants--Circumnutation of flower-stems--Circumnutation of Dicotyledonous leaves--Singular oscillatory movement of leaves of Dionaea--Leaves of Cannabis sinkat night--Leaves of Gymnosperms--Of Monocotyledons--Cryptogams--Concludingremarks on the circumnutation of leaves; generally rise in the evening andsink in the morning. 

WE have seen in the first chapter that the stems of all seedlings, whetherhypocotyls or epicotyls, as well as the cotyledons and the radicles, arecontinually circumnutating--that is they grow first on one side and then onanother, such growth being probably preceded by increased turgescence ofthe cells. As it was unlikely that plants should change their manner ofgrowth with advancing age, it seemed probable that the various organs ofall plants at all ages, as long as they continued to grow, would be foundto circumnutate, though perhaps to an extremely small extent. As it wasimportant for us to discover whether this was the case, we determined toobserve carefully a certain number of plants which were growing vigorously, and which were not known to move in any manner. We commenced with stems. Observations of this kind are tedious, and it appeared to us that it wouldbe sufficient to observe the stems in about a score of genera, belonging towidely distinct families and inhabitants of various countries. Severalplants[page 202]were selected which, from being woody, or for other reasons, seemed theleast likely to circumnutate. The observations and the diagrams were madein the manner described in the Introduction. Plants in pots were subjectedto a proper temperature, and whilst being observed, were kept either indarkness or were feebly illuminated from above. They are arranged in theorder adopted by Hooker in Le Maout and Decaisne's 'System of Botany. ' Thenumber of the family to which each genus belongs is appended, as thisserves to show the place of each in the series. 

[(1. ) Iberis umbellata (Cruciferae, Fam. 14). --The movement of the stem ofa young plant, 4 inches in height, consisting of four internodes (thehypocotyl included) besides a large bud

Fig. 70. Iberis umbellata: circumnutation of stem of young plant, tracedfrom 8. 30 A. M. Sept. 13th to same hour on following morning. Distance ofsummit of stem beneath the horizontal glass 7. 6 inches. Diagram reduced tohalf of original size. Movement as here shown magnified between 4 and 5times. 

on the summit, was traced, as here shown, during 24 h. (Fig. 70). As far aswe could judge the uppermost inch alone of the stem circumnutated, and thisin a simple manner. The movement was slow, and the rate very unequal atdifferent times. In part of its course an irregular ellipse, or rathertriangle, was completed in 6 h. 30 m. 

 (2. ) Brassica oleracea (Cruciferae). --A very young plant, bearing threeleaves, of which the longest was only three-quarters of an inch in length, was placed under a microscope, furnished with an eye-piece micrometer, andthe tip of the largest leaf was[page 203]found to be in constant movement. It crossed five divisions of themicrometer, that is, 1/100th of an inch, in 6 m. 20 s. There could hardlybe a doubt that it was the stem which chiefly moved, for the tip did notget quickly out of focus; and this would have occurred had the movementbeen confined to the leaf, which moves up or down in nearly the samevertical plane. 

(3. ) Linum usitatissimum (Lineae, Fam. 39). --The stems of this plant, shortly before the flowering period, are stated by Fritz Müller ('JenaischeZeitschrift, ' B. V. P. 137) to revolve, or circumnutate. 

(4. ) Pelargonium zonale (Geraniaceae, Fam. 47). --A young plant, 7 ½ inchesin height, was observed in the usual manner; but, in order to see the beadat the end of the glass filament

Fig. 71. Pelargonium zonale: circumnutation of stem of young plant, feeblyilluminated from above. Movement of bead magnified about 11 times; tracedon a horizontal glass from noon on March 9th to 8 A. M. On the 11th. 

and at the same time the mark beneath, it was necessary to cut off threeleaves on one side. We do not know whether it was owing to this cause, orto the plant having previously become bent to one side throughheliotropism, but from the morning of the 7th of March to 10. 30 P. M. On the8th, the stem moved a considerable distance in a zigzag line in the samegeneral direction. During the night of the 8th it moved to some distance atright angles to its former course, and next morning (9th) stood for a timealmost still. At noon on the 9th a new tracing was begun (see Fig. 71), which was continued till 8 A. M. On the 11th. Between noon on the 9th and 5P. M. On the 10th (i. E. In the course of 29 h. ), the stem described acircle. This plant therefore circumnutates, but at a very slow rate, and toa small extent. 

 (5. ) Tropaeolum majus (?) (dwarfed var. Called Tom Thumb); (Geraniaceae, Fam. 47). --The species of this genus climb by the[page 204]aid of their sensitive petioles, but some of them also twine roundsupports; but even these latter species do not begin to circumnutate in aconspicuous manner whilst young. The

Fig. 72. Tropaeolum majus (?): circumnutation of stem of young plant, traced on a horizontal glass from 9 A. M. Dec. 26th to 10 A. M. On 27th. Movement of bead magnified about 5 times, and here reduced to half oforiginal scale. 

variety here treated of has a rather thick stem, and is so dwarf thatapparently it does not climb in any manner. We therefore wished toascertain whether the stem of a young plant, consisting of two internodes, together 3. 2 inches in height, circumnutated. It was observed during 25 h. , and we see in Fig. 72 that the stem moved in a zigzag course, indicatingcircumnutation. 

Fig. 73. Trifolium resupinatum: circumnutation of stem, traced on verticalglass from 9. 30 A. M. To 4. 30 P. M. Nov. 3rd. Tracing not greatly magnified, reduced to half of original size. Plant feebly illuminated from above. 

 (6. ) Trifolium resupinatum (Leguminosae, Fam. 75). --When we treat of thesleep of plants, we shall see that the stems in several Leguminous genera, for instance, those of Hedysarum, Mimosa, Melilotus, etc. , which are notclimbers, circumnutate in a conspicuous manner. We will here give only asingle instance (Fig. 73), showing the circumnutation of the stem of alarge plant of a clover, Trifolium resupinatum. In the course of 7 h. Thestem changed[page 205]its course greatly eight times and completed three irregular circles orellipses. It therefore circumnutated rapidly. Some of the lines run atright angles to one another. 

Fig. 74. Rubus (hybrid): circumnutation of stem, traced on horizontalglass, from 4 P. M. March 14th to 8. 30 A. M. 16th. Tracing much magnified, reduced to half of original size. Plant illuminated feebly from above. 

 (7. ) Rubus idaeus (hybrid) (Rosaceae, Fam. 76). --As we happened to have ayoung plant, 11 inches in height and growing vigorously, which had beenraised from a cross between the raspberry (Rubus idaeus) and a NorthAmerican Rubus, it was observed in the usual manner. During the morning ofMarch 14th the stem almost completed a circle, and then moved far to theright. At 4 P. M. It reversed its course, and now a fresh tracing was begun, which was continued during 40 ½ h. , and is given in Fig. 74. We here havewell-marked circumnutation. 

(8. ) Deutzia gracilis (Saxifrageae, Fam. 77). --A shoot on a bush about 18inches in height was observed. The bead changed its course greatly eleventimes in the course of 10 h. 30 m. (Fig. 75), and there could be no doubtabout the circumnutation of the stem. 

Fig. 75. Deutzia gracilis: circumnutation of stem, kept in darkness, tracedon horizontal glass, from 8. 30 A. M. To 7 P. M. March 20th. Movement of beadoriginally magnified about 20 times, here reduced to half scale. 

(9. ) Fuchsia (greenhouse var. , with large flowers, probably a hybrid)(Onagrarieae, Fam. 100). --A young plant, 15 inches in height, was observedduring nearly 48 h. The[page 206]accompanying figure (Fig. 76) gives the necessary particulars, and showsthat the stem circumnutated, though rather slowly. 

Fig. 76. Fuchsia (garden var. ): circumnutation of stem, kept in darkness, traced on horizontal glass, from 8. 30 A. M. To 7 P. M. March 20th. Movementof bead originally magnified about 40 times, here reduced to half scale. 

(10. ) Cereus speciocissimus (garden var. , sometimes called Phyllocactusmultiflorus) (Cacteae, Fam. 109). --This plant, which was growing vigorouslyfrom having been removed a few days before from the greenhouse to thehot-house, was observed with especial interest, as it seemed so littleprobable that the stem would circumnutate. The branches are flat, orflabelliform; but some of them are triangular in section, with the threesides hollowed out. A branch of this latter shape, 9 inches in length and 1½ in diameter, was chosen for observation, as less likely to circumnutatethan a flabelliform branch. The movement of the bead at the end of theglass filament, affixed to the summit of the branch, was traced (A, Fig. 77) from 9. 23 A. M. To 4. 30 P. M. On Nov. 23rd, during which time it changedits course greatly six times. On the 24th another tracing was made (see B), and the bead on this day changed its course oftener, making in 8 h. Whatmay be considered as four ellipses, with their longer axes differentlydirected. The position of the stem and its commencing course on thefollowing morning are likewise shown. There can be no doubt that thisbranch, though appearing quite rigid, circumnutated; but the[page 207]extreme amount of movement during the time was very small, probably ratherless than the 1/20th of an inch. 

Fig 77. Cereus speciocissimus: circumnutation of stem, illuminated fromabove, traced on a horizontal glass, in A from 9 A. M. To 4. 30 P. M. On Nov. 23rd; and in B from 8. 30 A. M. On the 24th to 8 A. M. On the 25th. Movementof the bead in B magnified about 38 times. 

(11. ) Hedera helix (Araliaceae, Fam. 114). --The stem is known to beapheliotropic, and several seedlings growing in a pot in the greenhousebecame bent in the middle of the summer at right angles from the light. OnSept. 2nd some of these stems were tied up so as to stand vertically, andwere placed before a north-east window; but to our surprise they were nowdecidedly heliotropic, for during 4 days they curved themselves towards thelight, and their course being traced on a horizontal glass, was stronglyzigzag. During the 6 succeeding days they circumnutated over the same smallspace at a slow rate, but there could be no doubt about theircircumnutation. The plants were kept exactly in the same place before thewindow, and after an interval of 15 days the stems were again observedduring 2 days and their movements traced, and[page 208]they were found to be still circumnutating, but on a yet smaller scale. 

(12. ) Gazania ringens (Compositae, Fam. 122). --The circumnutation of thestem of a young plant, 7 inches in height, as measured to the tip of thehighest leaf, was traced during 33 h. , and is shown in the accompanyingfigure (Fig. 78). Two

Fig. 78. Gazania ringens: circumnutation of stem traced from 9 A. M. March21st to 6 P. M. On 22nd; plant kept in darkness. Movement of bead at theclose of the observations magnified 34 times, here reduced to half theoriginal scale. 

main lines may be observed running at nearly right angles to two other mainlines; but these are interrupted by small loops. 

(13. ) Azalea Indica (Ericineae, Fam. 128). --A bush 21 inches in height wasselected for observation, and the circumnutation of its leading shoot wastraced during 26 h. 40 m. , as shown in the following figure (Fig. 79). 

 (14. ) Plumbago Capensis (Plumbagineae, Fam. 134). --A small lateral branchwhich projected from a tall freely growing bush, at an angle of 35o abovethe horizon, was selected for observation. For the first 11 h. It moved toa considerable distance in a nearly straight line to one side, owingprobably to its having been previously deflected by the light whilststanding in the greenhouse. At 7. 20 P. M. On March 7th a fresh tracing wasbegun and continued for the next 43 h. 40 m. (see Fig. 80). During thefirst 2 h. It followed nearly the same direction as before, and thenchanged it a little; during the night it moved at nearly right angles toits previous course. Next[page 209]day (8th) it zigzagged greatly, and on the 9th moved irregularly round andround a small circular space. By 3 P. M. On the 9th the figure had become socomplicated that no more dots could be made; but the shoot continued duringthe evening of the 9th, the whole of the 10th, and the morning of the 11thto

Fig. 79. Azalea Indica: circumnutation of stem, illuminated from above, traced on horizontal glass, from 9. 30 A. M. March 9th to 12. 10 P. M. On the10th. But on the morning of the 10th only four dots were made between 8. 30A. M. And 12. 10 P. M. , both hours included, so that the circumnutation is notfairly represented in this part of the diagram. Movement of the bead heremagnified about 30 times. 

Fig. 80. Plumbago Capensis: circumnutation of tip of a lateral branch, traced on horizontal glass, from 7. 20 P. M. On March 7th to 3 P. M. On the9th. Movement of bead magnified 13 times. Plant feebly illuminated fromabove. 

circumnutate over the same small space, which was only about the 1/26th ofan inch (. 97 mm. ) in diameter. Although this branch circumnutated to a verysmall extent, yet it changed its course frequently. The movements ought tohave been more magnified. 

(15. ) Aloysia citriodora (Verbenaceae, Fam. 173). --The following figure(Fig. 81) gives the movements of a shoot during[page 210]31 h. 40 m. , and shows that it circumnutated. The bush was 15 inches inheight. 

Fig. 81. Aloysia citriodora: circumnutation of stem, traced from 8. 20 A. M. On March 22nd to 4 P. M. On 23rd. Plant kept in darkness. Movement magnifiedabout 40 times. 

(16. ) Verbena melindres (?) (a scarlet-flowered herbaceous var. )(Verbenaceae). --A shoot 8 inches in height had been laid horizontally, forthe sake of observing its apogeotropism, and the terminal portion had grownvertically upwards for a length of 1 ½ inch. A glass filament, with a beadat the end, was fixed

Fig. 82. Verbena melindres: circumnutation of stem in darkness, traced onvertical glass, from 5. 30 P. M. On June 5th to 11 A. M. June 7th. Movement ofbead magnified 9 times. 

upright to the tip, and its movements were traced during 41 h. 30 m. On avertical glass (Fig. 82). Under these circumstances the lateral movementswere chiefly shown; but as the lines from side to side are not on the samelevel, the shoot[page 211]must have moved in a plane at right angles to that of the lateral movement, that is, it must have circumnutated. On the next day (6th) the shoot movedin the course of 16 h. Four times to the right, and four times to the left;and this apparently represents the formation of four ellipses, so that eachwas completed in 4 h. (17. ) Ceratophyllum demersum (Ceratophylleae, Fam. 220). --An interestingaccount of the movements of the stem of this water-plant has been publishedby M. E. Rodier. * The movements are confined to the young internodes, becoming less and less lower down the stem; and they are extraordinary fromtheir amplitude. The stems sometimes moved through an angle of above 200oin 6 h. , and in one instance through 220o in 3 h. They generally bent fromright to left in the morning, and in an opposite direction in theafternoon; but the movement was sometimes temporarily reversed or quitearrested. It was not affected by light. It does not appear that M. Rodiermade any diagram on a horizontal plane representing the actual coursepursued by the apex, but he speaks of the "branches executing round theiraxes of growth a movement of torsion. " From the particulars above given, and remembering in the case of twining plants and of tendrils, howdifficult it is not to mistake their bending to all points of the compassfor true torsion, we are led to believe that the stems of thisCeratophyllum circumnutate, probably in the shape of narrow ellipses, eachcompleted in about 26 h. The following statement, however, seems toindicate something different from ordinary circumnutation, but we cannotfully understand it. M. Rodier says: "Il est alors facile de voir que lemouvement de flexion se produit d'abord dans les mérithalles supérieurs, qu'il se propage ensuite, en s'amoindrissant du haut en bas; tandis qu'aucontraire le movement de redressement commence par la partie inférieur pourse terminer a la partie supérieure qui, quelquefois, peu de temps avant dese relever tout à fait, forme avec l'axe un angle très aigu. "

 (18. ) Coniferae. --Dr. Maxwell Masters states ('Journal Linn. Soc. , ' Dec. 2nd, 1879) that the leading shoots of many Coniferae during the season oftheir active growth exhibit very remarkable movements of revolvingnutation, that is, they circumnutate. We may feel sure that the lateralshoots whilst growing would exhibit the same movement if carefullyobserved. 

* 'Comptes Rendus, ' April 30th, 1877. Also a second notice publishedseparately in Bourdeaux, Nov. 12th, 1877. [page 212]

(19. ) Lilium auratum (Fam. Liliaceae). --The circumnutation

Fig. 83. Lilium auratum: circumnutation of a stem in darkness, traced on ahorizontal glass, from 8 A. M. On March 14th to 8. 35 A. M. On 16th. But itshould be noted that our observations were interrupted between 6 P. M. Onthe 14th and 12. 15 P. M. On the 15th, and the movements during this intervalof 18 h. 15 m. Are represented by a long broken line. Diagram reduced tohalf original scale. 

of the stem of a plant 24 inches in height is represented in the abovefigure (Fig. 83). 

Fig. 84. Cyperus alternifolius: circumnutation of stem, illuminated fromabove, traced on horizontal glass, from 9. 45 A. M. March 9th to 9 P. M. On10th. The stem grew so rapidly whilst being observed, that it was notpossible to estimate how much its movements were magnified in the tracing. 

 (20. ) Cyperus alternifolius (Fam. Cyperaceae. )--A glass[page 213]filament, with a bead at the end, was fixed across the summit of a youngstem 10 inches in height, close beneath the crown of elongated leaves. OnMarch 8th, between 12. 20 and 7. 20 P. M. The stem described an ellipse, openat one end. On the following day a new tracing was begun (Fig. 84), whichplainly shows that the stem completed three irregular figures in the courseof 35 h. 15 m. ]

Concluding Remarks on the Circumnutation of Stems. --Any one who willinspect the diagrams now given, and will bear in mind the widely separatedposition of the plants described in the series, --remembering that we havegood grounds for the belief that the hypocotyls and epicotyls of allseedlings circumnutate, --not forgetting the number of plants distributed inthe most distinct families which climb by a similar movement, --willprobably admit that the growing stems of all plants, if carefully observed, would be found to circumnutate to a greater or less extent. When we treatof the sleep and other movements of plants, many other cases ofcircumnutating stems will be incidentally given. In looking at thediagrams, we should remember that the stems were always growing, so that ineach case the circumnutating apex as it rose will have described a spire ofsome kind. The dots were made on the glasses generally at intervals of anhour, or hour and a half, and were then joined by straight lines. If theyhad been made at intervals of 2 or 3 minutes, the lines would have beenmore curvilinear, as in the case of the tracks left on the smokedglass-plates by the tips of the circumnutating radicles of seedling plants. The diagrams generally approach in form to a succession of more or lessirregular ellipses or ovals, with their longer axes directed to differentpoints of the compass during the same day or on succeeding days. The stemsthere-[page 214]fore, sooner or later, bend to all sides; but after a stem has bent in anyone direction, it commonly bends back at first in nearly, though not quite, the opposite direction; and this gives the tendency to the formation ofellipses, which are generally narrow, but not so narrow as those describedby stolons and leaves. On the other hand, the figures sometimes approach inshape to circles. Whatever the figure may be, the course pursued is ofteninterrupted by zigzags, small triangles, loops, or ellipses. A stem maydescribe a single large ellipse one day, and two on the next. Withdifferent plants the complexity, rate, and amount of movement differ much. The stems, for instance, of Iberis and Azalea described only a single largeellipse in 24 h. ; whereas those of the Deutzia made four or five deepzigzags or narrow ellipses in 11 ½ h. , and those of the Trifolium threetriangular or quadrilateral figures in 7 h. 

CIRCUMNUTATION OF STOLONS OR RUNNERS. 

Stolons consist of much elongated, flexible branches, which run along thesurface of the ground and form roots at a distance from the parent-plant. They are therefore of the same homological nature as stems; and the threefollowing cases may be added to the twenty previously given cases. 

[Fragaria (cultivated garden var. ): Rosaceae. --A plant growing in a pot hademitted a long stolon; this was supported by a stick, so that it projectedfor the length of several inches horizontally. A glass filament bearing twominute triangles of paper was affixed to the terminal bud, which was alittle upturned; and its movements were traced during 21 h. , as shown inFig. 85. In the course of the first 12 h. It moved twice up and twice downin somewhat zigzag lines, and no doubt travelled in the same manner duringthe night. On the following[page 215]morning after an interval of 20 h. The apex stood a little higher than itdid at first, and this shows that the stolon had not beenFig. 85. Fragaria: circumnutation of stolon, kept in darkness, traced onvertical glass, from 10. 45 A. M. May 18th to 7. 45 A. M. On 19th. 

acted on within this time by geotropism;* nor had its own weight caused itto bend downwards. 

On the following morning (19th) the glass filament was detached and refixedclose behind the bud, as it appeared possible that the circumnutation ofthe terminal bud and of the adjoining part of the stolon might bedifferent. The movement was now traced during two consecutive days (Fig. 86). During the first day the filament travelled in the course of 14 h. 30m. Five times up and four times down, besides some lateral movement. On the20th the course was even more complicated, and can hardly be followed inthe figure; but the filament moved in 16 h. At least five times up and fivetimes down, with very little

* Dr. A. B. Frank states ('Die Naturliche wagerechte Richtung vonPflanzentheilen, ' 1870, p. 20) that the stolons of this plant are acted onby geotropism, but only after a considerable interval of time. [page 216]

lateral deflection. The first and last dots made on this second day, viz. , at 7 A. M. And 11 P. M. , were close together, showing that the stolon had notfallen or risen. Nevertheless, by comparing its position on the morning ofthe 19th and 21st, it is obvious that the stolon had sunk; and this may beattributed to slow bending down either from its own weight or fromgeotropism. 

Fig. 86. Fragaria: circumnutation of the same stolon as in the last figure, observed in the same manner, and traced from 8 A. M. May 19th to 8 A. M. 21st. 

During a part of the 20th an orthogonal tracing was made by applying a cubeof wood to the vertical glass and bringing the apex of the stolon atsuccessive periods into a line with one edge; a dot being made each time onthe glass. This tracing therefore represented very nearly the actual amountof movement of the apex; and in the course of 9 h. The distance of theextreme dots from one another was . 45 inch. By the same method it wasascertained that the apex moved between 7 A. M. On the 20th and 8 A. M. Onthe 21st a distance of . 82 inch. 

A younger and shorter stolon was supported so that it projected at about45o above the horizon, and its movement was traced by the same orthogonalmethod. On the first day the apex soon rose above the field of vision. Bythe next morning it had sunk, and the course pursued was now traced during14 h. 30 m. (Fig. 87). The amount of movement was almost the same, [page 217]from side to side as up and down; and differed in this respect remarkablyfrom the movement in the previous cases. During the latter part of the day, viz. , between 3 and 10. 30 P. M. , the

Fig. 87. Fragaria: circumnutation of another and younger stolon, tracedfrom 8 A. M. To 10. 30 P. M. Figure reduced to one-half of original scale. 

actual distance travelled by the apex amounted to 1. 15 inch; and in thecourse of the whole day to at least 2. 67 inches. This is an amount ofmovement almost comparable with that of some climbing plants. The samestolon was observed on the following day, and now it moved in a somewhatless complex manner, in a plane not far from vertical. The extreme amountof actual movement was 1. 55 inch in one direction, and . 6 inch in anotherdirection at right angles. During neither of these days did the stolon benddownwards through geotropism or its own weight. 

Four stolons still attached to the plant were laid on damp sand in the backof a room, with their tips facing the north-east windows. They were thusplaced because De Vries says* that they are apheliotropic when exposed tothe light of the sun; but we could not perceive any effect from the abovefeeble degree of illumination. We may add that on another occasion, late inthe summer, some stolons, placed upright before a south-west window

* 'Arbeiten Bot Inst. , Würzburg, ' 1872, p. 434. [page 218]

on a cloudy day, became distinctly curved towards the light, and weretherefore heliotropic. Close in front of the tips of the prostrate stolons, a crowd of very thin sticks and the dried haulms of grasses were driveninto the sand, to represent the crowded stems of surrounding plants in astate of nature. This was done for the sake of observing how the growingstolons would pass through them. They did so easily in the course of 6days, and their circumnutation apparently facilitated their passage. Whenthe tips encountered sticks so close together that they could not passbetween them, they rose up and passed over them. The sticks and haulms wereremoved after the passage of the four stolons, two of which were found tohave assumed a permanently sinuous shape, and two were still straight. Butto this subject we shall recur under Saxifraga. 

Saxifraga sarmentosa (Saxifrageae). --A plant in a suspended pot had emittedlong branched stolons, which depended like

Fig. 88. Saxifraga sarmentosa: circumnutation of an inclined stolon, tracedin darkness on a horizontal glass, from 7. 45 A. M. April 18th to 9 A. M. On19th. Movement of end of stolon magnified 2. 2 times. 

threads on all sides. Two were tied up so as to stand vertically, and theirupper ends became gradually bent downwards, but so slowly in the course ofseveral days, that the bending was probably due to their weight and not togeotropism. A glass filament with little triangles of paper was fixed tothe end of one of these stolons, which was 17 ½ inches in length, and hadalready become much bent down, but still projected at a considerable angleabove the horizon. It moved only slightly three times from side to side andthen upwards; on the following day[page 219]the movement was even less. As this stolon was so long we thought that itsgrowth was nearly completed, so we tried another which was thicker andshorter, viz. , 10 1/4 inches in length. It moved greatly, chiefly upwards, and changed its course five times in the course of the day. During thenight it curved so much upwards in opposition to gravity, that the movementcould no longer be traced on the vertical glass, and a horizontal one hadto be used. The movement was followed during the next 25 h. , as shown inFig. 88. Three irregular ellipses, with their longer axes somewhatdifferently directed, were almost completed in the first 15 h. The extremeactual amount of movement of the tip during the 25 h. Was . 75 inch. Several stolons were laid on a flat surface of damp sand, in the samemanner as with those of the strawberry. The friction of the sand did notinterfere with their circumnutation; nor could we detect any evidence oftheir being sensitive to contact. In order to see how in a state of naturethey would act, when encountering a stone or other obstacle on the ground, short pieces of smoked glass, an inch in height, were stuck upright intothe sand in front of two thin lateral branches. Their tips scratched thesmoked surface in various directions; one made three upward and twodownward lines, besides a nearly horizontal one; the other curled quiteaway from the glass; but ultimately both surmounted the glass and pursuedtheir original course. The apex of a third thick stolon swept up the glassin a curved line, recoiled and again came into contact with it; it thenmoved to the right, and after ascending, descended vertically; ultimatelyit passed round one end of the glass instead of over it. 

Many long pins were next driven rather close together into the sand, so asto form a crowd in front of the same two thin lateral branches; but theseeasily wound their way through the crowd. A thick stolon was much delayedin its passage; at one place it was forced to turn at right angles to itsformer course; at another place it could not pass through the pins, and thehinder part became bowed; it then curved upwards and passed through anopening between the upper part of some pins which happened to diverge; itthen descended and finally emerged through the crowd. This stolon wasrendered permanently sinuous to a slight degree, and was thicker wheresinuous than elsewhere, apparently from its longitudinal growth having beenchecked. 

Cotyledon umbilicus (Crassulaceae). --A plant growing in a pan[page 220]of damp moss had emitted 2 stolons, 22 and 20 inches in length. One ofthese was supported, so that a length of 4 ½ inches projected in a straightand horizontal line, and the movement of the apex was traced. The first dotwas made at 9. 10 A. M. ;

Fig. 89. Cotyledon umbilicus: circumnutation of stolon, traced from 11. 15A. M. Aug. 25th to 11 A. M. 27th. Plant illuminated from above. The terminalinternode was . 25 inch in length, the penultimate 2. 25 and the third 3. 0inches in length. Apex of stolon stood at a distance of 5. 75 inches fromthe vertical glass; but it was not possible to ascertain how much thetracing was magnified, as it was not known how great a length of theinternode circumnutated. 

the terminal portion soon began to bend downwards and continued to do sountil noon. Therefore a straight line, very nearly as long as the wholefigure here given (Fig. 89), was first traced on the glass; but the upperpart of this line has not been copied in the diagram. The curvatureoccurred in the middle[page 221]of the penultimate internode; and its chief seat was at the distance of 11/4 inch from the apex; it appeared due to the weight of the terminalportion, acting on the more flexible part of the internode, and not togeotropism. The apex after thus sinking down from 9. 10 A. M. To noon, moveda little to the left; it then rose up and circumnutated in a nearlyvertical plane until 10. 35 P. M. On the following day (26th) it was ob-

Fig. 90. Cotyledon umbilicus: circumnutation and downward movement ofanother stolon, traced on vertical glass, from 9. 11 A. M. Aug. 25th to 11A. M. 27th. Apex close to glass, so that figure but little magnified, andhere reduced to two-thirds of original size. 

served from 6. 40 A. M. To 5. 20 P. M. , and within this time it moved twice upand twice down. On the morning of the 27th the apex stood as high as it didat 11. 30 A. M. On the 25th. Nor did it sink down during the 28th, butcontinued to circumnutate about the same place. 

Another stolon, which resembled the last in almost every[page 222]respect, was observed during the same two days, but only two inches of theterminal portion was allowed to project freely and horizontally. On the25th it continued from 9. 10 A. M. To 1. 30 P. M. To bend straight downwards, apparently owing to its weight (Fig. 90); but after this hour until 10. 35P. M. It zigzagged. This fact deserves notice, for we here probably see thecombined effects of the bending down from weight and of circumnutation. Thestolon, however, did not circumnutate when it first began to bend down, asmay be observed in the present diagram, and as was still more evident inthe last case, when a longer portion of the stolon was left unsupported. Onthe following day (26th) the stolon moved twice up and twice down, butstill continued to fall; in the evening and during the night it travelledfrom some unknown cause in an oblique direction. ]

We see from these three cases that stolons or runners circumnutate in avery complex manner. The lines generally extend in a vertical plane, andthis may probably be attributed to the effect of the weight of theunsupported end of the stolon; but there is always some, and occasionally aconsiderable, amount of lateral movement. The circumnutation is so great inamplitude that it may almost be compared with that of climbing plants. Thatthe stolons are thus aided in passing over obstacles and in winding betweenthe stems of the surrounding plants, the observations above given renderalmost certain. If they had not circumnutated, their tips would have beenliable to have been doubled up, as often as they met with obstacles intheir path; but as it is, they easily avoid them. This must be aconsiderable advantage to the plant in spreading from its parent-stock; butwe are far from supposing that the power has been gained by the stolons forthis purpose, for circumnutation seems to be of universal occurrence withall growing parts; but it is not improbable that the amplitude of themovement may have been specially increased for this purpose. [page 223]

CIRCUMNUTATION OF FLOWER-STEMS. 

We did not think it necessary to make any special observations on thecircumnutation of flower-stems, these being axial in their nature, likestems or stolons; but some were incidentally made whilst attending to othersubjects, and these we will here briefly give. A few observations have alsobeen made by other botanists. These taken together suffice to render itprobable that all peduncles and sub-peduncles circumnutate whilst growing. 

[Oxalis carnosa. --The peduncle which springs from the thick and woody stemof this plant bears three or four sub-peduncles. 

Fig. 91. Oxalis carnosa: flower-stem, feebly illuminated from above, itscircumnutation traced from 9 A. M. April 13th to 9 A. M. 15th. Summit offlower 8 inches beneath the horizontal glass. Movement probably magnifiedabout 6 times. 

A filament with little triangles of paper was fixed within the calyx of aflower which stood upright. Its movements were observed for 48 h. ; duringthe first half of this time the flower was fully expanded, and during thesecond half withered. The figure here given (Fig. 91) represents 8 or 9ellipses. Although the main peduncle circumnutated, and described one largeand[page 224]two smaller ellipses in the course of 24 h. , yet the chief seat of movementlies in the sub-peduncles, which ultimately bend vertically downwards, aswill be described in a future chapter. The peduncles of Oxalis acetosellalikewise bend downwards, and afterwards, when the pods are nearly mature, upwards; and this is effected by a circumnutating movement. 

It may be seen in the above figure that the flower-stem of O. Carnosacircumnutated during two days about the same spot. On the other hand, theflower-stem of O. Sensitiva undergoes a strongly marked, daily, periodicalchange of position, when kept at a proper temperature. In the middle of theday it stands vertically up, or at a high angle; in the afternoon it sinks, and in the evening projects horizontally, or almost horizontally, risingagain during the night. This movement continues from the period when theflowers are in bud to when, as we believe, the pods are mature: and itought perhaps to have been included amongst the so-called sleep-movementsof plants. A tracing was not made, but the angles were measured atsuccessive periods during one whole day; and these showed that the movementwas not continuous, but that the peduncle oscillated up and down. We maytherefore conclude that it circumnutated. At the base of the peduncle thereis a mass of small cells, forming a well-developed pulvinus, which isexteriorly coloured purple and hairy. In no other genus, as far as we know, is the peduncle furnished with a pulvinus. The peduncle of O. Ortegesiibehaved differently from that of O. Sensitiva, for it stood at a less angleabove the horizon in the middle of the day, then in the morning or evening. By 10. 20 P. M. It had risen greatly. During the middle of the day itoscillated much up and down. 

Trifolium subterraneum. --A filament was fixed vertically to the uppermostpart of the peduncle of a young and upright flower-head (the stem of theplant having been secured to a stick); and its movements were traced during36 h. Within this time it described (see Fig. 92) a figure which representsfour ellipses; but during the latter part of the time the peduncle began tobend downwards, and after 10. 30 P. M. On the 24th it curved so rapidly down, that by 6. 45 A. M. On the 25th it stood only 19o above the horizon. It wenton circumnutating in nearly the same position for two days. Even after theflower-heads have buried themselves in the ground they continue, as willhereafter be shown, to circumnutate. It will also be seen in the nextchapter that the sub-peduncles of the separate flowers of[page 225]Trifolium repens circumnutate in a complicated course during several days. I may add that the gynophore of Arachis hypogoea, Fig. 92. Trifolium subterraneum: main flower-peduncle, illuminated fromabove, circumnutation traced on horizontal glass, from 8. 40 A. M. July 23rdto 10. 30 P. M. 24th. 

which looks exactly like a peduncle, circumnutates whilst growingvertically downwards, in order to bury the young pod in the ground. 

The movements of the flowers of Cyclamen Persicum were not observed; butthe peduncle, whilst the pod is forming, increases much in length, and bowsitself down by a circumnutating movement. A young peduncle of Maurandiasemperflorens, 1 ½ inch in length, was carefully observed during a wholeday, and it made 4 ½ narrow, vertical, irregular and short ellipses, eachat an average rate of about 2 h. 25 m. An adjoining peduncle describedduring the same time similar, though fewer, ellipses. * According to Sachs**the flower-stems, whilst growing, * 'The Movements and Habits of Climbing Plants, ' 2nd edit. , 1875, p. 68. 

** 'Text-Book of Botany, ' 1875, [[page 226]]p. 766. Linnaeus and Treviranus (according to Pfeffer, 'Die PeriodischenBewegungen, ' etc. , p. 162) state that the flower-stalks of many plantsoccupy different positions by night and day, and we shall see in thechapter on the Sleep of Plants that this implies circumnutation. [page 226]

of many plants, for instance, those of Brassica napus, revolve orcircumnutate; those of Allium porrum bend from side to side, and, if thismovement had been traced on a horizontal glass, no doubt ellipses wouldhave been formed. Fritz Müller has described* the spontaneous revolvingmovements of the flower-stems of an Alisma, which he compares with those ofa climbing plant. 

We made no observations on the movements of the different parts of flowers. Morren, however, has observed** in the stamens of Sparmannia and Cereus a"fremissement spontané, " which, it may be suspected, is a circumnutatingmovement. The circumnutation of the gynostemium of Stylidium, as describedby Gad, *** is highly remarkable, and apparently aids in the fertilisationof the flowers. The gynostemium, whilst spontaneously moving, comes intocontact with the viscid labellum, to which it adheres, until freed by theincreasing tension of the parts or by being touched. ]

We have now seen that the flower-stems of plants belonging to such widelydifferent families as the Cruciferae, Oxalidae, Leguminosae, Primulaceae, Scrophularineae, Alismaceae, and Liliaceae, circumnutate; and that thereare indications of this movement in many other families. With these factsbefore us, bearing also in mind that the tendrils of not a few plantsconsist of modified peduncles, we may admit without much doubt that allgrowing flower-stems circumnutate. 

CIRCUMNUTATION OF LEAVES: DICOTYLEDONS. 

Several distinguished botanists, Hofmeister, Sachs, Pfeffer, De Vries, Batalin, Millardet, etc. , have ob-

* 'Jenaische Zeitsch. , ' B. V. P. 133. 

** 'N. Mem. De l'Acad. R. De Bruxelles, ' tom. Xiv. 1841, p. 3. 

*** 'Sitzungbericht des bot. Vereins der P. Brandenburg, ' xxi. P. 84. [page 227]served, and some of them with the greatest care, the periodical movementsof leaves; but their attention has been chiefly, though not exclusively, directed to those which move largely and are commonly said to sleep atnight. From considerations hereafter to be given, plants of this nature arehere excluded, and will be treated of separately. As we wished to ascertainwhether all young and growing leaves circumnutated, we thought that itwould be sufficient if we observed between 30 and 40 genera, widelydistributed throughout the vegetable series, selecting some unusual formsand others on woody plants. All the plants were healthy and grew in pots. They were illuminated from above, but the light perhaps was not alwayssufficiently bright, as many of them were observed under a skylight ofground-glass. Except in a few specified cases, a fine glass filament withtwo minute triangles of paper was fixed to the leaves, and their movementswere traced on a vertical glass (when not stated to the contrary) in themanner already described. I may repeat that the broken lines represent thenocturnal course. The stem was always secured to a stick, close to the baseof the leaf under observation. The arrangement of the species, with thenumber of the Family appended, is the same as in the case of stems. 

Fig. 93. Sarracenia purpurea: circumnutation of young pitcher, traced from8 A. M. July 3rd to 10. 15 A. M. 4th. Temp. 17o - 18o C. Apex of pitcher 20inches from glass, so movement greatly magnified. 

(1. ) Sarracenia purpurea (Sarraceneae, Fam. 11). --A young leaf, or pitcher, 8 ½ inches in height, with the bladder swollen but with the hood not as yetopen, had a filament fixed transversely[page 228]across its apex; it was observed for 48 h. , and during the whole of thistime it circumnutated in a nearly similar manner, but to a very smallextent. The tracing given (Fig. 93) relates only to the movement during thefirst 26 h. 

(2) Glaucium luteum (Papaveraceae, Fam. 12). --A young plant, bearing only 8leaves, had a filament attached to the youngest leaf but one, which was 3inches in length, including the petiole. The circumnutating movement wastraced during 47 h. On both days the leaf descended from before 7 A. M. Until about 11 A. M. , and then ascended slightly during the rest of the dayand the early part of the night. During the latter part of the night itfell greatly. It did not ascend so much during the second as during thefirst day, and it descended considerably lower on the second night than onthe first. This difference was probably due to the illumination from abovehaving been insufficient during the two days of observation. Its courseduring the two days is shown in Fig. 94. 

Fig. 94. Glaucium luteum: circumnutation of young leaf, traced from 9. 30A. M. June 14th to 8. 30 A. M. 16th. Tracing not much magnified, as apex ofleaf stood only 5 ½ inches from the glass. 

(3. ) Crambe maritima (Cruciferae, Fam. 14). --A leaf 9 ½ inches in length ona plant not growing vigorously was first observed. Its apex was in constantmovement, but this could hardly be traced, from being so small in extent. The apex, however, certainly changed its course at least 6 times in thecourse of 14 h. A more vigorous young plant, bearing only 4 leaves, wasthen selected, and a filament was affixed to the midrib of the third leaffrom the base, which, with the petiole, was 5 inches in length. The leafstood up almost vertically, but the tip[page 229]was deflected, so that the filament projected almost horizontally, and itsmovements were traced during 48 h. On a vertical glass as shown in theaccompanying figure (Fig. 95). We here plainly see that the leaf wascontinually circumnutating; but the proper periodicity of its movements wasdisturbed by its being only dimly illuminated from above through a doubleskylight. We infer that this was the case, because two leaves on plantsgrowing out of doors, had their angles above the horizon measured in themiddle of the day and at 9 to about 10 P. M. On successive nights, and theywere found at this latter hour to have risen by an average angle of 9oabove their mid-day position: on the following morning they fell to theirformer position. Now it may be observed in the diagram that the leaf roseduring the second night, so that it stood at 6. 40 A. M. Higher than at 10. 20P. M. On the preceding night; and this may be attributed to the leafadjusting itself to the dim light, coming exclusively from above. 

Fig. 95. Crambe maritima: circumnutation of leaf, disturbed by beinginsufficiently illuminated from above, traced from 7. 50 A. M. June 23rd to 8A. M. 25th. Apex of leaf 15 1/4 inches from the vertical glass, so that thetracing was much magnified, but is here reduced to one-fourth of originalscale. 

(4. ) Brassica oleracea (Cruciferae). --Hofmeister and Batalin* state thatthe leaves of the cabbage rise at night, and fall by day. We covered ayoung plant, bearing 8 leaves, under a large bell-glass, placing it in thesame position with respect to the

* 'Flora, ' 1873, p. 437. [page 230]

light in which it had long remained, and a filament was fixed at thedistance of . 4 of an inch from the apex of a young leaf nearly 4 inches inlength. Its movements were then traced during three days, but the tracingis not worth giving. The leaf fell during the whole morning, and rose inthe evening and during the early part of the night. The ascending anddescending lines did not coincide, so that an irregular ellipse was formedeach 24 h. The basal part of the midrib did not move, as was ascertained bymeasuring at successive periods the angle which it formed with the horizon, so that the movement was confined to the terminal portion of the leaf, which moved through an angle of 11o in the course of 24 h. , and thedistance travelled by the apex, up and down, was between . 8 and . 9 of aninch. 

In order to ascertain the effect of darkness, a filament was fixed to aleaf 5 ½ inches in length, borne by a plant which after forming a head hadproduced a stem. The leaf was inclined 44o above the horizon, and itsmovements were traced on a vertical glass every hour by the aid of a taper. During the first day the leaf rose from 8 A. M. To 10. 40 P. M. In a slightlyzigzag course, the actual distance travelled by the apex being . 67 of aninch. During the night the leaf fell, whereas it ought to have risen; andby 7 A. M. On the following morning it had fallen . 23 of an inch, and itcontinued falling until 9. 40 A. M. It then rose until 10. 50 P. M. , but therise was interrupted by one considerable oscillation, that is, by a falland re-ascent. During the second night it again fell, but only to a veryshort distance, and on the following morning re-ascended to a very shortdistance. Thus the normal course of the leaf was greatly disturbed, orrather completely inverted, by the absence of light; and the movements werelikewise greatly diminished in amplitude. 

We may add that, according to Mr. A. Stephen Wilson, * the young leaves ofthe Swedish turnip, which is a hybrid between B. Oleracea and rapa, drawtogether in the evening so much "that the horizontal breadth diminishesabout 30 per cent. Of the daylight breadth. " Therefore the leaves must riseconsiderably at night. 

(5. ) Dianthus caryophyllus (Caryophylleae, Fam. 26). --The

* 'Trans. Bot. Soc. Edinburgh, ' vol. Xiii. P. 32. With respect to theorigin of the Swedish turnip, see Darwin, 'Animals and Plants underDomestication, ' 2nd edit. Vol. I. P. 344. [page 231]

terminal shoot of a young plant, growing very vigorously, was selected forobservation. The young leaves at first stand up vertically and closetogether, but they soon bend outwards and downwards, so as to becomehorizontal, and often at the same time a little to one side. A filament wasfixed to the tip of a young leaf whilst still highly inclined, and thefirst dot was made on the vertical glass at 8. 30 A. M. June 13th, but itcurved downwards so quickly that by 6. 40 A. M. On the following morning itstood only a little above the horizon. In Fig. 96

Fig. 96. Dianthus caryophyllus: circumnutation of young leaf, traced from10. 15 P. M. June 13th to 10. 35 P. M. 16th. Apex of leaf stood, at the closeof our observations, 8 3/4 inches from the vertical glass, so tracing notgreatly magnified. The leaf was 5 1/4 inches long. Temp. 15 1/2o - 17 1/2oC. 

the long, slightly zigzag line representing this rapid downward course, which was somewhat inclined to the left, is not given; but the figure showsthe highly tortuous and zigzag course, together with some loops, pursuedduring the next 2 ½ days. As the leaf continued to move all the time to theleft, it is evident that the zigzag line represents many circumnutations. 

(6. ) Camellia Japonica (Camelliaceae, Fam. 32). --A youngish leaf, whichtogether with its petiole was 2 3/4 inches in length and which arose from aside branch on a tall bush, had a filament attached to its apex. This leafsloped downwards at an angle of 40o beneath the horizon. As it was thickand rigid, and its[page 232]petiole very short, much movement could not be expected. Nevertheless, theapex changed its course completely seven times in the course of 11 ½ h. , but moved to only a very small distance. On the next day the movement ofthe apex was traced during 26 h. 20 m. (as shown in Fig. 97), and wasnearly of the same nature, but rather less complex. The movement seems tobe periodical, for on both days the leaf circumnutated in the forenoon, fell in the afternoon (on the first day until between 3 and 4 P. M. , and onthe second day until 6 P. M. ), and then rose, falling again during the nightor early morning. 

Fig. 97. Camellia Japonica: circumnutation of leaf, traced from 6. 40 A. M. June 14th to 6. 50 A. M. 15th. Apex of leaf 12 inches from the verticalglass, so figure considerably magnified. Temp. 16o - 16 1/2o C. 

In the chapter on the Sleep of Plants we shall see that the leaves inseveral Malvaceous genera sink

Fig. 98. Pelargonium zonale: circumnutation and downward movement of youngleaf, traced from 9. 30 A. M. June 14th to 6. 30 P. M. 16th. Apex of leaf 9 1. 4inches from the vertical glass, so figure moderately magnified. Temp. 15o -16 1/2o C. 

at night; and as they often do not then occupy a vertical position, especially if they have not been well illuminated during[page 233]the day, it is doubtful whether some of these cases ought not to have beenincluded in the present chapter. 

(7. ) Pelargonium zonale (Geraniaceae, Fam. 47). --A young leaf, 1 1/4 inchin breadth, with its petiole 1 inch long, borne on a young plant, wasobserved in the usual manner during 61 h. ; and its course is shown in thepreceding figure (Fig. 98). During the first day and night the leaf moveddownwards, but circumnutated between 10 A. M. And 4. 30 P. M. On the secondday it sank and rose again, but between 10 A. M. And 6 P. M. It circumnutatedon an extremely small scale. On the third day the circumnutation was moreplainly marked. 

(8. ) Cissus discolor (Ampelideae, Fam. 67). --A leaf, not nearly full-grown, the third from the apex of a shoot on a cut-down plant, was observed during31 h. 30 m. (see Fig. 99). The day was cold (15o - 16o C. ), and if theplant had been observed in the hot-house, the circumnutation, though plainenough as it was, would probably have been far more conspicuous. 

Fig. 99. Cissus discolor: circumnutation of leaf, traced from 10. 35 A. M. May 28th to 6 P. M. 29th. Apex of leaf 8 3/4 inches from the vertical glass. 

(9. ) Vicia faba (Leguminosae, Fam. 75). --A young leaf, 3. 1 inches inlength, measured from base of petiole to end of leaflets, had a filamentaffixed to the midrib of one of the two terminal leaflets, and itsmovements were traced during 51 ½ h. The filament fell all morning (July2nd) till 3 P. M. , and then rose greatly till 10. 35 P. M. ; but the rise thisday was so great, compared with that which subsequently occurred, that itwas probably due in part to the plant being illuminated from above. Thelatter part of the course on July 2nd is alone given in the followingfigure (Fig. 100). On the next day (July 3rd) the leaf again fell in themorning, then circumnutated in a conspicuous manner, and rose till late atnight; but the movement was not traced after 7. 15 P. M. , as by that time thefilament pointed towards the upper edge of the glass. During the latterpart of the night or early morning it again fell in the same manner asbefore. [page 234]

As the evening rise and the early morning fall were unusually large, theangle of the petiole above the horizon was measured at the two periods, andthe leaf was found to have risen 19o

Fig. 100. Vicia faba: circumnutation of leaf, traced from 7. 15 P. M. July2nd to 10. 15 A. M. 4th. Apex of the two terminal leaflets 7 1/4 inches fromthe vertical glass. Figure here reduced to two-thirds of original scale. Temp. 17o - 18o C. 

between 12. 20 P. M. And 10. 45 P. M. , and to have fallen 23o 30 secondsbetween the latter hour and 10. 20 A. M. On the following morning. 

The main petiole was now secured to a stick close to the base[page 235]of the two terminal leaflets, which were 1. 4 inch in length; and themovements of one of them were traced during 48 h. (see Fig. 101). Thecourse pursued is closely analogous to that of the whole leaf. The zigzagline between 8. 30 A. M. And 3. 30 P. M. On the second day represents 5 verysmall ellipses, with theirFig 101. Vicia faba: circumnutation of one of the two terminal leaflets, the main petiole having been secured, traced from 10. 40 A. M. July 4th to10. 30 A. M. 6th. Apex of leaflet 6 5/8 inches from the vertical glass. Tracing here reduced to one-half of original scale. Temp. 16o - 18o C. 

longer axes differently directed. From these observations it follows thatboth the whole leaf and the terminal leaflets undergo a well-marked dailyperiodical movement, rising in the evening and falling during the latterpart of the night or early morning; whilst in the middle of the day theygenerally circumnutate round the same small space. [page 236]

(10. ) Acacia retinoides (Leguminosae). --The movement of a young phyllode, 23/8 inches in length, and inclined at a considerable angle above thehorizon, was traced during 45 h. 30 m. ; but in the figure here given (Fig. 102), its circumnutation is shown during only 21 h. 30 m. During part ofthis time (viz. , 14 h. 30 m. ) the phyllode described a figure representing5 or 6 small ellipses. The actual amount of movement in a verticaldirection was . 3 inch. The phyllode rose considerably between 1. 30 P. M. And4 P. M. , but there was no evidence on either day of a regular periodicmovement. 

Fig. 102. Acacia retinoides: circumnutation of a young phyllode, tracedfrom 10. 45 A. M. July 18th to 8. 15 A. M. 19th. Apex of phyllode 9 inches fromthe vertical glass; temp. 16 1/2o - 17 1/2o C. 

(11. ) Lupinus speciosus (Leguminosae). --Plants were raised from seedpurchased under this name. This is one of the species in this large genus, the leaves of which do not sleep at night. The petioles rise direct fromthe ground, and are from 5 to 7 inches in length. A filament was fixed tothe midrib of one of the longer leaflets, and the movement of the wholeleaf was traced, as shown in Fig. 103. In the course of 6 h. 30 m. Thefilament went four times up and three times down. A new tracing was thenbegun (not here given), and during 12 ½ h. The leaf moved eight times upand seven times down; so that it described 7 ½ ellipses in this time, andthis is an extraordinary rate of movement. The summit of the petiole wasthen secured to a stick, and the separate leaflets were found to becontinually circumnutating. 

Fig. 103. Lupinus speciosus: circumnutation of leaf, traced on verticalglass, from 10. 15 A. M. To 5. 45 P. M. ; i. E. , during 6 h. 30 m. [page 237]

(12. ) Echeveria stolonifera (Crassulaceae, Fam. 84). --The older leaves ofthis plant are so thick and fleshy, and the young ones so short and broad, that it seemed very improbable that any circumnutation could be detected. Afilament was fixed to a young upwardly inclined leaf, . 75 inch in lengthand . 28 in breadth, which stood on the outside of a terminal rosette ofleaves, produced by a plant growing very vigorously. Its movement wastraced during 3 days, as here shown (Fig. 104). The course was chiefly inan upward direction, and this may be attributed to the elongation of theleaf through growth; but we see that the lines are strongly zigzag, andthat occasionally there was distinct circumnutation, though on a very smallscale. 

Fig. 104. Echeveria stolonifera: circumnutation of leaf, traced from 8. 20A. M. June 25th to 8. 45 A. M. 28th. Apex of leaf 12 1/4 inches from theglass, so that the movement was much magnified; temp. 23o - 24 1/2o C. (13. ) Bryophyllum (vel Calanchae) calycinum (Crassulaceae). --Duval-Jouve('Bull. Soc. Bot. De France, ' Feb. 14th, 1868) measured the distancebetween the tips of the upper pair of leaves on this plant, with the resultshown in the following Table. It should be noted that the measurements onDec. 2nd were made on a different pair of leaves: --

 8 A. M. 2 P. M. 7 P. M. Nov. 16. . . . . . . . . . . . . . . . . . . 15 mm.. . . . . . 25 mm. . . ... . . (?) " 19. . . . . . . . . . . . . . . . . . . 48 " . . . . . . . 60 ". .. . . . . 48 mm. Dec. 2. . . . . . . . . . . . . . . . . . . 22 ". . . . . . . . 43 ". . .. . . . . 28 "

We see from this Table that the leaves stood considerably further apart at2 P. M. Than at either 8 A. M. Or 7 P. M. ; and this shows that they rise alittle in the evening and fall or open in the forenoon. 

(14. ) Drosera rotundifolia (Droseraceae, Fam. 85). --The movements of ayoung leaf, having a long petiole but with its tentacles (or gland-bearinghairs) as yet unfolded, were traced during 47 h. 15 m. The figure (Fig. 105) shows that it circumnutated largely, chiefly in a vertical direction, making two ellipses each[page 238]day. On both days the leaf began to descend after 12 or 1 o'clock, andcontinued to do so all night, though to a very unequal distance on the twooccasions. We therefore thought that the movement was periodic; but onobserving three other leaves during several successive days and nights, wefound this to be an error; and the case is given merely as a caution. Onthe third morning the above leaf occupied almost exactly the same positionas on the first morning; and the tentacles by this time had unfoldedsufficiently to project at right angles to the blade or disc. 

Fig. 105. Drosera rotundifolia: circumnutation of young leaf, with filamentfixed to back of blade, traced from 9. 15 A. M. June 7th to 8. 30 A. M. June9th. Figure here reduced to one-half original scale. 

The leaves as they grow older generally sink more and more downwards. Themovement of an oldish leaf, the glands of which were still secretingfreely, was traced for 24 h. , during which time it continued to sink alittle in a slightly zigzag line. On the following morning, at 7 A. M. , adrop of a solution of carbonate of ammonia (2 gr. To 1 oz. Of water) wasplaced on the disc, and this blackened the glands and induced inflection ofmany of the tentacles. The weight of the drop caused the leaf at first tosink a little; but immediately afterwards it began to rise in a somewhatzigzag course, and continued to do so till 3 P. M. It then circumnutatedabout the same spot on a very small scale for 21 h. ; and during the next 21h. It sank in a zigzag line to nearly the same level which it had held whenthe ammonia was first administered. By this time the tentacles hadre-expanded, and the glands had recovered their proper colour. We thuslearn that an old leaf[page 239]circumnutates on a small scale, at least whilst absorbing carbonate ofammonia; for it is probable that this absorption may stimulate growth andthus re-excite circumnutation. Whether the rising of the glass filamentwhich was attached to the back of the leaf, resulted from its marginbecoming slightly inflected (as generally occurs), or from the rising ofthe petiole, was not ascertained. 

In order to learn whether the tentacles or gland-bearing hairscircumnutate, the back of a young leaf, with the innermost tentacles as yetincurved, was firmly cemented with shellac to a flat stick driven intocompact damp argillaceous sand. The plant was placed under a microscopewith the stage removed and with an eye-piece micrometer, of which eachdivision equalled 1/500 of an inch. It should be stated that as the leavesgrow older the tentacles of the exterior rows bend outwards and downwards, so as ultimately to become deflected considerably beneath the horizon. Atentacle in the second row from the margin was selected for observation, and was found to be moving outwards at a rate of 1/500 of an inch in 20 m. , or 1/100 of inch in 1 h. 40 m. ; but as it likewise moved from side to sideto an extent of above 1/500 of inch, the movement was probably one ofmodified circumnutation. A tentacle on an old leaf was next observed in thesame manner. In 15 m. After being placed under the microscope it had movedabout 1/1000 of an inch. During the next 7 ½ h. It was looked atrepeatedly, and during this whole time it moved only another 1/1000 of aninch; and this small movement may have been due to the settling of the dampsand (on which the plant rested), though the sand had been firmly presseddown. We may therefore conclude that the tentacles when old do notcircumnutate; yet this tentacle was so sensitive, that in 23 seconds afterits gland had been merely touched with a bit of raw meat, it began to curlinwards. This fact is of some importance, as it apparently shows that theinflection of the tentacles from the stimulus of absorbed animal matter(and no doubt from that of contact with any object) is not due to modifiedcircumnutation. 

(15. ) Dionoea muscipula (Droseraceae). --It should be premised that theleaves at an early stage of their development have the two lobes pressedclosely together. These are at first directed back towards the centre ofthe plant; but they gradually rise up and soon stand at right angles to thepetiole, and ultimately in nearly a straight line with it. A young leaf, which with the[page 240]petiole was only 1. 2 inch in length, had a filament fixed externally alongthe midrib of the still closed lobes, which projected at right angles tothe petiole. In the evening this leaf completed an ellipse in the course of2 h. On the following day (Sept. 25th) its movements were traced during 22h. ; and we see in Fig. 106 that it moved in the same general direction, dueto the straightening of the leaf, but in an extremely zigzag line. Thisline represents several drawn-out or modified ellipses. There can thereforebe no doubt that this young leaf circumnutated. 

Fig. 106. Dionaea muscipula: circumnutation of a young and expanding leaf, traced on a horizontal glass in darkness, from noon Sept. 24th to 10 A. M. 25th. Apex of leaf 13 ½ inches from the glass, so tracing considerablymagnified. 

A rather old, horizontally extended leaf, with a filament attached alongthe under side of the midrib, was next observed during 7 h. It hardlymoved, but when one of its sensitive hairs was touched, the blades closed, though not very quickly. A new dot was now made on the glass, but in thecourse of 14 h. 20 m. There was hardly any change in the position of thefilament. We may therefore infer that an old and only moderately sensitiveleaf does not circumnutate plainly; but we shall soon see that it by nomeans follows that such a leaf is absolutely motionless. We may furtherinfer that the stimulus from a touch does not re-excite plaincircumnutation. 

Another full-grown leaf had a filament attached externally along one sideof the midrib and parallel to it, so that the filament would move if thelobes closed. It should be first stated that, although a touch on one ofthe sensitive hairs of a vigorous leaf causes it to close quickly, oftenalmost instantly, yet when a bit of damp meat or some solution of carbonateof ammonia is placed on the lobes, they close so slowly that generally 24h. Is required for the completion of the act. The above leaf was firstobserved for 2 h. 30 m. , and did not circumnutate, but it ought to havebeen observed for a[page 241]longer period; although, as we have seen, a young leaf completed a fairlylarge ellipse in 2 h. A drop of an infusion of raw meat was then placed onthe leaf, and within 2 h. The glass filament rose a little; and thisimplies that the lobes had begun to close, and perhaps the petiole to rise. It continued to rise with extreme slowness for the next 8 h. 30 m. Theposition of the pot was then (7. 15 P. M. , Sept. 24th) slightly changed andan additional drop of the infusion given, and a new tracing was begun (Fig. 107). By 10. 50 P. M. The filament had risen only a little more, and it fellduring the night. On the following morning the lobes were closing morequickly, and by 5 P. M. It was evident to the eye that they had closedconsiderably; by 8. 48. P. M. This was still plainer, and by 10. 45 P. M. Themarginal spikes were interlocked. The leaf fell a little during the night, and next morning (25th) at 7 A. M. The lobes were completely shut. Thecourse pursued, as may be seen in the figure, was

Fig. 107. Dionoea muscipula: closure of the lobes and circumnutation of afull-grown leaf, whilst absorbing an infusion of raw meat, traced indarkness, from 7. 15 P. M. Sept. 24th to 9 A. M. 26th. Apex of leaf 8 ½ inchesfrom the vertical glass. Figure here reduced to two-thirds of originalscale. 

strongly zigzag, and this indicates that the closing of the lobes wascombined with the circumnutation of the whole leaf; and there cannot bemuch doubt, considering how motionless the leaf was during 2 h. 30 m. Before it received the infusion, that the absorption of the animal matterhad excited it to circumnutate. The leaf was occasionally observed for thenext four days, but was kept in rather too cool a place; nevertheless, itcontinued to circumnutate to a small extent, and the lobes remained closed. 

It is sometimes stated in botanical works that the lobes close or sleep atnight; but this is an error. To test the statement, very long glassfilaments were fixed inside the two lobes of three leaves, and thedistances between their tips were measured in the middle of the day and atnight; but no difference could be detected. 

The previous observations relate to the movements of the whole leaf, butthe lobes move independently of the petiole, and[page 242]seem to be continually opening and shutting to a very small extent. Anearly full-grown leaf (afterwards proved to be highly sensitive tocontact) stood almost horizontally, so that by driving a long thin pinthrough the foliaceous petiole close to the blade, it was renderedmotionless. The plant, with a little triangle of paper attached to one ofthe marginal spikes, was placed under a microscope with an eye-piecemicrometer, each division of which equalled 1/500 of an inch. The apex ofthe paper-triangle was now seen to be in constant slight movement; for in 4h. It crossed nine divisions, or 9/500 of an inch, and after ten additionalhours it moved back and had crossed 5/500 in an opposite direction. Theplant was kept in rather too cool a place, and on the following day itmoved rather less, namely, 1/500 in 3 h. , and 2/500 in an oppositedirection during the next 6 h. The two lobes, therefore, seem to beconstantly closing or opening, though to a very small distance; for we mustremember that the little triangle of paper affixed to the marginal spikeincreased its length, and thus exaggerated somewhat the movement. Similarobservations, with the important difference that the petiole was left freeand the plant kept under a high temperature, were made on a leaf, which washealthy, but so old that it did not close when its sensitive hairs wererepeatedly touched, though judging from other cases it would have slowlyclosed if it had been stimulated by animal matter. The apex of the trianglewas in almost, though not quite, constant movement, sometimes in onedirection and sometimes in an opposite one; and it thrice crossed fivedivisions of the micrometer (i. E. 1/100 of an inch) in 30 m. This movementon so small a scale is hardly comparable with ordinary circumnutation; butit may perhaps be compared with the zigzag lines and little loops, by whichthe larger ellipses made by other plants are often interrupted. 

In the first chapter of this volume, the remarkable oscillatory movementsof the circumnutating hypocotyl of the cabbage have been described. Theleaves of Dionaea present the same phenomenon, which is a wonderful one, asviewed under a low power (2-inch object-glass), with an eye-piecemicrometer of which each division (1/500 of an inch) appeared as a ratherwide space. The young unexpanded leaf, of which the circumnutatingmovements were traced (Fig. 106), had a glass filament fixedperpendicularly to it; and the movement of the apex was observed in thehot-house (temp. 84o to 86o F. ), with light admitted only from above, andwith any lateral currents of air[page 243]excluded. The apex sometimes crossed one or two divisions of the micrometerat an imperceptibly slow rate, but generally it moved onwards by rapidstarts or jerks of 2/1000 or 3/1000, and in one instance of 4/1000 of aninch. After each jerk forwards, the apex drew itself backwards withcomparative slowness for part of the distance which had just been gained;and then after a very short time made another jerk forwards. Fourconspicuous jerks forwards, with slower retreats, were seen on one occasionto occur in exactly one minute, besides some minor oscillations. As far aswe could judge, the advancing and retreating lines did not coincide, and ifso, extremely minute ellipses were each time described. Sometimes the apexremained quite motionless for a short period. Its general course during theseveral hours of observation was in two opposite directions, so that theleaf was probably circumnutating. 

An older leaf with the lobes fully expanded, and which was afterwardsproved to be highly sensitive to contact, was next observed in a similarmanner, except that the plant was exposed to a lower temperature in a room. The apex oscillated forwards and backwards in the same manner as before;but the jerks forward were less in extent, viz. About 1/1000 inch; andthere were longer motionless periods. As it appeared possible that themovements might be due to currents of air, a wax taper was held close tothe leaf during one of the motionless periods, but no oscillations werethus caused. After 10 m. , however, vigorous oscillations commenced, perhapsowing to the plant having been warmed and thus stimulated. The candle wasthen removed and before long the oscillations ceased; nevertheless, whenlooked at again after an interval of 1 h. 30 m. , it was again oscillating. The plant was taken back into the hot-house, and on the following morningwas seen to be oscillating, though not very vigorously. Another old buthealthy leaf, which was not in the least sensitive to a touch, was likewiseobserved during two days in the hot-house, and the attached filament mademany little jerks forwards of about 2/1000 or only 1/1000 of an inch. 

Finally, to ascertain whether the lobes independently of the petioleoscillated, the petiole of an old leaf was cemented close to the blade withshellac to the top of a little stick driven into the soil. But before thiswas done the leaf was observed, and found to be vigorously oscillating orjerking; and after it had been cemented to the stick, the oscillations ofabout 2/1000 of an inch still continued. On the following day a littleinfusion[page 244]of raw meat was placed on the leaf, which caused the lobes to closetogether very slowly in the course of two days; and the oscillationscontinued during this whole time and for the next two days. After nineadditional days the leaf began to open and the margins were a littleeverted, and now the apex of the glass filament remained for long periodsmotionless, and then moved backwards and forwards for a distance of about1/1000 of an inch slowly, without any jerks. Nevertheless, after warmingthe leaf with a taper held close to it, the jerking movement recommenced. 

This same leaf had been observed 2 ½ months previously, and was then foundto be oscillating or jerking. We may therefore infer that this kind ofmovement goes on night and day for a very long period; and it is common toyoung unexpanded leaves and to leaves so old as to have lost theirsensitiveness to a touch, but which were still capable of absorbingnitrogenous matter. The phenomenon when well displayed, as in the youngleaf just described, is a very interesting one. It often brought before ourminds the idea of effort, or of a small animal struggling to escape fromsome constraint. 

(16. ) Eucalyptus resinifera (Myrtaceae, Fam. 94). --A young leaf, two inchesin length together with the petiole, produced by a lateral shoot from acut-down tree, was observed in the usual manner. The blade had not as yetassumed its vertical position. On June 7th only a few observations weremade, and the tracing merely showed that the leaf had moved three timesupwards and three downwards. On the following day it was observed morefrequently; and two tracings were made (see A and B, Fig. 108), as a singleone would have been too complicated. The apex changed its course 13 timesin the course of 16 h. , chiefly up and down, but with some lateralmovement. The actual amount of movement in any one direction was small. 

Fig. 108. Eucalyptus resinifera: circumnutation of a leaf, traced, A, from6. 40 A. M. To 1 P. M. June 8th; B, from 1 P. M. 8th to 8. 30 A. M. 9th. Apex ofleaf 14 ½ inches from the horizontal glass, so figures considerablymagnified. 

(17. ) Dahlia (garden var. ) (Compositae, Fam. 122). --A fine young[page 245]leaf 5 3/4 inches in length, produced by a young plant 2 feet high, growingvigorously in a large pot, was directed at an angle of about 45o beneaththe horizon. On June 18th the leaf descended from 10 A. M. Till 11. 35 A. M. (see Fig. 109); it then ascended greatly till 6 P. M. , this ascent beingprobably due to the light

Fig. 109. Dahlia: circumnutation of leaf, traced from 10 A. M. June 18th to8. 10 A. M. 20th, but with a break of 1 h. 40 m. On the morning of the 19th, as, owing to the glass filament pointing too much to one side, the pot hadto be slightly moved; therefore the relative position of the two tracingsis somewhat arbitrary. The figure here given is reduced to one-fifth of theoriginal scale. Apex of leaf 9 inches from the glass in the line of itsinclination, and 4 3/4 in a horizontal line. Coming only from above. It zigzagged between 6 P. M. And 10. 35 P. M. , andascended a little during the night. It should be remarked that the verticaldistances in the lower part of the diagram are much exaggerated, as theleaf was at first deflected beneath the horizon, and after it had sunkdownwards, the filament pointed in a very oblique line towards the glass. Next[page 246]day the leaf descended from 8. 20 A. M. Till 7. 15 P. M. , then zigzagged andascended greatly during the night. On the morning of the 20th the leaf wasprobably beginning to descend, though the short line in the diagram ishorizontal. The actual distances travelled by the apex of the leaf wereconsiderable, but could not be calculated with safety. From the coursepursued on the second day, when the plant had accommodated itself to thelight from above, there cannot be much doubt that the leaves undergo adaily periodic movement, sinking during the day and rising at night. 

(18. ) Mutisia clematis (Compositae). --The leaves terminate in tendrils andcircumnutate like those of other tendril-bearers; but this plant is herementioned, on account of an erroneous statement* which has been published, namely, that the leaves sink at night and rise during the day. The leaveswhich behaved in this manner had been kept for some days in a northern roomand had not been sufficiently illuminated. A plant therefore was leftundisturbed in the hot-house, and three leaves had their angles measured atnoon and at 10 P. M. All three were inclined a little beneath the horizon atnoon, but one stood at night 2o, the second 21o, and the third 10o higherthan in the middle of the day; so that instead of sinking they rise alittle at night. 

(19. ) Cyclamen Persicum (Primulaceae, Fam. 135). --A young leaf, 1. 8 of aninch in length, petiole included, produced by an old root-stock, wasobserved during three days in the usual manner (Fig. 110). On the first daythe leaf fell more than afterwards, apparently from adjusting itself to thelight from above. On all three days it fell from the early morning to about7 P. M. , and from that hour rose during the night, the course being slightlyzigzag. The movement therefore is strictly periodic. It should be notedthat the leaf would have sunk each evening a little lower down than it did, had not the glass filament rested between 5 and 6 P. M. On the rim of thepot. The amount of movement was considerable; for if we assume that thewhole leaf to the base of the petiole became bent, the tracing would bemagnified rather less than five times, and this would give to the apex arise and fall of half an inch, with some lateral movement. This amount, however, would not attract attention without the aid of a tracing ormeasurement of some kind. 

* 'The Movements and Habits of Climbing Plants, ' 1875, p. 118. [page 247]

 (20. ) Allamanda Schottii (Apocyneae, Fam. 144). --The young leaves of thisshrub are elongated, with the blade bowed so much

Fig. 110. Cyclamen Persicum: circumnutation of leaf, traced from 6. 45 A. M. June 2nd to 6. 40 A. M. 5th. Apex of leaf 7 inches from the vertical glass. 

downwards as almost to form a semicircle. The chord--that is, a line drawnfrom the apex of the blade to the base of the petiole--of a young leaf, 43/4 inches in length, stood at 2. 50 P. M. On[page 248]Dec. 5th at an angle of 13o beneath the horizon, but by 9. 30 P. M. The bladehad straightened itself so much, which implies the raising of the apex, that the chord now stood at 37o above the horizon, and had therefore risen50o. On the next day similar angular measurements of the same leaf weremade; and at noon the chord stood 36o beneath the horizon, and 9. 30 P. M. 31/2o above it, so had risen 39 1/2o. The chief cause of the rising movementlies in the straightening of the blade, but the short petiole rises between4o and 5o. On the third night the chord stood at 35o above the horizon, andif the leaf occupied the same position at noon, as on the previous day, ithad risen 71o. With older leaves no such change of curvature could bedetected. The plant was then brought into the house and kept in anorth-east room, but at night there was no change in the curvature of theyoung leaves; so that previous exposure to a strong light is apparentlyrequisite for the periodical change of curvature in the blade, and for theslight rising of the petiole. 

(21. ) Wigandia (Hydroleaceae, Fam. 149). --Professor Pfeffer informs us thatthe leaves of this plant rise in the evening; but as we do not know whetheror not the rising is great, this species ought perhaps to be classedamongst sleeping plants. 

Fig. 111. Petunia violacea: downward movement and circumnutation of a veryyoung leaf, traced from 10 A. M. June 2nd to 9. 20 A. M. June 6th. N. B. --At6. 40 A. M. On the 5th it was necessary to move the pot a little, and a newtracing was begun at the point where two dots are not joined in thediagram. Apex of leaf 7 inches from the vertical glass. Temp. Generally 171/2o C. [page 249]

(22. ) Petunia violacea (Solaneae, Fam. 157). --A very young leaf, only 3/4inch in length, highly inclined upwards, was observed for four days. Duringthe whole of this time it bent outwards and downwards, so as to become moreand more nearly horizontal. The strongly marked zigzag line in the figureon p. 248 (Fig. 111), shows that this was effected by modifiedcircumnutation; and during the latter part of the time there was muchordinary circumnutation on a small scale. The movement in the diagram ismagnified between 10 and 11 times. It exhibits a clear trace ofperiodicity, as the leaf rose a little each evening; but this upwardtendency appeared to be almost conquered by the leaf striving to becomemore and more horizontal as it grew older. The angles which two olderleaves formed together, were measured in the evening and about noon on 3successive days, and each night the angle decreased a little, thoughirregularly. 

Fig. 112. Acanthus mollis: circumnutation of young leaf, traced from 9. 20A. M. June 14th to 8. 30 A. M. 16th. Apex of leaf 11 inches from the verticalglass, so movement considerably magnified. Figure here reduced to one-halfof original scale. Temp. 15o - 16 1/2o C. 

(23. ) Acanthus mollis (Acanthaceae, Fam. 168). --The younger of two leaves, 2 1/4 inches in length, petiole included, produced by a seedling plant, wasobserved during 47 h. Early on each of the three mornings, the apex of theleaf fell; and it continued to fall till 3 P. M. , on the two afternoons whenobserved. After 3 P. M. It rose considerably, and continued to rise on thesecond night until the early morning. But on the first night it fellinstead of rising, and we have little doubt that this was owing to the leafbeing very young and becoming through epinastic growth more and morehorizontal; for it may be seen in the diagram (Fig. 112), that the leafstood on a higher level on the first than on the second day. The leaves ofan allied species ('A. Spinosus') certainly rose every night; and the risebetween noon and 10. 15 P. M. , when measured on one occasion, was 10o. Thisrise was chiefly[page 250]or exclusively due to the straightening of the blade, and not to themovement of the petiole. We may therefore conclude that the leaves ofAcanthus circumnutate periodically, falling in the morning and rising inthe afternoon and night. 

(24. ) Cannabis sativa (Cannabineae, Fam. 195). --We have here the rare caseof leaves moving downwards in the evening, but not to a sufficient degreeto be called sleep. * In the early morning, or in the latter part of thenight, they move upwards. For instance, all the young leaves near thesummits of several stems stood almost horizontally at 8 A. M. May 29th andat 10. 30 P. M. Were considerably declined. On a subsequent day two leavesstood at 2 P. M. At 21o and 12o beneath the horizon, and at 10 P. M. At 38obeneath it. Two other leaves on a younger plant were horizontal at 2 P. M. , and at 10 P. M. Had sunk to 36o beneath the horizon. With respect to thisdownward movement of the leaves, Kraus believes that it is due to theirepinastic growth. He adds, that the leaves are relaxed during the day, andtense at night, both in sunny and rainy weather. 

(25. ) Pinus pinaster (Coniferae, Fam. 223). --The leaves on the summits ofthe terminal shoots stand at first in a bundle almost upright, but theysoon diverge and ultimately become almost horizontal. The movements of ayoung leaf, nearly one inch in length, on the summit of a seedling plantonly 3 inches high, were traced from the early morning of June 2nd to theevening of the 7th. During these five days the leaf diverged, and its apexdescended at first in an almost straight line; but during the two latterdays it zigzagged so much that it was evidently circumnutating. The samelittle plant, when grown to a height of 5 inches, was again observed duringfour days. A filament was fixed transversely to the apex of a leaf, oneinch in length, and which had already diverged considerably from itsoriginally upright position. It continued to diverge (see A, Fig. 113), andto descend from 11. 45 A. M. July 31st to 6. 40 A. M. Aug. 1st. On August 1stit circumnutated about the same small space, and again descended at night. Next morning the pot was moved nearly one inch to the right, and a newtracing was begun (B). From this time, viz. , 7 A. M. August 2nd to 8. 20 A. M. On the 4th, * We were led to observe this plant by Dr. Carl Kraus' paper, 'Beiträge zurKentniss der Bewegungen Wachsender Laubblätter, ' Flora, 1879, p. 66. Weregret that we cannot fully understand parts of this paper. [page 251]

the leaf manifestly circumnutated. It does not appear from the diagram thatthe leaves move periodically, for the descending course during the firsttwo nights, was clearly due to epinastic

Fig. 113. Pinus pinaster: circumnutation of young leaf, traced from 11. 45A. M. July 31st to 8. 20 A. M. Aug. 4th. At 7 A. M. Aug. 2nd the pot was movedan inch to one side, so that the tracing consists of two figures. Apex ofleaf 14 ½ inches from the vertical glass, so movements much magnified. 

growth, and at the close of our observations the leaf was not nearly sohorizontal as it would ultimately become. 

Pinus austriaca. --Two leaves, 3 inches in length, but not[page 252]quite fully grown, produced by a lateral shoot, on a young tree 3 feet inheight, were observed during 29 h. (July 31st), in the same manner as theleaves of the previous species. Both these leaves certainly circumnutated, making within the above period two, or two and a half, small, irregularellipses. 

(26. ) Cycas pectinata (Cycadeae, Fam. 224). --A young leaf, 11 ½ inches inlength, of which the leaflets had only recently become uncurled, wasobserved during 47 h. 30 m. The main petiole was secured to a stick at thebase of the two terminal leaflets. To one of the latter, 3 3/4 inches inlength, a filament was fixed; the leaflet was much bowed downward, but asthe terminal part was upturned, the filament projected almost horizontally. The leaflet moved (see Fig. 114) largely and periodically, for it felluntil about 7 P. M. And rose during the night, falling again next morningafter 6. 40 A. M. The descending lines are in a marked manner zigzag, and soprobably would have been the ascending lines, if they had been tracedthroughout the night. 

Fig. 114. Cycas pectinata: circumnutation of one of the terminal leaflets, traced from 8. 30 A. M. June 22nd to 8 A. M. June 24th. Apex of leaflet 7 3/4inches from the vertical glass, so tracing not greatly magnified, and herereduced to one-third of original scale; temp. 19o - 21o C. 

CIRCUMNUTATION OF LEAVES: MONOCOTYLEDONS. 

(27. ) Canna Warscewiczii (Cannaceae, Fam. 2). --The movements of a youngleaf, 8 inches in length and 3 ½ in breadth, produced by a vigorous youngplant, were observed during 45 h. 50 m. , as shown in Fig. 115. The pot wasslided about an inch to the right on the morning of the 11th, as a singlefigure would have been too complicated; but the two figures are continuousin time. The movement is periodical, as the leaf descended from the earlymorning until about 5 P. M. , and ascended during the rest of the evening and[page 253]part of the night. On the evening of the 11th it circumnutated on a smallscale for some time about the same spot. 

Fig. 115. Canna Warscewiczii: circumnutation of leaf, traced (A) from 11. 30A. M. June 10th to 6. 40 A. M. 11th; and (B) from 6. 40 A. M. 11th to 8. 40 A. M. 12th. Apex of leaf 9 inches from the vertical glass. 

(28. ) Iris pseudo-acorus (Irideae, Fam. 10). --The movements of a youngleaf, rising 13 inches above the water in which the plant grew, were tracedas shown in the figure (Fig. 116), during 27 h. 30 m. It manifestlycircumnutated, though only to a small extent. On the second morning, between 6. 40 A. M. And 2 P. M. (at which latter hour the figure here givenends), the apex changed its course five times. During the next 8 h. 40 m. It zigzagged much, and descended as far as the lowest dot in the figure, making in its course two very small ellipses; but if these lines had beenadded to the diagram it would have been too complex. 

Fig. 116. Iris pseudo-acorus: circumnutation of leaf, traced from 10. 30A. M. May 28th to 2 P. M. 29th. Tracing continued to 11 P. M. , but not herecopied. Apex of leaf 12 inches beneath the horizontal glass, so figureconsiderably magnified. Temp. 15o - 16o C. (29. ) Crinum Capense (Amaryllideae, Fam. 11). --The leaves of this plant areremarkable for their great length and narrowness: one was measured andfound to be 53 inches long and only 1. 4 broad at the base. Whilst quiteyoung they stand up almost vertically to the height of about a foot;afterwards[page 254]their tips begin to bend over, and subsequently hang vertically down, andthus continue to grow. A rather young leaf was selected, of which thedependent tapering point was as yet only 5 ½ inches in length, the uprightbasal part being 20 inches high, though this part would ultimately becomeshorter by being more bent over. A large bell-glass was placed over theplant, with a black dot on one side; and by bringing the dependent apex ofthe leaf into a line with this dot, the accompanying figure (Fig. 117) wastraced on the other side of the bell, during 2 ½ days. During the first day(22nd) the tip travelled laterally far to the left, perhaps in consequenceof the plant having been

Fig. 117. Crinum Capense: circumnutation of dependent tip of young leaf, traced on a bell-glass, from 10. 30 P. M. May 22nd to 10. 15 A. M. 25th. Figurenot greatly magnified. 

disturbed; and the last dot made at 10. 30 P. M. On this day is alone heregiven. As we see in the figure, there can be no doubt that the apex of thisleaf circumnutated. 

A glass filament with little triangles of paper was at the same time fixedobliquely across the tip of a still younger leaf, which stood vertically upand was as yet straight. Its movements were traced from 3 P. M. May 22nd to10. 15 A. M. 25th. The leaf was growing rapidly, so that the apex ascendedgreatly during this period; as it zigzagged much it was clearlycircumnutating, and it apparently tended to form one ellipse each day. Thelines traced during the night were much more vertical than those tracedduring the day; and this indicates that the tracing would have exhibited anocturnal rise and a diurnal fall, if the leaf had not grown so quickly. The movement of this same leaf after an interval of six days (May 31st), bywhich time the tip had curved outwards into a horizontal position, [page 255]and had thus made the first step towards becoming dependent, was tracedorthogonically by the aid of a cube of wood (in the manner beforeexplained); and it was thus ascertained that the actual distance travelledby the apex, and due to circumnutation, was 3 1/8 inches in the course of20 ½ h. During the next 24 h. It travelled 2 ½ inches. The circumnutatingmovement, therefore, of this young leaf was strongly marked. 

(30. ) Pancratium littorale (Amaryllideae). --The movements, much magnified, of a leaf, 9 inches in length and inclined at about 45o above the horizon, were traced during two days. On the first day it changed its coursecompletely, upwards and downwards and laterally, 9 times in 12 h. ; and thefigure traced apparently represented five ellipses. On the second day itwas observed seldomer, and was therefore not seen to change its course sooften, viz. , only 6 times, but in the same complex manner as before. Themovements were small in extent, but there could be no doubt about thecircumnutation of the leaf. 

(31. ) Imatophyllum vel Clivia (sp. ?) (Amaryllideae). --A long glass filamentwas fixed to a leaf, and the angle formed by it with the horizon wasmeasured occasionally during three successive days. It fell each morninguntil between 3 and 4 P. M. , and rose at night. The smallest angle at anytime above the horizon was 48o, and the largest 50o; so that it rose only 2oat night; but as this was observed each day, and as similar observationswere nightly made on another leaf on a distinct plant, there can be nodoubt that the leaves move periodically, though to a very small extent. Theposition of the apex when it stood highest was . 8 of an inch above itslowest point. 

(32. ) Pistia stratiotes (Aroideae, Fam. 30). --Hofmeister remarks that theleaves of this floating water-plant are more highly inclined at night thanby day. * We therefore fastened a fine glass filament to the midrib of amoderately young leaf, and on Sept. 19th measured the angle which it formedwith the horizon 14 times between 9 A. M. And 11. 50 P. M. The temperature ofthe hot-house varied during the two days of observation between 18 1/2o and23 1/2o C. At 9 A. M. The filament stood at 32o above the horizon; at 3. 34P. M. At 10o and at 11. 50 P. M. At 55o; these two latter angles being thehighest and the lowest observed during the day, showing a difference of45o. The rising did not become strongly marked until between

* 'Die Lehre von der Pflanzenzelle, ' 1867, p. 327. [page 256]

5 and 6 P. M. On the next day the leaf stood at only 10o above the horizonat 8. 25 A. M. , and it remained at about 15o till past 3 P. M. ; at 5. 40 P. M. It was 23o, and at 9. 30 P. M. 58o; so that the rise was more sudden thisevening than on the previous one, and the difference in the angle amountedto 48o. The movement is obviously periodical, and as the leaf stood on thefirst night at 55o, and on the second night at 58o above the horizon, itappeared very steeply inclined. This case, as we shall see in a futurechapter, ought perhaps to have been included under the head of sleepingplants. 

(33. ) Pontederia (sp. ?) (from the highlands of St. Catharina, Fig. 118. Pontederia (sp. ?): circumnutation of leaf, traced from 4. 50 P. M. July 2nd to 10. 15 A. M. 4th. Apex of leaf 16 ½ inches from the verticalglass, so tracing greatly magnified. Temp. About 17o C. , and thereforerather too low. 

Brazil) (Pontederiaceae, Fam. 46). --A filament was fixed across the apex ofa moderately young leaf, 7 ½ inches in height, and its movements weretraced during 42 ½ h. (see Fig. 118). On the first evening, when thetracing was begun, and during the night, the leaf descended considerably. On the next morning it ascended in a strongly marked zigzag line, anddescended again in the evening and during the night. The movement, therefore, seems to be periodic, but some doubt is thrown on thisconclusion, because another leaf, 8 inches in height, appearing older andstanding more highly inclined, behaved differently. During the first 12 h. It circumnutated over a[page 257]small space, but during the night and the whole following day it ascendedin the same general direction; the ascent being effected by repeated up anddown well-pronounced oscillations. 

CRYPTOGAMS. 

(34. ) Nephrodium molle (Filices, Fam. 1). --A filament was fixed near theapex of a young frond of this Fern, 17 inches in height, which was not asyet fully uncurled; and its movements were traced during 24 h. We see inFig. 119 that it

Fig. 119. Nephrodium molle: circumnutation of rachis, traced from 9. 15 A. M. May 28th to 9 A. M. 29th. Figure here given two-thirds of original scale. 

plainly circumnutated. The movement was not greatly magnified as the frondwas placed near to the vertical glass, and would probably have been greaterand more rapid had the day been warmer. For the plant was brought out of awarm greenhouse and observed under a skylight, where the temperature wasbetween 15o and 16o C. We have seen in Chap. I. That a frond of this Fern, as yet only slightly lobed and with a rachis only . 23 inch in height, plainly circumnutated. *

* Mr. Loomis and Prof. Asa Gray have described ('Botanical Gazette, ' 1880, pp. 27, 43), an extremely curious case of movement in the fronds, but onlyin the fruiting fronds, of Asplenium trichomanes. They move almost asrapidly as the little leaflets of Desmodium gyrans, alternately backwardsand forwards through from 20 to 40 degrees, in a plane at right angles tothat of the frond. The apex of the frond describes "a long and very narrowellipse, " so that it circumnutates. But the movement differs from ordinary[[page 258]]circumnutation as it occurs only when the plant is exposed to the light;even artificial light "is sufficient to excite motion for a few minutes. "[page 258]

In the chapter on the Sleep of Plants the conspicuous circumnutation ofMarsilea quadrifoliata (Marsileaceae, Fam. 4) will be described. 

It has also been shown in Chap. I. That a very young Selaginella(Lycopodiaceae, Fam. 6), only . 4 inch in height, plainly circumnutated; wemay therefore conclude that older plants, whilst growing, would do thesame. 

Fig. 120. Lunularia vulgaris: circumnutation of a frond, traced from 9 A. M. Oct 25th to 8 A. M. 27th. 

(35. ) Lunularia vulgaris (Hepaticae, Fam. 11, Muscales). --The earth in anold flower-pot was coated with this plant, bearing gemmae. A highlyinclined frond, which projected . 3 inch above the soil and was . 4 inch inbreadth, was selected for observation. A glass hair of extreme tenuity, . 75inch in length, with its end whitened, was cemented with shellac to thefrond at right angles to its breadth; and a white stick with a minute blackspot was driven into the soil close behind the end of the hair. The whiteend could be accurately brought into a line with the black spot, and dotscould thus be successively made on the vertical glass-plate in front. Anymovement of the frond would of course be exhibited and increased by thelong glass hair; and the black spot was placed so close behind the end ofthe hair, relatively to the distance of the glass-plate in front, that themovement of the end was magnified about 40 times. Nevertheless, we areconvinced that our tracing gives a fairly faithful representation of themovements of the frond. In the intervals between each observation, theplant was covered by a small bell-glass. The frond, as already stated, [page 259]was highly inclined, and the pot stood in front of a north-east window. During the five first days the frond moved downwards or became lessinclined; and the long line which was traced was strongly zigzag, withloops occasionally formed or nearly formed; and this indicatedcircumnutation. Whether the sinking was due to epinastic growth, orapheliotropism, we do not know. As the sinking was slight on the fifth day, a new tracing was begun on the sixth day (Oct. 25th), and was continued for47 h. ; it is here given (Fig. 120). Another tracing was made on the nextday (27th) and the frond was found to be still circumnutating, for during14 h. 30 m. It changed its course completely (besides minor changes) 10times. It was casually observed for two more days, and was seen to becontinually moving. 

The lowest members of the vegetable series, the Thallogens, apparentlycircumnutate. If an Oscillaria be watched under the microscope, it may beseen to describe circles about every 40 seconds. After it has bent to oneside, the tip first begins to bend back to the opposite side and then thewhole filament curves over in the same direction. Hofmeister* has given aminute account of the curious, but less regular though constant, movementsof Spirogyra: during 2 ½ h. The filament moved 4 times to the left and 3times to the right, and he refers to a movement at right angles to theabove. The tip moved at the rate of about 0. 1 mm. In five minutes. Hecompares the movement with the nutation of the higher plants. ** We shallhereafter see that heliotropic movements result from modifiedcircumnutation, and as unicellular Moulds bend to the light we may inferthat they also circumnutate. ]

CONCLUDING REMARKS ON THE CIRCUMNUTATION OF LEAVES. 

The circumnutating movements of young leaves in 33 genera, belonging to 25families, widely distributed

* 'Ueber die Bewegungen der Faden der Spirogyra princeps: Jahreshefte desVereins für vaterländische Naturkunde in Württemberg, ' 1874, p. 211. 

** Zukal also remarks (as quoted in 'Journal R. Microscop. Soc. , ' 1880, vol. Iii. P. 320) that the movements of Spirulina, a member of theOscillatorieae, are closely analogous "to the well-known rotation ofgrowing shoots and tendrils. "[page 260]

amongst ordinary and gymnospermous Dicotyledons and amongst Monocotyledons, together with several Cryptogams, have now been described. It would, therefore, not be rash to assume that the growing leaves of all plantscircumnutate, as we have seen reason to conclude is the case withcotyledons. The seat of movement generally lies in the petiole, butsometimes both in the petiole and blade, or in the blade alone. The extentof the movement differed much in different plants; but the distance passedover was never great, except with Pistia, which ought perhaps to have beenincluded amongst sleeping plants. The angular movement of the leaves wasonly occasionally measured; it commonly varied from only 2o (and probablyeven less in some instances) to about 10o; but it amounted to 23o in thecommon bean. The movement is chiefly in a vertical plane, but as theascending and descending lines never coincided, there was always somelateral movement, and thus irregular ellipses were formed. The movement, therefore, deserves to be called one of circumnutation; for allcircumnutating organs tend to describe ellipses, --that is, growth on oneside is succeeded by growth on nearly but not quite the opposite side. Theellipses, or the zigzag lines representing drawn-out ellipses, aregenerally very narrow; yet with the Camellia, their minor axes were half aslong, and with the Eucalyptus more than half as long as their major axes. In the case of Cissus, parts of the figure more nearly represented circlesthan ellipses. The amount of lateral movement is therefore sometimesconsiderable. Moreover, the longer axes of the successively formed ellipses(as with the Bean, Cissus, and Sea-kale), and in several instances thezigzag lines representing ellipses, were extended in very differentdirections during the same day or on[page 261]the next day. The course followed was curvilinear or straight, or slightlyor strongly zigzag, and little loops or triangles were often formed. Asingle large irregular ellipse may be described on one day, and two smallerones by the same plant on the next day. With Drosera two, and with Lupinus, Eucalyptus and Pancratium, several were formed each day. 

The oscillatory and jerking movements of the leaves of Dionaea, whichresemble those of the hypocotyl of the cabbage, are highly remarkable, asseen under the microscope. They continue night and day for some months, andare displayed by young unexpanded leaves, and by old ones which have losttheir sensibility to a touch, but which, after absorbing animal matter, close their lobes. We shall hereafter meet with the same kind of movementin the joints of certain Gramineae, and it is probably common to manyplants while circumnutating. It is, therefore, a strange fact that no suchmovement could be detected in the tentacles of Drosera rotundifolia, thougha member of the same family with Dionaea; yet the tentacle which wasobserved was so sensitive, that it began to curl inwards in 23 secondsafter being touched by a bit of raw meat. 

One of the most interesting facts with respect to the circumnutation ofleaves is the periodicity of their movements; for they often, or evengenerally, rise a little in the evening and early part of the night, andsink again on the following morning. Exactly the same phenomenon wasobserved in the case of cotyledons. The leaves in 16 genera out of the 33which were observed behaved in this manner, as did probably 2 others. Normust it be supposed that in the remaining 15 genera there was noperiodicity in their movements; for 6 of them were observed during tooshort a period for any judgment to be formed on this head, [page 262]and 3 were so young that their epinastic growth, which serves to bring themdown into a horizontal position, overpowered every other kind of movement. In only one genus, Cannabis, did the leaves sink in the evening, and Krausattributes this movement to the prepotency of their epinastic growth. Thatthe periodicity is determined by the daily alternations of light anddarkness there can hardly be a doubt, as will hereafter be shown. Insectivorous plants are very little affected, as far as their movementsare concerned, by light; and hence probably it is that their leaves, atleast in the cases of Sarracenia, Drosera, and Dionaea, do not moveperiodically. The upward movement in the evening is at first slow, and withdifferent plants begins at very different hours;--with Glaucium as early as11 A. M. , commonly between 3 and 5 P. M. , but sometimes as late as 7 P. M. Itshould be observed that none of the leaves described in this chapter(except, as we believe, those of Lupinus speciosus) possess a pulvinus; forthe periodical movements of leaves thus provided have generally beenamplified into so-called sleep-movements, with which we are not hereconcerned. The fact of leaves and cotyledons frequently, or even generally, rising a little in the evening and sinking in the morning, is of interestas giving the foundation from which the specialised sleep-movements of manyleaves and cotyledons, not provided with a pulvinus, have been developed. The above periodicity should be kept in mind, by any one considering theproblem of the horizontal position of leaves and cotyledons during the day, whilst illuminated from above. [page 263]

CHAPTER V. 

MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTICMOVEMENTS. 

Circumnutation modified through innate causes or through the action ofexternal conditions--Innate causes--Climbing plants; similarity of theirmovements with those of ordinary plants; increased amplitude; occasionalpoints of difference--Epinastic growth of young leaves--Hyponastic growthof the hypocotyls and epicotyls of seedlings--Hooked tips of climbing andother plants due to modified circumnutation--Ampelopsis tricuspidata--Smithia Pfundii--Straightening of the tip due to hyponasty--Epinasticgrowth and circumnutation of the flower-peduncles of Trifolium repens andOxalis carnosa. 

THE radicles, hypocotyls and epicotyls of seedling plants, even before theyemerge from the ground, and afterwards the cotyledons, are all continuallycircumnutating. So it is with the stems, stolons, flower-peduncles, andleaves of older plants. We may, therefore, infer with a considerable degreeof safety that all the growing parts of all plants circumnutate. Althoughthis movement, in its ordinary or unmodified state, appears in some casesto be of service to plants, either directly or indirectly--for instance, the circumnutation of the radicle in penetrating the ground, or that of thearched hypocotyl and epicotyl in breaking through the surface--yetcircumnutation is so general, or rather so universal a phenomenon, that wecannot suppose it to have been gained for any special purpose. We mustbelieve that it follows in some unknown way from the manner in whichvegetable tissues grow. [page 264]

We shall now consider the many cases in which circumnutation has beenmodified for various special purposes; that is, a movement already inprogress is temporarily increased in some one direction, and temporarilydiminished or quite arrested in other directions. These cases may bedivided in two sub-classes; in one of which the modification depends oninnate or constitutional causes, and is independent of external conditions, excepting in so far that the proper ones for growth must be present. In thesecond sub-class the modification depends to a large extent on externalagencies, such as the daily alternations of light and darkness, or lightalone, temperature, or the attraction of gravity. The first small sub-classwill be considered in the present chapter, and the second sub-class in theremainder of this volume. 

THE CIRCUMNUTATION OF CLIMBING PLANTS. 

The simplest case of modified circumnutation is that offered by climbingplants, with the exception of those which climb by the aid of motionlesshooks or of rootlets: for the modification consists chiefly in the greatlyincreased amplitude of the movement. This would follow either from greatlyincreased growth over a small length, or more probably from moderatelyincreased growth spread over a considerable length of the moving organ, preceded by turgescence, and acting successively on all sides. Thecircumnutation of climbers is more regular than that of ordinary plants;but in almost every other respect there is a close similarity between theirmovements, namely, in their tendency to describe ellipses directedsuccessively to all points of the compass--in their courses being ofteninterrupted by zigzag lines, triangles, loops, or small[page 265]ellipses--in the rate of movement, and in different species revolving onceor several times within the same length of time. In the same internode, themovements cease first in the lower part and then slowly upwards. In bothsets of cases the movement may be modified in a closely analogous manner bygeotropism and by heliotropism; though few climbing plants are heliotropic. Other points of similarity might be pointed out. 

That the movements of climbing plants consist of ordinary circumnutation, modified by being increased in amplitude, is well exhibited whilst theplants are very young; for at this early age they move like otherseedlings, but as they grow older their movements gradually increasewithout undergoing any other change. That this power is innate, and is notexcited by any external agencies, beyond those necessary for growth andvigour, is obvious. No one doubts that this power has been gained for thesake of enabling climbing plants to ascend to a height, and thus to reachthe light. This is effected by two very different methods; first, bytwining spirally round a support, but to do so their stems must be long andflexible; and, secondly, in the case of leaf-climbers and tendril-bearers, by bringing these organs into contact with a support, which is then seizedby the aid of their sensitiveness. It may be here remarked that theselatter movements have no relation, as far as we can judge, withcircumnutation. In other cases the tips of tendrils, after having beenbrought into contact with a support, become developed into little discswhich adhere firmly to it. 

We have said that the circumnutation of climbing plants differs from thatof ordinary plants chiefly by its greater amplitude. But most leavescircumnutate[page 266]in an almost vertical plane, and therefore describe very narrow ellipses, whereas the many kinds of tendrils which consist of metamorphosed leaves, make much broader ellipses or nearly circular figures; and thus they have afar better chance of catching hold of a support on any side. The movementsof climbing plants have also been modified in some few other special ways. Thus the circumnutating stems of Solnanum dulcamara can twine round asupport only when this is as thin and flexible as a string or thread. Thetwining stems of several British plants cannot twine round a support whenit is more than a few inches in thickness; whilst in tropical forests somecan embrace thick trunks;* and this great difference in power depends onsome unknown difference in their manner of circumnutation. The mostremarkable special modification of this movement which we have observed isin the tendrils of Echinocystis lobata; these are usually inclined at about45o above the horizon, but they stiffen and straighten themselves so as tostand upright in a part of their circular course, namely, when theyapproach and have to pass over the summit or the shoot from which theyarise. If they had not possessed and exercised this curious power, theywould infallibly have struck against the summit of the shoot and beenarrested in their course. As soon as one of these tendrils with its threebranches begins to stiffen itself and rise up vertically, the revolvingmotion becomes more rapid; and as soon as it has passed over the point ofdifficulty, its motion coinciding with that from its own weight, causes itto fall into its previously inclined position so quickly, that the apex canbe seen travelling like the hand of a gigantic clock. 

* 'The Movements and Habits of Climbing Plants, ' p. 36. [page 267]

A large number of ordinary leaves and leaflets and a few flower-pedunclesare provided with pulvini; but this is not the case with a single tendrilat present known. The cause of this difference probably lies in the fact, that the chief service of a pulvinus is to prolong the movement of the partthus provided after growth has ceased; and as tendrils or otherclimbing-organs are of use only whilst the plant is increasing in height orgrowing, a pulvinus which served to prolong their movements would beuseless. 

It was shown in the last chapter that the stolons or runners of certainplants circumnutate largely, and that this movement apparently aids them infinding a passage between the crowded stems of adjoining plants. If itcould be proved that their movements had been modified and increased forthis special purpose, they ought to have been included in the presentchapter; but as the amplitude of their revolutions is not so conspicuouslydifferent from that of ordinary plants, as in the case of climbers, we haveno evidence on this head. We encounter the same doubt in the case of someplants which bury their pods in the ground. This burying process iscertainly favoured by the circumnutation of the flower-peduncle; but we donot know whether it has been increased for this special purpose. 

EPINASTY--HYPONASTY. 

The term epinasty is used by De Vries* to express greater longitudinalgrowth along the upper than

* 'Arbeiten des Bot. Inst. , in Würzburg, ' Heft ii. 1872, p. 223. De Vrieshas slightly modified (p. 252) the meaning of the above two terms as firstused by Schimper, and they have been adopted in this sense by Sachs. [page 268]

along the lower side of a part, which is thus caused to bend downwards; andhyponasty is used for the reversed process, by which the part is made tobend upwards. These actions come into play so frequently that the use ofthe above two terms is highly convenient. The movements thus induced resultfrom a modified form of circumnutation; for, as we shall immediately see, an organ under the influence of epinasty does not generally move in astraight line downwards, or under that of hyponasty upwards, but oscillatesup and down with some lateral movement: it moves, however, in apreponderant manner in one direction. This shows that there is some growthon all sides of the part, but more on the upper side in the case ofepinasty, and more on the lower side in that of hyponasty, than on theother sides. At the same time there may be in addition, as De Vriesinsists, increased growth on one side due to geotropism, and on anotherside due to heliotropism; and thus the effects of epinasty or of hyponastymay be either increased or lessened. 

He who likes, may speak of ordinary circumnutation as being combined withepinasty, hyponasty, the effects of gravitation, light, etc. ; but it seemsto us, from reasons hereafter to be given, to be more correct to say thatcircumnutation is modified by these several agencies. We will thereforespeak of circumnutation, which is always in progress, as modified byepinasty, hyponasty, geotropism, or other agencies, whether internal orexternal. 

[One of the commonest and simplest cases of epinasty is that offered byleaves, which at an early age are crowded together round the buds, anddiverge as they grow older. Sachs first remarked that this was due toincreased growth along the upper side of the petiole and blade; and DeVries has now shown in more detail that the movement is thus caused, aidedslightly by[page 269]the weight of the leaf, and resisted as he believes by apogeotropism, atleast after the leaf has somewhat diverged. In our observations on thecircumnutation of leaves, some were selected which were rather too young, so that they continued to diverge or sink downwards whilst their movementswere being traced. This may be seen in the diagrams (Figs. 98 and 112, pp. 232 and 249) representing the circumnutation of the young leaves ofAcanthus mollis and Pelargonium zonale. Similar cases were observed withDrosera. The movements of a young leaf, only 3/4 inch in length, of Petuniaviolacea were traced during four days, and offers a better instance (Fig. 111, p. 248) as it diverged during the whole of this time in a curiouslyzigzag line with some of the angles sharply acute, and during the latterdays plainly circumnutated. Some young leaves of about the same age on aplant of this Petunia, which had been laid horizontally, and on anotherplant which was left upright, both being kept in complete darkness, diverged in the same manner for 48 h. , and apparently were not affected byapogeotropism; though their stems were in a state of high tension, for whenfreed from the sticks to which they had been tied, they instantly curledupwards. 

The leaves, whilst very young, on the leading shoots of the Carnation(Dianthus caryophyllus) are highly inclined or vertical; and if the plantis growing vigorously they diverge so quickly that they become almosthorizontal in a day. But they move downwards in a rather oblique line andcontinue for some time afterwards to move in the same direction, inconnection, we presume, with their spiral arrangement on the stem. Thecourse pursued by a young leaf whilst thus obliquely descending was traced, and the line was distinctly yet not strongly zigzag; the larger anglesformed by the successive lines amounting only to 135o, 154o, and 163o. Thesubsequent lateral movement (shown in Fig. 96, p. 231) was strongly zigzagwith occasional circumnutations. The divergence and sinking of the youngleaves of this plant seem to be very little affected by geotropism orheliotropism; for a plant, the leaves of which were growing rather slowly(as ascertained by measurement) was laid horizontally, and the oppositeyoung leaves diverged from one another symmetrically in the usual manner, without any upturning in the direction of gravitation or towards the light. 

The needle-like leaves of Pinus pinaster form a bundle whilst young;afterwards they slowly diverge, so that those on the upright shoots becomehorizontal. The movements of one such[page 270]young leaf was traced during 4 ½ days, and the tracing here given (Fig. 121) shows that it descended at first in a nearly straight line, butafterwards zigzagged, making one or two little loops. The diverging anddescending movements of a rather older leaf were also traced (see formerFig. 113, p. 251): it descended during the first day and night in asomewhat zigzag line; it then circumnutated round a small space and againdescended. By this time the leaf had nearly assumed its final position, andnow plainly circumnutated. As in the case of the Carnation, the leaves, whilst very young, do not seem to be much affected by geotropism orheliotropism, for those on a young plant laid horizontally, and those onanother plant left upright, both kept in the dark, continued to diverge inthe usual manner without bending to either side. 

Fig. 121. Pinus pinaster: epinastic downward movement of a young leaf, produced by a young plant in a pot, traced on a vertical glass under askylight, from 6. 45 A. M. June 2nd to 10. 40 P. M. 6th. 

With Coboea scandens, the young leaves, as they successively diverge fromthe leading shoot which is bent to one side, rise up so as to projectvertically, and they retain this position for some time whilst the tendrilis revolving. The diverging and ascending movements of the petiole of onesuch a leaf, were traced on a vertical glass under a skylight; and thecourse pursued was in most parts nearly straight, but there were two[page 271]well-marked zigzags (one of them forming an angle of 112o), and thisindicates circumnutation. 

The still closed lobes of a young leaf of Dionaea projected at right anglesto the petiole, and were in the act of slowly rising. A glass filament wasattached to the under side of the midrib, and its movements were traced ona vertical glass. It circumnutated once in the evening, and on the next dayrose, as already described (see Fig. 106, p. 240), by a number of acutelyzigzag lines, closely approaching in character to ellipses. This movementno doubt was due to epinasty, aided by apogeotropism, for the closed lobesof a very young leaf on a plant which had been placed horizontally, movedinto nearly the same line with the petiole, as if the plant had stoodupright; but at the same time the lobes curved laterally upwards, and thusoccupied an unnatural position, obliquely to the plane of the foliaceouspetiole. 

As the hypocotyls and epicotyls of some plants protrude from the seed-coatsin an arched form, it is doubtful whether the arching of these parts, whichis invariably present when they break through the ground, ought always tobe attributed to epinasty; but when they are at first straight andafterwards become arched, as often happens, the arching is certainly due toepinasty. As long as the arch is surrounded by compact earth it must retainits form; but as soon as it rises above the surface, or even before thisperiod if artificially freed from the surrounding pressure, it begins tostraighten itself, and this no doubt is mainly due to hyponasty. Themovement of the upper and lower half of the arch, and of the crown, wasoccasionally traced; and the course was more or less zigzag, showingmodified circumnutation. 

With not a few plants, especially climbers, the summit of the shoot ishooked, so that the apex points vertically downwards. In seven genera oftwining plants* the hooking, or as it has been called by Sachs, thenutation of the tip, is mainly due to an exaggerated form ofcircumnutation. That is, the growth is so great along one side that itbends the shoot completely over to the opposite side, thus forming a hook;the longitudinal line or zone of growth then travels a little laterallyround the shoot, and the hook points in a slightly different direction, andso onwards until the hook is completely reversed. Ultimately it

* 'The Movements and Habits of Climbing Plants, ' 2nd edit. P. 13. [page 272]

comes back to the point whence it started. This was ascertained by paintingnarrow lines with Indian ink along the convex surface of several hooks, andthe line was found slowly to become at first lateral, then to appear alongthe concave surface, and ultimately back again on the convex surface. Inthe case of Lonicera brachypoda the hooked terminal part of the revolvingshoot straightens itself periodically, but is never reversed; that is, theperiodically increased growth of the concave side of the hook is sufficientonly to straighten it, and not to bend it over to the opposite side. Thehooking of the tip is of service to twining plants by aiding them to catchhold of a support, and afterwards by enabling this part to embrace thesupport much more closely than it could otherwise have done at first, thuspreventing it, as we often observed, from being blown away by a strongwind. Whether the advantage thus gained by twining plants accounts fortheir summits being so frequently hooked, we do not know, as this structureis not very rare with plants which do not climb, and with some climbers(for instance, Vitis, Ampelopsis, Cissus, etc. ) to whom it does not affordany assistance in climbing. 

With respect to those cases in which the tip remains always bent or hookedtowards the same side, as in the genera just named, the most obviousexplanation is that the bending is due to continued growth in excess alongthe convex side. Wiesner, however, maintains* that in all cases the hookingof the tip is the result of its plasticity and weight, --a conclusion whichfrom what we have already seen with several climbing plants is certainlyerroneous. Nevertheless, we fully admit that the weight of the part, aswell as geotropism, etc. , sometimes come into play. 

Ampelopsis tricuspidata. --This plant climbs by the aid of adhesivetendrils, and the hooked tips of the shoots do not appear to be of anyservice to it. The hooking depends chiefly, as far as we could ascertain, on the tip being affected by epinasty and geotropism; the lower and olderparts continually straightening themselves through hyponasty andapogeotropism. We believe that the weight of the apex is an unimportantelement, because on horizontal or inclined shoots the hook is oftenextended horizontally or even faces upwards. Moreover shoots frequentlyform loops instead of hooks; and in this case the

* 'Sitzb. Der k. Akad. Der Wissensch. , ' Vienna, Jan. 1880, p. 16. [page 273]

Fig. 122. Ampelopsis tricuspidata: hyponastic movement of hooked tip ofleading shoot, traced from 8. 10 A. M. July 13th to 8 A. M. 15th. Apex ofshoot 5 ½ inches from the vertical glass. Plant illuminated through askylight. Temp. 17 1/2o - 19o C. Diagram reduced to one-third of originalscale. 

extreme part, instead of hanging vertically down as would follow if weightwas the efficient cause, extends horizontally or even points upwards. Ashoot, which terminated in a rather open hook, was fastened in a highlyinclined downward position, so that the concave side faced upwards, and theresult was that the apex at first curved upwards. This apparently was dueto epinasty and not to apogeotropism, for the apex, soon after passing theperpendicular, curved so rapidly downwards that we could not doubt that themovement was at least aided by geotropism. In the course of a few hours thehook was thus converted into a loop with the apex of the shoot pointingstraight downwards. The longer axis of the loop was at first horizontal, but afterwards became vertical. During this same time the basal part of thehook (and subsequently of the loop) curved itself slowly upwards; and thismust have been wholly due to apogeotropism in opposition to hyponasty. Theloop was then fastened upside down, so that its basal half would besimultaneously acted on by hyponasty (if present) and by apogeotropism; andnow it curved itself so greatly upwards in the course of only 4 h. Thatthere could hardly be a doubt that both forces were acting[page 274]together. At the same time the loop became open and was thus reconvertedinto a hook, and this apparently was effected by the geotropic movement ofthe apex in opposition to epinasty. In the case of Ampelopsis hederacea, weight plays, as far as we could judge, a more important part in thehooking of the tip. 

In order to ascertain whether the shoots of A. Tricuspidata instraightening themselves under the combined action of hyponasty andapogeotropism moved in a simple straight course, or whether theycircumnutated, glass filaments were fixed to the crowns of four hooked tipsstanding in their natural position; and the movements of the filaments weretraced on a vertical glass. All four tracings resembled each other in ageneral manner; but we will give only one (see Fig. 122, p. 273). Thefilament rose at first, which shows that the hook was straightening itself;it then zigzagged, moving a little to the left between 9. 25 A. M. And 9 P. M. From this latter hour on the 13th to 10. 50 A. M. On the following morning(14th) the hook continued to straighten itself, and then zigzagged a shortdistance to the right. But from 1 P. M. To 10. 40 P. M. On the 14th themovement

Fig. 123. Smithia Pfundii: hyponastic movement of the curved summit of astem, whilst straightening itself, traced from 9 A. M. July 10th to 3 P. M. 13th. Apex 9 ½ inches from the vertical glass. Diagram reduced to one-fifthof original scale. Plant illuminated through skylight; temp. 17 1/2o - 19oC. [page 275]

was reversed and the shoot became more hooked. During the night, after10. 40 P. M. To 8. 15 A. M. On the 15th, the hook again opened or straighteneditself. By this time the glass filament had become so highly inclined thatits movements could no longer be traced with accuracy; and by 1. 30 P. M. Onthis same day, the crown of the former arch or hook had become perfectlystraight and vertical. There can therefore be no doubt that thestraightening of the hooked shoot of this plant is effected by thecircumnutation of the arched portion--that is, by growth alternatingbetween the upper and lower surface, but preponderant on the lower surface, with some little lateral movement. 

We were enabled to trace the movement of another straightening shoot for alonger period (owing to its slower growth and to its having been placedfurther from the vertical glass), namely, from the early morning on July13th to late in the evening of the 16th. During the whole daytime of the14th, the hook straightened itself very little, but zigzagged and plainlycircumnutated about nearly the same spot. By the 16th it had become nearlystraight, and the tracing was no longer accurate, yet it was manifest thatthere was still a considerable amount of movement both up and down andlaterally; for the crown whilst continuing to straighten itselfoccasionally became for a short time more curved, causing the filament todescend twice during the day. 

Smithia Pfundii. --The stiff terminal shoots of this Leguminous water-plantfrom Africa project so as to make a rectangle with the stem below; but thisoccurs only when the plants are growing vigorously, for when kept in a coolplace, the summits of the stems become straight, as they likewise did atthe close of the growing season. The direction of the rectangularly bentpart is independent of the chief source of light. But from observing theeffects of placing plants in the dark, in which case several shoots becamein two or three days upright or nearly upright, and when brought back intothe light again became rectangularly curved, we believe that the bending isin part due to apheliotropism, apparently somewhat opposed byapogeotropism. On the other hand, from observing the effects of tying ashoot downwards, so that the rectangle faced upwards, we are led to believethat the curvature is partly due to epinasty. As the rectangularly bentportion of an upright stem grows older, the lower part straightens itself;and this is effected through hyponasty. He who has read Sachs' recent Essayon the vertical[page 276]and inclined positions of the parts of plants* will see how difficult asubject this is, and will feel no surprise at our expressing ourselvesdoubtfully in this and other such cases. 

A plant, 20 inches in height, was secured to a stick close beneath thecurved summit, which formed rather less than a rectangle with the stembelow. The shoot pointed away from the observer; and a glass filamentpointing towards the vertical glass on which the tracing was made, wasfixed to the convex surface of the curved portion. Therefore the descendinglines in the figure represent the straightening of the curved portion as itgrew older. The tracing (Fig. 123, p. 274) was begun at 9 A. M. On July10th; the filament at first moved but little in a zigzag line, but at 2P. M. It began rising and continued to do so till 9 P. M. ; and this provesthat the terminal portion was being more bent downwards. After 9 P. M. Onthe 10th an opposite movement commenced, and the curved portion began tostraighten itself, and this continued till 11. 10 A. M. On the 12th, but wasinterrupted by some small oscillations and zigzags, showing movement indifferent directions. After 11. 10 A. M. On the 12th this part of the stem, still considerably curved, circumnutated in a conspicuous manner untilnearly 3 P. M. On the 13th; but during all this time a downward movement ofthe filament prevailed, caused by the continued straightening of the stem. By the afternoon of the 13th, the summit, which had originally beendeflected more than a right angle from the perpendicular, had grown sonearly straight that the tracing could no longer be continued on thevertical glass. There can therefore be no doubt that the straightening ofthe abruptly curved portion of the growing stem of this plant, whichappears to be wholly due to hyponasty, is the result of modifiedcircumnutation. We will only add that a filament was fixed in a differentmanner across the curved summit of another plant, and the same general kindof movement was observed. 

Trifolium repens. --In many, but not in all the species of Trifolium, as theseparate little flowers wither, the sub-peduncles bend downwards, so as todepend parallel to the upper part of the main peduncle. In Tr. Subterraneumthe main peduncle curves downwards for the sake of burying its capsules, and in this species the sub-peduncles of the separate flowers bend

* 'Ueber Orthotrope und Plagiotrope Pflanzentheile;' 'Arbeiten des Bot. Inst. , in Würzburg, ' Heft ii. 1879, p. 226. [page 277]

Fig. 124. Trifolium repens: circumnutating and epinastic movements of thesub-peduncle of a single flower, traced on a vertical glass under askylight, in A from 11. 30 A. M. Aug. 27th to 7 A. M. 30th; in B from 7 A. M. Aug. 30th to a little after 6 P. M. Sept. 8th. [page 278]

upwards, so as to occupy the same position relatively to the upper part ofthe main peduncle as in Tr. Repens. This fact alone would render itprobable that the movements of the sub-peduncles in Tr. Repens wereindependent of geotropism. Nevertheless, to make sure, some flower-headswere tied to little sticks upside down and others in a horizontal position;their sub-peduncles, however, all quickly curved upwards through the actionof heliotropism. We therefore protected some flower-heads, similarlysecured to sticks, from the light, and although some of them rotted, manyof their sub-peduncles turned very slowly from their reversed or from theirhorizontal positions, so as to stand in the normal manner parallel to theupper part of the main peduncle. These facts show that the movement isindependent of geotropism or apheliotropism; it must there[fore] beattributed to epinasty, which however is checked, at least as long as theflowers are young, by heliotropism. Most of the above flowers were neverfertilised owing to the exclusion of bees; they consequently withered veryslowly, and the movements of the sub-peduncles were in like manner muchretarded. 

To ascertain the nature of the movement of the sub-peduncle, whilst bendingdownwards, a filament was fixed across the summit of the calyx of a notfully expanded and almost upright flower, nearly in the centre of the head. The main peduncle was secured to a stick close beneath the head. In orderto see the marks on the glass filament, a few flowers had to be cut away onthe lower side of the head. The flower under observation at first divergeda little from its upright position, so as to occupy the open space causedby the removal of the adjoining flowers. This required two days, afterwhich time a new tracing was begun (Fig. 124). In A we see the complexcircumnutating course pursued from 11. 30 A. M. Aug. 26th to 7 A. M. On the30th. The pot was then moved a very little to the right, and the tracing(B) was continued without interruption from 7 A. M. Aug. 30th to after 6P. M. Sept. 8th. It should be observed that on most of these days, only asingle dot was made each morning at the same hour. Whenever the flower wasobserved carefully, as on Aug. 30th and Sept. 5th and 6th, it was found tobe circumnutating over a small space. At last, on Sept. 7th, it began tobend downwards, and continued to do so until after 6 P. M. On the 8th, andindeed until the morning of the 9th, when its movements could no longer betraced on the vertical glass. It was carefully observed during the whole ofthe 8th, and by[page 279]10. 30 P. M. It had descended to a point lower down by two-thirds of thelength of the figure as here given; but from want of space the tracing hasbeen copied in B, only to a little after 6 P. M. On the morning of the 9ththe flower was withered, and the sub-peduncle now stood at an angle of 57obeneath the horizon. If the flower had been fertilised it would havewithered much sooner, and have moved much more quickly. We thus see thatthe sub-peduncle oscillated up and down, or circumnutated, during its wholedownward epinastic course. 

The sub-peduncles of the fertilised and withered flowers of Oxalis carnosalikewise bend downwards through epinasty, as will be shown in a futurechapter; and their downward course is strongly zigzag, indicatingcircumnutation. ]

The number of instances in which various organs move through epinasty orhyponasty, often in combination with other forces, for the most diversifiedpurposes, seems to be inexhaustibly great; and from the several cases whichhave been here given, we may safely infer that such movements are due tomodified circumnutation. [page 280]

CHAPTER VI. 

MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEPOF COTYLEDONS. 

Preliminary sketch of the sleep or nyctitropic movements of leaves--Presence of pulvini--The lessening of radiation the final cause ofnyctitropic movements--Manner of trying experiments on leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea and on the cotyledons ofMimosa--Concluding remarks on radiation from leaves--Small differences inthe conditions make a great difference in the result - Description of thenyctitropic position and movements of the cotyledons of various plants--List of species--Concluding remarks--Independence of the nyctitropicmovements of the leaves and cotyledons of the same species--Reasons forbelieving that the movements have been acquired for a special purpose. 

The so-called sleep of leaves is so conspicuous a phenomenon that it wasobserved as early as the time of Pliny;* and since Linnaeus published hisfamous Essay, 'Somnus Plantarum, ' it has been the subject of severalmemoirs. Many flowers close at night, and these are likewise said to sleep;but we are not here concerned with their movements, for although effectedby the same mechanism as in the case of young leaves, namely, unequalgrowth on the opposite sides (as first proved by Pfeffer), yet they differessentially in being excited chiefly by changes of temperature instead oflight; and in being effected, as far as we can judge, for a differentpurpose. Hardly any one supposes that there is any real analogy

* Pfeffer has given a clear and interesting sketch of the history of thissubject in his 'Die Periodischen Bewegungen der Blattorgane, ' 1875, P. 163. [page 281]

between the sleep of animals and that of plants, * whether of leaves orflowers. It seems therefore, advisable to give a distinct name to theso-called sleep-movements of plants. These have also generally beenconfounded, under the term "periodic, " with the slight daily rise and fallof leaves, as described in the fourth chapter; and this makes it all themore desirable to give some distinct name to sleep-movements. Nyctitropismand nyctitropic, i. E. Night-turning, may be applied both to leaves andflowers, and will be occasionally used by us; but it would be best toconfine the term to leaves. The leaves of some few plants move eitherupwards or downwards when the sun shines intensely on them, and thismovement has sometimes been called diurnal sleep; but we believe it to beof an essentially different nature from the nocturnal movement, and it willbe briefly considered in a future chapter. 

The sleep or nyctitropism of leaves is a large subject, and we think thatthe most convenient plan will be first to give a brief account of theposition which leaves assume at night, and of the advantages apparentlythus gained. Afterwards the more remarkable cases will be described indetail, with respect to cotyledons in the present chapter, and to leaves inthe next chapter. Finally, it will be shown that these movements resultfrom circumnutation, much modified and regulated by the alternations of dayand night, or light and darkness; but that they are also to a certainextent inherited. 

Leaves, when they go to sleep, move either upwards or downwards, or in thecase of the leaflets of com-

* Ch. Royer must, however, be excepted; see 'Annales des Sc. Nat. ' (5thseries), Bot. Vol. Ix. 1868, p. 378. 

[page 282]pound leaves, forwards, that is, towards the apex of the leaf, orbackwards, that is, towards its base; or, again, they may rotate on theirown axes without moving either upwards or downwards. But in almost everycase the plane of the blade is so placed as to stand nearly or quitevertically at night. Therefore the apex, or the base, or either lateraledge, may be directed towards the zenith. Moreover, the upper surface ofeach leaf, and more especially of each leaflet, is often brought into closecontact with that of the opposite one; and this is sometimes effected bysingularly complicated movements. This fact suggests that the upper surfacerequires more protection than the lower one. For instance, the terminalleaflet in Trifolium, after turning up at night so as to stand vertically, often continues to bend over until the upper surface is directed downwardswhilst the lower surface is fully exposed to the sky; and an arched roof isthus formed over the two lateral leaflets, which have their upper surfacespressed closely together. Here we have the unusual case of one of theleaflets not standing vertically, or almost vertically, at night. 

Considering that leaves in assuming their nyctitropic positions often movethrough an angle of 90o; that the movement is rapid in the evening; that insome cases, as we shall see in the next chapter, it is extraordinarilycomplicated; that with certain seedlings, old enough to bear true leaves, the cotyledons move vertically upwards at night, whilst at the same timethe leaflets move vertically downwards; and that in the same genus theleaves or cotyledons of some species move upwards, whilst those of otherspecies move downwards;--from these and other such facts, it is hardlypossible to doubt that plants must derive some[page 283]great advantage from such remarkable powers of movement. 

The nyctitropic movements of leaves and cotyledons are effected in twoways, * firstly, by means of pulvini which become, as Pfeffer has shown, alternately more turgescent on opposite sides; and secondly, by increasedgrowth along one side of the petiole or midrib, and then on the oppositeside, as was first proved by Batalin. ** But as it has been shown by DeVries*** that in these latter cases increased growth is preceded by theincreased turgescence of the cells, the difference between the above twomeans of movement is much diminished, and consists chiefly in theturgescence of the cells of a fully developed pulvinus, not being followedby growth. When the movements of leaves or cotyledons, furnished with apulvinus and destitute of one, are compared, they are seen to be closelysimilar, and are apparently effected for the same purpose. Therefore, withour object in view, it does not appear advisable to separate the above twosets of cases into two distinct classes. There is, however, one importantdistinction between them, namely, that movements effected by growth on thealternate sides, are confined to young growing leaves, whilst thoseeffected by means of a pulvinus last for a long time. We have already seenwell-marked instances of this latter fact with cotyledons, and so it iswith leaves, as has been observed by Pfeffer and by ourselves. The longendurance of the nyctitropic movements when effected by the aid of pulviniindicates, in addition to the evidence already advanced, the functionalimport-

* This distinction was first pointed out (according to Pfeffer, 'DiePeriodischen Bewegungen der Blattorgane, ' 1875, p. 161) by Dassen in 1837. 

** 'Flora, ' 1873, p. 433. 

*** 'Bot. Zeitung, ' 1879, Dec. 19th, p. 830. 

[page 284]ance of such movements to the plant. There is another difference betweenthe two sets of cases, namely, that there is never, or very rarely, anytorsion of the leaves, excepting when a pulvinus is present;* but thisstatement applies only to periodic and nyctitropic movements as may beinferred from other cases given by Frank. **The fact that the leaves of many plants place themselves at night in widelydifferent positions from what they hold during the day, but with the onepoint in common, that their upper surfaces avoid facing the zenith, oftenwith the additional fact that they come into close contact with oppositeleaves or leaflets, clearly indicates, as it seems to us, that the objectgained is the protection of the upper surfaces from being chilled at nightby radiation. There is nothing improbable in the upper surface needingprotection more than the lower, as the two differ in function andstructure. All gardeners know that plants suffer from radiation. It is thisand not cold winds which the peasants of Southern Europe fear for theirolives. *** Seedlings are often protected from radiation by a very thincovering of straw; and fruit-trees on walls by a few fir-branches, or evenby a fishing-net, suspended over them. There is a variety of thegooseberry, **** the flowers of which from being produced before the leaves, are not protected by them from radiation, and consequently often fail toyield fruit. An excellent observer***** has remarked

* Pfeffer, 'Die Period. Beweg. Der Blattorgane. ' 1875, p. 159. 

** 'Die Nat. Wagerechte Richtung von Pflanzentheilen, ' 1870, p. 52

*** Martins in 'Bull. Soc. Bot. De France, ' tom. Xix. 1872. Wells, in hisfamous 'Essay on Dew, ' remarks that an exposed thermometer rises as soon aseven a fleecy cloud, high in the sky, passes over the zenith. 

**** 'Loudon's Gardener's Mag. , ' vol. Iv. 1828, p. 112. 

***** Mr. Rivers in 'Gardener's Chron. , ' 1866, p. 732. [page 285]

that one variety of the cherry has the petals of its flowers much curledbackwards, and after a severe frost all the stigmas were killed; whilst atthe same time, in another variety with incurved petals, the stigmas werenot in the least injured. 

This view that the sleep of leaves saves them from being chilled at nightby radiation, would no doubt have occurred to Linnaeus, had the principleof radiation been then discovered; for he suggests in many parts of his'Somnus Plantarum' that the position of the leaves at night protects theyoung stems and buds, and often the young inflorescence, against coldwinds. We are far from doubting that an additional advantage may be thusgained; and we have observed with several plants, for instance, Desmodiumgyrans, that whilst the blade of the leaf sinks vertically down at night, the petiole rises, so that the blade has to move through a greater angle inorder to assume its vertical position than would otherwise have beennecessary; but with the result that all the leaves on the same plant arecrowded together as if for mutual protection. 

We doubted at first whether radiation would affect in any important mannerobjects so thin as are many cotyledons and leaves, and more especiallyaffect differently their upper and lower surfaces; for although thetemperature of their upper surfaces would undoubtedly fall when freelyexposed to a clear sky, yet we thought that they would so quickly acquireby conduction the temperature of the surrounding air, that it could hardlymake any sensible difference to them, whether they stood horizontally andradiated into the open sky, or vertically and radiated chiefly in a lateraldirection towards neighbouring plants and other objects. We endeavoured, therefore, to ascertain something on this head by preventing the leaves[page 286]of several plants from going to sleep, and by exposing to a clear sky whenthe temperature was beneath the freezing-point, these, as well as the otherleaves on the same plants which had already assumed their nocturnalvertical position. Our experiments show that leaves thus compelled toremain horizontal at night, suffered much more injury from frost than thosewhich were allowed to assume their normal vertical position. It may, however, be said that conclusions drawn from such observations are notapplicable to sleeping plants, the inhabitants of countries where frosts donot occur. But in every country, and at all seasons, leaves must be exposedto nocturnal chills through radiation, which might be in some degreeinjurious to them, and which they would escape by assuming a verticalposition. 

In our experiments, leaves were prevented from assuming their nyctitropicposition, generally by being fastened with the finest entomological pins(which did not sensibly injure them) to thin sheets of cork supported onsticks. But in some instances they were fastened down by narrow strips ofcard, and in others by their petioles being passed through slits in thecork. The leaves were at first fastened close to the cork, for as this is abad conductor, and as the leaves were not exposed for long periods, wethought that the cork, which had been kept in the house, would veryslightly warm them; so that if they were injured by the frost in a greaterdegree than the free vertical leaves, the evidence would be so much thestronger that the horizontal position was injurious. But we found that whenthere was any slight difference in the result, which could be detected onlyoccasionally, the leaves which had been fastened closely down sufferedrather more than those fastened with very long and[page 287]thin pins, so as to stand from ½ to 3/4 inch above the cork. Thisdifference in the result, which is in itself curious as showing what a veryslight difference in the conditions influences the amount of injuryinflicted, may be attributed, as we believe, to the surrounding warmer airnot circulating freely beneath the closely pinned leaves and thus slightlywarming them. This conclusion is supported by some analogous factshereafter to be given. 

We will now describe in detail the experiments which were tried. These weretroublesome from our not being able to predict how much cold the leaves ofthe several species could endure. Many plants had every leaf killed, boththose which were secured in a horizontal position and those which wereallowed to sleep--that is, to rise up or sink down vertically. Others againhad not a single leaf in the least injured, and these had to be re-exposedeither for a longer time or to a lower temperature. 

[Oxalis acetosella. --A very large pot, thickly covered with between 300 and400 leaves, had been kept all winter in the greenhouse. Seven leaves werepinned horizontally open, and were exposed on March 16th for 2 h. To aclear sky, the temperature on the surrounding grass being -4o C. (24o to 25oF. ). Next morning all seven leaves were found quite killed, so were many ofthe free ones which had previously gone to sleep, and about 100 of them, either dead or browned and injured were picked off. Some leaves showed thatthey had been slightly injured by not expanding during the whole of thenext day, though they afterwards recovered. As all the leaves which werepinned open were killed, and only about a third or fourth of the otherswere either killed or injured, we had some little evidence that those whichwere prevented from assuming their vertically dependent position sufferedmost. 

The following night (17th) was clear and almost equally cold (-3o to -4o C. On the grass), and the pot was again exposed, but this time for only 30 m. Eight leaves had been pinned out, [page 288]and in the morning two of them were dead, whilst not a single other leaf onthe many plants was even injured. 

On the 23rd the pot was exposed for 1 h. 30 m. , the temperature on thegrass being only -2o C. , and not one leaf was injured: the pinned openleaves, however, all stood from ½ to 3/4 of an inch above the cork. 

On the 24th the pot was again placed on the ground and exposed to a clearsky for between 35 m. And 40 m. By a mistake the thermometer was left on anadjoining sun-dial 3 feet high, instead of being placed on the grass; itrecorded 25o to 26o F. (-3. 3o to -3. 8o C. ), but when looked at after 1 h. Had fallen to 22o F. (-5. 5o C. ); so that the pot was perhaps exposed torather a lower temperature than on the two first occasions. Eight leaveshad been pinned out, some close to the cork and some above it, and on thefollowing morning five of them (i. E. 63 per cent. ) were found killed. Bycounting a portion of the leaves we estimated that about 250 had beenallowed to go to sleep, and of these about 20 were killed (i. E. Only 8 percent. ), and about 30 injured. 

Considering these cases, there can be no doubt that the leaves of thisOxalis, when allowed to assume their normal vertically dependent positionat night, suffer much less from frost than those (23 in number) which hadtheir upper surfaces exposed to the zenith. 

Oxalis carnosa. --A plant of this Chilian species was exposed for 30 m. To aclear sky, the thermometer on the grass standing at -2o C. , with some ofits leaves pinned open, and not one leaf on the whole bushy plant was inthe least injured. On the 16th of March another plant was similarly exposedfor 30 m. , when the temperature on the grass was only a little lower, viz. , -3o to -4o C. Six of the leaves had been pinned open, and next morning fiveof them were found much browned. The plant was a large one, and none of thefree leaves, which were asleep and depended vertically, were browned, excepting four very young ones. But three other leaves, though not browned, were in a rather flaccid condition, and retained their nocturnal positionduring the whole of the following day. In this case it was obvious that theleaves which were exposed horizontally to the zenith suffered most. Thissame pot was afterwards exposed for 35 - 40 m. On a slightly colder night, and every leaf, both the pinned open and the free ones, was killed. It maybe added that two pots of O. Corniculata (var. Atro-[page 289]purpurea) were exposed for 2 h. And 3 h. To a clear sky with the temp. Ongrass -2o C. , and none of the leaves, whether free or pinned open, were atall injured. 

Arachis hypogoea. --Some plants in a pot were exposed at night for 30 m. Toa clear sky, the temperature on the surrounding grass being -2o C. , and ontwo nights afterwards they were again exposed to the same temperature, butthis time during 1 h. 30 m. On neither occasion was a single leaf, whetherpinned open or free, injured; and this surprised us much, considering itsnative tropical African home. Two plants were next exposed (March 16th) for30 m. To a clear sky, the temperature of the surrounding grass being nowlower, viz. , between -3o and -4o C. , and all four pinned-open leaves werekilled and blackened. These two plants bore 22 other and free leaves(excluding some very young bud-like ones) and only two of these were killedand three somewhat injured; that is, 23 per cent. Were either killed orinjured, whereas all four pinned-open leaves were utterly killed. 

On another night two pots with several plants were exposed for between 35m. And 40 m. To a clear sky, and perhaps to a rather lower temperature, fora thermometer on a dial, 3 feet high, close by stood at -3. 3o to -3. 8o C. In one pot three leaves were pinned open, and all were badly injured; ofthe 44 free leaves, 26 were injured, that is, 59 per cent. In the other pot3 leaves were pinned open and all were killed; four other leaves wereprevented from sleeping by narrow strips of stiff paper gummed across them, and all were killed; of 24 free leaves, 10 were killed, 2 much injured, and12 unhurt; that is, 50 per cent. Of the free leaves were either killed ormuch injured. Taking the two pots together, we may say that rather morethan half of the free leaves, which were asleep, were either killed orinjured, whilst all the ten horizontally extended leaves, which had beenprevented from going to sleep, were either killed or much injured. 

Cassia floribunda. --A bush was exposed at night for 40 m. To a clear sky, the temperature on the surrounding grass being -2o C. , and not a leaf wasinjured. * It was again exposed on

* Cassia laevigata was exposed to a clear sky for 35 m. , and C. Calliantha(a Guiana species) for 60 m. , the temperature on the surrounding grassbeing -2o C. , and neither was in the least injured. But when C. Laevigatawas exposed for 1 h. , the temp. On the surrounding grass being between -3oand -4o C. , every leaf was killed. [page 290]

another night for 1 h. , when the temperature of the grass was -4o C. ; andnow all the leaves on a large bush, whether pinned flat open or free, werekilled, blackened, and shrivelled, with the exception of those on one smallbranch, low down, which was very slightly protected by the leaves on thebranches above. Another tall bush, with four of its large compound leavespinned out horizontally, was afterwards exposed (temp. Of surrounding grassexactly the same, viz. , -4o C. ), but only for 30 m. On the followingmorning every single leaflet on these four leaves was dead, with both theirupper and lower surfaces completely blackened. Of the many free leaves onthe bush, only seven were blackened, and of these only a single one (whichwas a younger and more tender leaf than any of the pinned ones) had bothsurfaces of the leaflets blackened. The contrast in this latter respect waswell shown by a free leaf, which stood between two pinned-open ones; forthese latter had the lower surfaces of their leaflets as black as ink, whilst the intermediate free leaf, though badly injured, still retained aplain tinge of green on the lower surface of the leaflets. This bushexhibited in a striking manner the evil effects of the leaves not beingallowed to assume at night their normal dependent position; for had theyall been prevented from doing so, assuredly every single leaf on the bushwould have been utterly killed by this exposure of only 30 m. The leaveswhilst sinking downwards in the evening twist round, so that the uppersurface is turned inwards, and is thus better protected than the outwardlyturned lower surface. Nevertheless, it was always the upper surface whichwas more blackened than the lower, whenever any difference could beperceived between them; but whether this was due to the cells near theupper surface being more tender, or merely to their containing morechlorophyll, we do not know. 

Melilotus officinalis. --A large pot with many plants, which had been keptduring the winter in the greenhouse, was exposed during 5 h. At night to aslight frost and clear sky. Four leaves had been pinned out, and these diedafter a few days; but so did many of the free leaves. Therefore nothingcertain could be inferred from this trial, though it indicated that thehorizontally extended leaves suffered most. Another large pot with manyplants was next exposed for 1 h. , the temperature on the surrounding grassbeing lower, viz. , -3o to -4o C. Ten leaves had been pinned out, and theresult was striking, for on the following morning all these were found muchinjured or[page 291]killed, and none of the many free leaves on the several plants were at allinjured, with the doubtful exception of two or three very young ones. 

Melilotus Italica. --Six leaves were pinned out horizontally, three withtheir upper and three with their lower surfaces turned to the zenith. Theplants were exposed for 5 h. To a clear sky, the temperature on groundbeing about -1o C. Next morning the six pinned-open leaves seemed moreinjured even than the younger and more tender free ones on the samebranches. The exposure, however, had been too long, for after an intervalof some days many of the free leaves seemed in almost as bad a condition asthe pinned-out ones. It was not possible to decide whether the leaves withtheir upper or those with their lower surfaces turned to the zenith hadsuffered most. 

Melilotus suaveolens. --Some plants with 8 leaves pinned out were exposed toa clear sky during 2 h. , the temperature on the surrounding grass being -2oC. Next morning 6 out of these 8 leaves were in a flaccid condition. Therewere about 150 free leaves on the plant, and none of these were injured, except 2 or 3 very young ones. But after two days, the plants having beenbrought back into the greenhouse, the 6 pinned-out leaves all recovered. 

Melilotus Taurica. --Several plants were exposed for 5 h. During two nightsto a clear sky and slight frost, accompanied by some wind; and 5 leaveswhich had been pinned out suffered more than those both above and below onthe same branches which had gone to sleep. Another pot, which had likewisebeen kept in the greenhouse, was exposed for 35 - 40 m. To a clear sky, thetemperature of the surrounding grass being between -3o and -4o C. Nineleaves had been pinned out, and all of these were killed. On the sameplants there were 210 free leaves, which had been allowed to go to sleep, and of these about 80 were killed, i. E. Only 38 per cent. 

Melilotus Petitpierreana. --The plants were exposed to a clear sky for 35 -40 m. : temperature on surrounding grass -3o to -4o C. Six leaves had beenpinned out so as to stand about ½ inch above the cork, and four had beenpinned close to it. These 10 leaves were all killed, but the closely pinnedones suffered most, as 4 of the 6 which stood above the cork still retainedsmall patches of a green colour. A considerable number, but not nearly all, of the free leaves, were killed or much injured, whereas all the pinned outones were killed. [page 292]

Melilotus macrorrhiza. --The plants were exposed in the same manner as inthe last case. Six leaves had been pinned out horizontally, and five ofthem were killed, that is, 83 percent. We estimated that there were 200free leaves on the plants, and of these about 50 were killed and 20 badlyinjured, so that about 35 per cent of the free leaves were killed orinjured. 

Lotus aristata. --Six plants were exposed for nearly 5 h. To a clear sky;temperature on surrounding grass -1. 5o C. Four leaves had been pinned outhorizontally, and 2 of these suffered more than those above or below on thesame branches, which had been allowed to go to sleep. It is rather aremarkable fact that some plants of Lotus Jacoboeus, an inhabitant of sohot a country as the Cape Verde Islands, were exposed one night to a clearsky, with the temperature of the surrounding grass -2o C. , and on a secondnight for 30 m. With the temperature of the grass between -3o and -4o C. , and not a single leaf, either the pinned-out or free ones, was in the leastinjured. 

Marsilea quadrifoliata. --A large plant of this species--the onlyCryptogamic plant known to sleep--with some leaves pinned open, was exposedfor 1 h. 35 m. To a clear sky, the temperature on the surrounding groundbeing -2o C. , and not a single leaf was injured. After an interval of somedays the plant was again exposed for 1 h. To a clear sky, with thetemperature on the surrounding ground lower, viz. , -4o C. Six leaves hadbeen pinned out horizontally, and all of them were utterly killed. Theplant had emitted long trailing stems, and these had been wrapped roundwith a blanket, so as to protect them from the frozen ground and fromradiation; but a very large number of leaves were left freely exposed, which had gone to sleep, and of these only 12 were killed. After anotherinterval, the plant, with 9 leaves pinned out, was again exposed for 1 h. , the temperature on the ground being again -4o C. Six of the leaves werekilled, and one which did not at first appear injured afterwards becamestreaked with brown. The trailing branches, which rested on the frozenground, had one-half or three-quarters of their leaves killed, but of themany other leaves on the plant, which alone could be fairly compared withthe pinned-out ones, none appeared at first sight to have been killed, buton careful search 12 were found in this state. After another interval, theplant with 9 leaves pinned out, was exposed for 35 - 40 m. To a clear skyand to nearly the same, or perhaps a rather lower, temperature (for thethermometer by an accident had been left on a[page 293]sun-dial close by), and 8 of these leaves were killed. Of the free leaves(those on the trailing branches not being considered), a good many werekilled, but their number, compared with the uninjured ones, was small. Finally, taking the three trials together, 24 leaves, extendedhorizontally, were exposed to the zenith and to unobstructed radiation, andof these 20 were killed and 1 injured; whilst a relatively very smallproportion of the leaves, which had been allowed to go to sleep with theirleaflets vertically dependent, were killed or injured. 

The cotyledons of several plants were prepared for trial, but the weatherwas mild and we succeeded only in a single instance in having seedlings ofthe proper age on nights which were clear and cold. The cotyledons of 6seedlings of Mimosa pudica were fastened open on cork and were thus exposedfor 1 h. 45 m. To a clear sky, with the temperature on the surroundingground at 29o F. ; of these, 3 were killed. Two other seedlings, after theircotyledons had risen up and had closed together, were bent over andfastened so that they stood horizontally, with the lower surface of onecotyledon fully exposed to the zenith, and both were killed. Therefore ofthe 8 seedlings thus tried 5, or more than half, were killed. Seven otherseedlings with their cotyledons in their normal nocturnal position, viz. , vertical and closed, were exposed at the same time, and of these only 2were killed. * Hence it appears, as far as these few trials tell anything, that the vertical position at night of the cotyledons of Mimosa pudicaprotects them to a certain degree from the evil effects of radiation andcold. ]

Concluding Remarks on the Radiation from Leaves at Night. --We exposed ontwo occasions during the summer to a clear sky several pinned-open leafletsof Trifolium pratense, which naturally rise at night, and of Oxalispurpurea, which naturally sink at night (the plants growing out of doors), and looked at

* We were surprised that young seedlings of so tropical a plant as Mimosapudica were able to resist, as well as they did, exposure for 1 hr. 45 m. To a clear sky, the temperature on the surrounding ground being 29o F. Itmay be added that seedlings of the Indian 'Cassia pubescens' were exposedfor 1 h. 30 m. To a clear sky, with the temp. On the surrounding ground at-2o C. , and they were not in the least injured. [page 294]

them early on several successive mornings, after they had assumed theirdiurnal positions. The difference in the amount of dew on the pinned-openleaflets and on those which had gone to sleep was generally conspicuous;the latter being sometimes absolutely dry, whilst the leaflets which hadbeen horizontal were coated with large beads of dew. This shows how muchcooler the leaflets fully exposed to the zenith must have become, thanthose which stood almost vertically, either upwards or downwards, duringthe night. 

From the several cases above given, there can be no doubt that the positionof the leaves at night affects their temperature through radiation to sucha degree, that when exposed to a clear sky during a frost, it is a questionof life and death. We may therefore admit as highly probable, seeing thattheir nocturnal position is so well adapted to lessen radiation, that theobject gained by their often complicated sleep movements, is to lessen thedegree to which they are chilled at night. It should be kept in mind thatit is especially the upper surface which is thus protected, as it is neverdirected towards the zenith, and is often brought into close contact withthe upper surface of an opposite leaf or leaflet. 

We failed to obtain sufficient evidence, whether the better protection ofthe upper surface has been gained from its being more easily injured thanthe lower surface, or from its injury being a greater evil to the plant. That there is some difference in constitution between the two surfaces isshown by the following cases. Cassia floribunda was exposed to a clear skyon a sharp frosty night, and several leaflets which had assumed theirnocturnal dependent position with their lower surfaces turned outwards soas to be[page 295]exposed obliquely to the zenith, nevertheless had these lower surfaces lessblackened than the upper surfaces which were turned inwards and were inclose contact with those of the opposite leaflets. Again, a pot full ofplants of Trifolium resupinatum, which had been kept in a warm room forthree days, was turned out of doors (Sept. 21st) on a clear and almostfrosty night. Next morning ten of the terminal leaflets were examined asopaque objects under the microscope. These leaflets, in going to sleep, either turn vertically upwards, or more commonly bend a little over thelateral leaflets, so that their lower surfaces are more exposed to thezenith than their upper surfaces. Nevertheless, six of these ten leafletswere distinctly yellower on the upper than on the lower and more exposedsurface. In the remaining four, the result was not so plain, but certainlywhatever difference there was leaned to the side of the upper surfacehaving suffered most. 

It has been stated that some of the leaflets experimented on were fastenedclose to the cork, and others at a height of from ½ to 3/4 of an inch aboveit; and that whenever, after exposure to a frost, any difference could bedetected in their states, the closely pinned ones had suffered most. Weattributed this difference to the air, not cooled by radiation, having beenprevented from circulating freely beneath the closely pinned leaflets. Thatthere was really a difference in the temperature of leaves treated in thesetwo different methods, was plainly shown on one occasion; for after theexposure of a pot with plants of Melilotus dentata for 2 h. To a clear sky(the temperature on the surrounding grass being -2o C. ), it was manifestthat more dew had congealed into hoar-frost on the closely pinned leaflets, than on those which stood horizontally[page 296]a little above the cork. Again, the tips of some few leaflets, which hadbeen pinned close to the cork, projected a little beyond the edge, so thatthe air could circulate freely round them. This occurred with six leafletsof Oxalis acetosella, and their tips certainly suffered rather less thenthe rest of the same leaflets; for on the following morning they were stillslightly green. The same result followed, even still more clearly, in twocases with leaflets of Melilotus officinalis which projected a littlebeyond the cork; and in two other cases some leaflets which were pinnedclose to the cork were injured, whilst other free leaflets on the sameleaves, which had not space to rotate and assume their proper verticalposition, were not at all injured. 

Another analogous fact deserves notice: we observed on several occasionsthat a greater number of free leaves were injured on the branches which hadbeen kept motionless by some of their leaves having been pinned to thecorks, than on the other branches. This was conspicuously the case withthose of Melilotus Petitpierreana, but the injured leaves in this instancewere not actually counted. With Arachis hypogaea, a young plant with 7stems bore 22 free leaves, and of these 5 were injured by the frost, all ofwhich were on two stems, bearing four leaves pinned to the cork-supports. With Oxalis carnosa, 7 free leaves were injured, and every one of thembelonged to a cluster of leaves, some of which had been pinned to the cork. We could account for these cases only by supposing that the branches whichwere quite free had been slightly waved about by the wind, and that theirleaves had thus been a little warmed by the surrounding warmer air. If wehold our hands motionless before a hot fire, and then wave them about, we[page 297]immediately feel relief; and this is evidently an analogous, thoughreversed, case. These several facts--in relation to leaves pinned close toor a little above the cork-supports--to their tips projecting beyond it--and to the leaves on branches kept motionless--seem to us curious, asshowing how a difference, apparently trifling, may determine the greater orless injury of the leaves. We may even infer as probable that the less orgreater destruction during a frost of the leaves on a plant which does notsleep, may often depend on the greater or less degree of flexibility oftheir petioles and of the branches which bear them. 

NYCTITROPIC OR SLEEP MOVEMENTS OF COTYLEDONS. 

We now come to the descriptive part of our work, and will begin withcotyledons, passing on to leaves in the next chapter. We have met with onlytwo brief notices of cotyledons sleeping. Hofmeister, * after stating thatthe cotyledons of all the observed seedlings of the Caryophylleae (Alsineaeand Sileneae) bend upwards at night (but to what angle he does not state), remarks that those of Stellaria media rise up so as to touch one another;they may therefore safely be said to sleep. Secondly, according to Ramey**, the cotyledons of Mimosa pudica and of Clianthus Dampieri rise up almostvertically at night and approach each other closely. It has been shown in aprevious chapter that the cotyledons of a large number of plants bend alittle upwards at night, and we here have to meet the difficult question atwhat inclination may they be said to sleep? According to the view which wemaintain, no movement deserves to be called

* 'Die Lehre von der Pflanzenzelle, ' 1867, p. 327. 

** 'Adansonia, ' March 10th, 1869. 

[page 298]nyctitropic, unless it has been acquired for the sake of lesseningradiation; but this could be discovered only by a long series ofexperiments, showing that the leaves of each species suffered from thiscause, if prevented from sleeping. We must therefore take an arbitrarylimit. If a cotyledon or leaf is inclined at 60o above or beneath thehorizon, it exposes to the zenith about one-half of its area; consequentlythe intensity of its radiation will be lessened by about half, comparedwith what it would have been if the cotyledon or leaf had remainedhorizontal. This degree of diminution certainly would make a greatdifference to a plant having a tender constitution. We will therefore speakof a cotyledon and hereafter of a leaf as sleeping, only when it rises atnight to an angle of about 60o, or to a still higher angle, above thehorizon, or sinks beneath it to the same amount. Not but that a lesserdiminution of radiation may be advantageous to a plant, as in the case ofDatura stramonium, the cotyledons of which rose from 31o at noon to 55o atnight above the horizon. The Swedish turnip may profit by the area of itsleaves being reduced at night by about 30 per cent. , as estimated by Mr. A. S. Wilson; though in this case the angle through which the leaves rose wasnot observed. On the other hand, when the angular rise of cotyledons or ofleaves is small, such as less than 30o, the diminution of radiation is soslight that it probably is of no significance to the plant in relation toradiation. For instance, the cotyledons of Geranium Ibericum rose at nightto 27o above the horizon, and this would lessen radiation by only 11 percent. : those of Linum Berendieri rose to 33o, and this would lessenradiation by 16 per cent. 

There are, however, some other sources of doubt with[page 299]respect to the sleep of cotyledons. In certain cases, the cotyledons whilstyoung diverge during the day to only a very moderate extent, so that asmall rise at night, which we know occurs with the cotyledons of manyplants, would necessarily cause them to assume a vertical or nearlyvertical position at night; and in this case it would be rash to infer thatthe movement was effected for any special purpose. On this account wehesitated long whether we should introduce several Cucurbitaceous plantsinto the following list; but from reasons, presently to be given, wethought that they had better be at least temporarily included. This samesource of doubt applies in some few other cases; for at the commencement ofour observations we did not always attend sufficiently to whether thecotyledons stood nearly horizontally in the middle of the day. With severalseedlings, the cotyledons assume a highly inclined position at night duringso short a period of their life, that a doubt naturally arises whether thiscan be of any service to the plant. Nevertheless, in most of the casesgiven in the following list, the cotyledons may be as certainly said tosleep as may the leaves of any plant. In two cases, namely with the cabbageand radish, the cotyledons of which rise almost vertically during the fewfirst nights of their life, it was ascertained by placing young seedlingsin the klinostat, that the upward movement was not due to apogeotropism. 

The names of the plants, the cotyledons of which stand at night at an angleof at least 60o with the horizon, are arranged in the appended list on thesame system as previously followed. The numbers of the Families, and withthe Leguminosae the numbers of the Tribes, have been added to show howwidely the plants in question are distributed throughout the[page 300]dicotyledonous series. A few remarks will have to be made about many of theplants in the list. In doing so, it will be convenient not to followstrictly any systematic order, but to treat of the Oxalidae and theLeguminosae at the close; for in these two Families the cotyledons aregenerally provided with a pulvinus, and their movements endure for a muchlonger time than those of the other plants in the list. 

List of Seedling Plants, the cotyledons of which rise or sink at night toan angle of at least 60o above or beneath the horizon. 

Brassica oleracea. Cruciferae (Fam. 14). -- napus (as we are informed by Prof. Pfeffer). Raphanus sativus. Cruciferae. Githago segetum. Caryophylleae (Fam. 26). Stellaria media (according to Hofmeister, as quoted). Caryophylleae. Anoda Wrightii. Malvaceae (Fam. 36). Gossypium (var. Nankin cotton). Malvaceae. Oxalis rosea. Oxalidae (Fam. 41). -- floribunda. -- articulata. -- Valdiviana. -- sensitiva. Geranium rotundifolium. Geraniaceae (Fam. 47). Trifolium subterraneum. Leguminosae (Fam. 75, Tribe 3). -- strictum. -- leucanthemum. Lotus ornithopopoides. Leguminosae (Tribe 4). -- peregrinus. -- Jacobaeus. Clianthus Dampieri. Leguminosae (Tribe 5)--according to M. Ramey. Smithia sensitiva. Leguminosae (Tribe 6). Haematoxylon Campechianum. Leguminosae (Tribe 13)--according to Mr. R. I. Lynch. Cassia mimosoides. Leguminosae (Tribe 14). -- glauca. -- florida. -- corymbosa. -- pubescens. -- tora. -- neglecta. -- 3 other Brazilian unnamed species. Bauhinia (sp. ?. Leguminosae (Tribe 15). Neptunia oleracea. Leguminosae (Tribe 20). Mimosa pudica. Leguminosae (Tribe 21). -- albida. Cucurbita ovifera. Cucurbitaceae (Fam. 106). -- aurantia. Lagenaria vulgaris. Cucurbitaceae. Cucumis dudaim. Cucurbitaceae. Apium petroselinum. Umbelliferae (Fam. 113). -- graveolens. Lactuca scariola. Compositae (Fam. 122). Helianthus annuus (?). Compositae. Ipomoea caerulea. Convolvulaceae (Fam. 151). -- purpurea. -- bona-nox. -- coccinea. [page 301]List of Seedling Plants (continued). Solanum lycopersicum. Solaneae (Fam. 157. )Mimulus, (sp. ?) Scrophularineae (Fam. 159)--from information given us byProf. Pfeffer. Mirabilis jalapa. Nyctagineae (Fam. 177). Mirabilis longiflora. Beta vulgaris. Polygoneae (Fam. 179). Amaranthus caudatus. Amaranthaceae (Fam. 180). Cannabis sativa (?). Cannabineae (Fam. 195). 

Brassica oleracea (Cruciferae). --It was shown in the first chapter that thecotyledons of the common cabbage rise in the evening and stand verticallyup at night with their petioles in contact. But as the two cotyledons areof unequal height, they frequently interfere a little with each other'smovements, the shorter one often not standing quite vertically. They awakeearly in the morning; thus at 6. 45 A. M. On Nov. 27th, whilst if was stilldark, the cotyledons, which had been vertical and in contact on theprevious evening, were reflexed, and thus presented a very differentappearance. It should be borne in mind that seedlings in germinating at theproper season, would not be subjected to darkness at this hour in themorning. The above amount of movement of the cotyledons is only temporary, lasting with plants kept in a warm greenhouse from four to six days; howlong it would last with seedlings growing out of doors we do not know. 

Raphanus sativus. --In the middle of the day the blades of the cotyledons of10 seedlings stood at right angles to their hypocotyls, with their petiolesa little divergent; at night the blades stood vertically, with their basesin contact and with their petioles parallel. Next morning, at 6. 45 A. M. , whilst it was still dark, the blades were horizontal. On the followingnight they were much raised, but hardly stood sufficiently vertical to besaid to be asleep, and so it was in a still less degree on the third night. Therefore the cotyledons of this plant (kept in the greenhouse) go to sleepfor even a shorter time than those of the cabbage. Similar observationswere made, but only during a single day and night, on 13 other seedlingslikewise raised in the greenhouse, with the same result. 

The petioles of the cotyledons of 11 young seedlings of Sinapis nigra wereslightly divergent at noon, and the blades stood at right angles to thehypocotyls; at night the petioles were in close contact, and the bladesconsiderably raised, with their bases in contact, but only a few stoodsufficiently upright to be called asleep. On the following morning, [page 302]the petioles diverged before it was light. The hypocotyl is slightlysensitive, so that if rubbed with a needle it bends towards the rubbedside. In the case of Lepidium sativum, the petioles of the cotyledons ofyoung seedlings diverge during the day and converge so as to touch eachother during the night, by which means the bases of the tripartite bladesare brought into contact; but the blades are so little raised that theycannot be said to sleep. The cotyledons of several other cruciferous plantswere observed, but they did not rise sufficiently during the night to besaid to sleep. 

Githago segetum (Caryophylleae). --On the first day after the cotyledons hadburst through the seed-coats, they stood at noon at an angle of 75o abovethe horizon; at night they moved upwards, each through an angle of 15o soas to stand quite vertical and in contact with one another. On the secondday they stood at noon at 59o above the horizon, and again at night werecompletely closed, each having risen 31o. On the fourth day the cotyledonsdid not quite close at night. The first and succeeding pairs of young trueleaves behaved in exactly the same manner. We think that the movement inthis case may be called nyctitropic, though the angle passed through wassmall. The cotyledons are very sensitive to light and will not expand ifexposed to an extremely dim one. 

Anoda Wrightii (Malvaceae). --The cotyledons whilst moderately young, andonly from . 2 to . 3 inch in diameter, sink in the evening from their mid-dayhorizontal position to about 35o beneath the horizon. But when the sameseedlings were older and had produced small true leaves, the almostorbicular cotyledons, now . 55 inch in diameter, moved vertically downwardsat night. This fact made us suspect that their sinking might be due merelyto their weight; but they were not in the least flaccid, and when lifted upsprang back through elasticity into their former dependent position. A potwith some old seedlings was turned upside down in the afternoon, before thenocturnal fall had commenced, and at night they assumed in opposition totheir own weight (and to any geotropic action) an upwardly directedvertical position. When pots were thus reversed, after the evening fall hadalready commenced, the sinking movement appeared to be somewhat disturbed;but all their movements were occasionally variable without any apparentcause. This latter fact, as well as that of the young cotyledons notsinking nearly so much as the older ones, deserves notice. [page 303]Although the movement of the cotyledons endured for a long time, nopulvinus was exteriorly visible; but their growth continued for a longtime. The cotyledons appear to be only slightly heliotropic, though thehypocotyl is strongly so. 

Gossypium arboreum (?) (var. Nankin cotton) (Malvaceae). --The cotyledonsbehave in nearly the same manner as those of the Anoda. On June 15th thecotyledons of two seedlings were . 65 inch in length (measured along themidrib) and stood horizontally at noon; at 10 P. M. They occupied the sameposition and had not fallen at all. On June 23rd, the cotyledons of one ofthese seedlings were 1. 1 inch in length, and by 10 P. M. They had fallenfrom a horizontal position to 62o beneath the horizon. The cotyledons ofthe other seedling were 1. 3 inch in length, and a minute true leaf had beenformed; they had fallen at 10 P. M. To 70o beneath the horizon. On June25th, the true leaf of this latter seedling was . 9 inch in length, and thecotyledons occupied nearly the same position at night. By July 9th thecotyledons appeared very old and showed signs of withering; but they stoodat noon almost horizontally, and at 10 P. M. Hung down vertically. 

Gossypium herbaceum. --It is remarkable that the cotyledons of this speciesbehave differently from those of the last. They were observed during 6weeks from their first development until they had grown to a very largesize (still appearing fresh and green), viz. 2 ½ inches in breadth. At thisage a true leaf had been formed, which with its petiole was 2 inches long. During the whole of these 6 weeks the cotyledons did not sink at night; yetwhen old their weight was considerable and they were borne by muchelongated petioles. Seedlings raised from some seed sent us from Naples, behaved in the same manner; as did those of a kind cultivated in Alabamaand of the Sea-island cotton. To what species these three latter formsbelong we do not know. We could not make out in the case of the Naplescotton, that the position of the cotyledons at night was influenced by thesoil being more or less dry; care being taken that they were not renderedflaccid by being too dry. The weight of the large cotyledons of the Alabamaand Sea-island kinds caused them to hang somewhat downwards, when the potsin which they grew were left for a time upside down. It should, however, beobserved that these three kinds were raised in the middle of the winter, which sometimes greatly interferes with the proper nyctitropic movements ofleaves and cotyledons. [page 304]

Cucurbitaceae. --The cotyledons of Cucurbita aurantia and ovifera, and ofLagenaria vulgaris, stand from the 1st to the 3rd day of their life atabout 60o above the horizon, and at night rise up so as to become verticaland in close contact with one another. With Cucumis dudaim they stood atnoon at 45o above the horizon, and closed at night. The tips of thecotyledons of all these species are, however, reflexed, so that this partis fully exposed to the zenith at night; and this fact is opposed to thebelief that the movement is of the same nature as that of sleeping plants. After the first two or three days the cotyledons diverge more during theday and cease to close at night. Those of Trichosanthes anguina aresomewhat thick and fleshy, and did not rise at night; and they couldperhaps hardly be expected to do so. On the other hand, those ofAcanthosicyos horrida* present nothing in their appearance opposed to theirmoving at night in the same manner as the preceding species; yet they didnot rise up in any plain manner. This fact leads to the belief that thenocturnal movements of the above-named species has been acquired for somespecial purpose, which may be to protect the young plumule from radiation, by the close contact of the whole basal portion of the two cotyledons. 

Geranium rotundifolium (Geraniaceae). --A single seedling came upaccidentally in a pot, and its cotyledons were observed to bendperpendicularly downwards during several successive nights, having beenhorizontal at noon. It grew into a fine plant but died before flowering: itwas sent to Kew and pronounced to be certainly a Geranium, and in allprobability the above-named species. This case is remarkable because thecotyledons of G. Cinereum, Endressii, Ibericum, Richardsoni, andsubcaulescens were observed during some weeks in the winter, and they didnot sink, whilst those of G. Ibericum rose 27o at night. 

Apium petroselinum (Umbelliferae). --A seedling had its cotyledons (Nov. 22nd) almost fully expanded during the day; by 8. 30 P. M. They had risenconsiderably, and at 10. 30 P. M. Were almost closed, their tips being only8/100 of an inch apart. On the following morning (23rd) the tips were58/100 of an inch apart, * This plant, from Dammara Land in S. Africa, is remarkable from being theone known member of the Family which is not a climber; it has beendescribed in 'Transact. Linn. Soc. , ' xxvii. P. 30. [page 305]

or more than seven times as much. On the next night the cotyledons occupiednearly the same position as before. On the morning of the 24th they stoodhorizontally, and at night were 60o above the horizon; and so it was on thenight of the 25th. But four days afterwards (on the 29th), when theseedlings were a week old, the cotyledons had ceased to rise at night toany plain degree. 

Apium graveolens. --The cotyledons at noon were horizontal, and at 10 P. M. Stood at an angle of 61o above the horizon. 

Lactuca scariola (Compositae). --The cotyledons whilst young stoodsub-horizontally during the day, and at night rose so as to be almostvertical, and some were quite vertical and closed; but this movement ceasedwhen they had grown old and large, after an interval of 11 days. 

Helianthus annuus (Compositae). --This case is rather doubtful; thecotyledons rise at night, and on one occasion they stood at 73o above thehorizon, so that they might then be said to have been asleep. 

Ipomoea caerulea vel Pharbitis nil (Convolvulaceae). --The cotyledons behavein nearly the same manner as those of the Anoda and Nankin cotton, and likethem grow to a large size. Whilst young and small, so that their bladeswere from . 5 to . 6 of an inch in length, measured along the middle to thebase of the central notch, they remained horizontal both during the middleof the day and at night. As they increased in size they began to sink moreand more in the evening and early night; and when they had grown to alength (measured in the above manner) of from 1 to 1. 25 inch, they sankbetween 55o and 70o beneath the horizon. They acted, however, in thismanner only when they had been well illuminated during the day. Nevertheless, the cotyledons have little or no power of bending towards alateral light, although the hypocotyl is strongly heliotropic. They are notprovided with a pulvinus, but continue to grow for a long time. 

Ipomoea purpurea (vel Pharbitis hispida). --The cotyledons behave in allrespects like those of I. Caerulea. A seedling with cotyledons . 75 inch inlength (measured as before) and 1. 65 inch in breadth, having a small trueleaf developed, was placed at 5. 30 P. M. On a klinostat in a darkened box, so that neither weight nor geotropism could act on them. At 10 P. M. Onecotyledon stood at 77o and the other at 82o beneath the horizon. Beforebeing placed in the klinostat they stood at 15o and 29o[page 306]beneath the horizon. The nocturnal position depends chiefly on thecurvature of the petiole close to the blade, but the whole petiole becomesslightly curved downwards. It deserves notice that seedlings of this andthe last-named species were raised at the end of February and another lotin the middle of March, and the cotyledons in neither case exhibited anynyctitropic movement. 

Ipomoea bona-nox. --The cotyledons after a few days grow to an enormoussize, those on a young seedling being 3 1/4 inches in breadth. They wereextended horizontally at noon, and at 10 P. M. Stood at 63o beneath thehorizon. Five days afterwards they were 4 ½ inches in breadth, and at nightone stood at 64o and the other 48o beneath the horizon. Though the bladesare thin, yet from their great size and from the petioles being long, weimagined that their depression at night might be determined by theirweight; but when the pot was laid horizontally, they became curved towardsthe hypocotyl, which movement could not have been in the least aided bytheir weight, at the same time they were somewhat twisted upwards throughapogeotropism. Nevertheless, the weight of the cotyledons is so farinfluential, that when on another night the pot was turned upside down, they were unable to rise and thus to assume their proper nocturnalposition. 

Ipomoea coccinea. --The cotyledons whilst young do not sink at night, butwhen grown a little older, but still only . 4 inch in length (measured asbefore) and . 82 in breadth, they became greatly depressed. In one case theywere horizontal at noon, and at 10 P. M. One of them stood at 64o and theother at 47o beneath the horizon. The blades are thin, and the petioles, which become much curved down at night, are short, so that here weight canhardly have produced any effect. With all the above species of Ipomoea, when the two cotyledons on the same seedling were unequally depressed atnight, this seemed to depend on the position which they had held during theday with reference to the light. 

Solanum lycopersicum (Solaneae). --The cotyledons rise so much at night asto come nearly in contact. Those of 'S. Palinacanthum' were horizontal atnoon, and by 10 P. M. Had risen only 27o 30 minutes; but on the followingmorning before it was light they stood at 59o above the horizon, and in theafternoon of the same day were again horizontal. The behaviour of thecotyledons of this latter species seems, therefore, to be anomalous. [page 307]

Mirabilis jalapa and longiflora (Nyctagineae). --The cotyledons, which areof unequal size, stand horizontally during the middle of the day, and atnight rise up vertically and come into close contact with one another. Butthis movement with M. Longiflora lasted for only the three first nights. 

Beta vulgaris (Polygoneae). --A large number of seedlings were observed onthree occasions. During the day the cotyledons sometimes stoodsub-horizontally, but more commonly at an angle of about 50o above thehorizon, and for the first two or three nights they rose up vertically soas to be completely closed. During the succeeding one or two nights theyrose only a little, and afterwards hardly at all. 

Amaranthus caudatus (Amaranthaceae). --At noon the cotyledons of manyseedlings, which had just germinated, stood at about 45o above the horizon, and at 10. 15 P. M. Some were nearly and the others quite closed. On thefollowing morning they were again well expanded or open. 

Cannabis sativa (Cannabineae). --We are very doubtful whether this plantought to be here included. The cotyledons of a large number of seedlings, after being well illuminated during the day, were curved downwards atnight, so that the tips of some pointed directly to the ground, but thebasal part did not appear to be at all depressed. On the following morningthey were again flat and horizontal. The cotyledons of many other seedlingswere at the same time not in any way affected. Therefore this case seemsvery different from that of ordinary sleep, and probably comes under thehead of epinasty, as is the case with the leaves of this plant according toKraus. The cotyledons are heliotropic, and so is the hypocotyl in a stillstronger degree. 

Oxalis. --We now come to cotyledons provided with a pulvinus, all of whichare remarkable from the continuance of the nocturnal movements duringseveral days or even weeks, and apparently after growth has ceased. Thecotyledons of O. Rosea, floribunda and articulata sink vertically down atnight and clasp the upper part of the hypocotyl. Those of O. Valdiviana andsensitiva, on the contrary, rise vertically up, so that their uppersurfaces come into close contact; and after the young leaves are developedthese are clasped by the cotyledons. As in the daytime they standhorizontally, or are even a little deflected beneath the horizon, they movein the evening through an angle of at least 90o. Their complicatedcircumnutating movements during the day have[page 308]been described in the first chapter. The experiment was a superfluous one, but pots with seedlings of O. Rosea and floribunda were turned upside down, as soon as the cotyledons began to show any signs of sleep, and this madeno difference in their movements. 

Leguminosae. --It may be seen in our list that the cotyledons of severalspecies in nine genera, widely distributed throughout the Family, sleep atnight; and this probably is the case with many others. The cotyledons ofall these species are provided with a pulvinus; and the movement in all iscontinued during many days or weeks. In Cassia the cotyledons of the tenspecies in the list rise up vertically at night and come into close contactwith one another. We observed that those of C. Florida opened in themorning rather later than those of C. Glauca and pubescens. The movement isexactly the same in C. Mimosoides as in the other species, though itssubsequently developed leaves sleep in a different manner. The cotyledonsof an eleventh species, namely, C. Nodosa, are thick and fleshy, and do notrise up at night. The circumnutation of the cotyledons during the day of C. Tora has been described in the first chapter. Although the cotyledons ofSmithia sensitiva rose from a horizontal position in the middle of the dayto a vertical one at night, those of S. Pfundii, which are thick andfleshy, did not sleep. When Mimosa pudica and albida have been kept at asufficiently high temperature during the day, the cotyledons come intoclose contact at night; otherwise they merely rise up almost vertically. The circumnutation of those of M. Pudica has been described. The cotyledonsof a Bauhinia from St. Catharina in Brazil stood during the day at an angleof about 50o above the horizon, and at night rose to 77o; but it isprobable that they would have closed completely, if the seedlings had beenkept in a warmer place. 

Lotus. --In three species of Lotus the cotyledons were observed to sleep. Those of L. Jacoboeus present the singular case of not rising at night inany conspicuous manner for the first 5 or 6 days of their life, and thepulvinus is not well developed at this period. Afterwards the sleepingmovement is well displayed, though to a variable degree, and is longcontinued. We shall hereafter meet with a nearly parallel case with theleaves of Sida rhombifolia. The cotyledons of L. Gebelii are only slightlyraised at night, and differ much in this respect from the three species inour list. [page 309]

Trifolium. --The germination of 21 species was observed. In most of them thecotyledons rise hardly at all, or only slightly, at night; but those of T. Glomeratum, striatum and incarnactum rose from 45o to 55o above thehorizon. With T. Subterraneum, leucanthemum and strictum, they stood upvertically; and with T. Strictum the rising movement is accompanied, as weshall see, by another movement, which makes us believe that the rising istruly nyctitropic. We did not carefully examine the cotyledons of all thespecies for a pulvinus, but this organ was distinctly present in those ofT. Subterraneum and strictum; whilst there was no trace of a pulvinus insome species, for instance, in T. Resupinatum, the cotyledons of which donot rise at night. 

Trifolium subterraneum. --The blades of the cotyledons on the first dayafter germination (Nov. 21st) were not fully expanded, being inclined atabout 35o above the horizon; at night they rose to about 75o. Two daysafterwards the blades at noon were horizontal, with the petioles highlyinclined upwards; and it is remarkable that the nocturnal movement isalmost wholly confined to the blades, being effected by the pulvinus attheir bases; whilst the petioles retain day and night nearly the sameinclination. On this night (Nov. 23rd), and for some few succeeding nights, the blades rose from a horizontal into a vertical position, and then becamebowed inwards at about an average angle of 10o; so that they had passedthrough an angle of 100o. Their tips now almost touched one another, theirbases being slightly divergent. The two blades thus formed a highlyinclined roof over the axis of the seedling. This movement is the same asthat of the terminal leaflet of the tripartite leaves of many species ofTrifolium. After an interval of 8 days (Nov. 29th) the blades werehorizontal during the day, and vertical at night, and now they were nolonger bowed inwards. They continued to move in the same manner for thefollowing two months, by which time they had increased greatly in size, their petioles being no less than . 8 of an inch in length, and two trueleaves had by this time been developed. 

Trifolium strictum. --On the first day after germination the cotyledons, which are provided with a pulvinus, stood at noon horizontally, and atnight rose to only about 45o above the horizon. Four days afterwards theseedlings were again observed at night, and now the blades stood verticallyand were in contact, excepting the tips, which were much deflexed, so thatthey faced the zenith. At this age the petioles are curved[page 310]upwards, and at night, when the bases of the blades are in contact, the twopetioles together form a vertical ring surrounding the plumule. Thecotyledons continued to act in nearly the same manner for 8 or 10 days fromthe period of germination; but the petioles had by this time becomestraight and had increased much in length. After from 12 to 14 days thefirst simple true leaf was formed, and during the ensuing fortnight aremarkable movement was repeatedly observed. At I. (Fig. 125) we have asketch, made in the middle of the day, of a seedling about a fortnight old. The two cotyledons, of which Rc is the right and Lc the left one, standdirectly opposite one another, Fig. 125. Trifolium strictum: diurnal and nocturnal positions of the twocotyledons and of the first leaf. I. Seedling viewed obliquely from above, during the day: Rc, right cotyledon; Lc, left cotyledon; F, first trueleaf. II. A rather younger seedling, viewed at night: Rc, right cotyledonraised, but its position not otherwise changed; Lc, left cotyledon raisedand laterally twisted; F, first leaf raised and twisted so as to face theleft twisted cotyledon. III. Same seedling viewed at night from theopposite side. The back of the first leaf, F, is here shown instead of thefront, as in II. 

and the first true leaf (F) projects at right angles to them. At night (seeII. And III. ) the right cotyledon (Rc) is greatly raised, but is nototherwise changed in position. The left cotyledon (Lc) is likewise raised, but it is also twisted so that its blade, instead of exactly facing theopposite one, now stands at nearly right angles to it. This nocturnaltwisting movement is effected not by means of the pulvinus, but by thetwisting of the whole length of the petiole, as could be seen by the curvedline of its upper concave surface. At the same time the true leaf (F) risesup, so as to stand vertically, or it even passes the vertical and isinclined a little inwards. It also twists a little, by which means theupper surface of its blade fronts, and almost comes into contact with, theupper surface of the twisted[page 311]left cotyledon. This seems to be the object gained by these singularmovements. Altogether 20 seedlings were examined on successive nights, andin 19 of them it was the left cotyledon alone which became twisted, withthe true leaf always so twisted that its upper surface approached closelyand fronted that of the left cotyledon. In only one instance was the rightcotyledon twisted, with the true leaf twisted towards it; but this seedlingwas in an abnormal condition, as the left cotyledon did not rise upproperly at night. This whole case is remarkable, as with the cotyledons ofno other plant have we seen any nocturnal movement except verticallyupwards or downwards. It is the more remarkable, because we shall meet withan analogous case in the leaves of the allied genus Melilotus, in which theterminal leaflet rotates at night so as to present one edge to the zenithand at the same time bends to one side, so that its upper surface comesinto contact with that of one of the two now vertical lateral leaflets. ]

Concluding Remarks on the Nyctitropic Movements of Cotyledons. --The sleepof cotyledons (though this is a subject which has been little attended to), seems to be a more common phenomenon than that of leaves. We observed theposition of the cotyledons during the day and night in 153 genera, widelydistributed throughout the dicotyledonous series, but otherwise selectedalmost by hazard; and one or more species in 26 of these genera placedtheir cotyledons at night so as to stand vertically or almost vertically, having generally moved through an angle of at least 60o. If we lay on oneside the Leguminosae, the cotyledons of which are particularly liable tosleep, 140 genera remain; and out of these, the cotyledons of at least onespecies in 19 genera slept. Now if we were to select by hazard 140 genera, excluding the Leguminosae, and observed their leaves at night, assuredlynot nearly so many as 19 would be found to include sleeping species. Wehere refer exclusively to the plants observed by ourselves. [page 312]

In our entire list of seedlings, there are 30 genera, belonging to 16Families, the cotyledons of which in some of the species rise or sink inthe evening or early night, so as to stand at least 60o above or beneaththe horizon. In a large majority of the genera, namely, 24, the movement isa rising one; so that the same direction prevails in these nyctitropicmovements as in the lesser periodic ones described in the second chapter. The cotyledons move downwards during the early part of the night in only 6of the genera; and in one of them, Cannabis, the curving down of the tip isprobably due to epinasty, as Kraus believes to be the case with the leaves. The downward movement to the amount of 90o is very decided in OxalisValdiviana and sensitiva, and in Geranium rotundifolium. It is a remarkablefact that with Anoda Wrightii, one species of Gossypium and at least 3species of Ipomoea, the cotyledons whilst young and light sink at nightvery little or not at all; although this movement becomes well pronouncedas soon as they have grown large and heavy. Although the downward movementcannot be attributed to the weight of the cotyledons in the several caseswhich were investigated, namely, in those of the Anoda, Ipomoea purpureaand bona-nox, nor in that of I. Coccinea, yet bearing in mind thatcotyledons are continually circumnutating, a slight cause might at firsthave determined whether the great nocturnal movement should be upwards ordownwards. We may therefore suspect that in some aboriginal member of thegroups in question, the weight of the cotyledons first determined thedownward direction. The fact of the cotyledons of these species not sinkingdown much whilst they are young and tender, seems opposed to the beliefthat the greater movement when they are[page 313]grown older, has been acquired for the sake of protecting them fromradiation at night; but then we should remember that there are many plants, the leaves of which sleep, whilst the cotyledons do not; and if in somecases the leaves are protected from cold at night whilst the cotyledons arenot protected, so in other cases it may be of more importance to thespecies that the nearly full-grown cotyledons should be better protectedthan the young ones. 

In all the species of Oxalis observed by us, the cotyledons are providedwith pulvini; but this organ has become more or less rudimentary in O. Corniculata, and the amount of upward movement of its cotyledons at nightis very variable, but is never enough to be called sleep. We omitted toascertain whether the cotyledons of Geranium rotundifolium possess pulvini. In the Leguminosae all the cotyledons which sleep, as far as we have seen, are provided with pulvini. But with Lotus Jacobaeus, these are not fullydeveloped during the first few days of the life of the seedling, and thecotyledons do not then rise much at night. With Trifolium strictum theblades of the cotyledons rise at night by the aid of their pulvini; whilstthe petiole of one cotyledon twists half-round at the same time, independently of its pulvinus. 

As a general rule, cotyledons which are provided with pulvini continue torise or sink at night during a much longer period than those destitute ofthis organ. In this latter case the movement no doubt depends onalternately greater growth on the upper and lower side of the petiole, orof the blade, or of both, preceded probably by the increased turgescence ofthe growing cells. Such movements generally last for a very short period--for instance, with Brassica and Githago for 4 or 5 nights, with Beta for 2or 3, and with[page 314]Raphanus for only a single night. There are, however, some strongexceptions to this rule, as the cotyledons of Gossypium, Anoda and Ipomoeado not possess pulvini, yet continue to move and to grow for a long time. We thought at first that when the movement lasted for only 2 or 3 nights, it could hardly be of any service to the plant, and hardly deserved to becalled sleep; but as many quickly-growing leaves sleep for only a fewnights, and as cotyledons are rapidly developed and soon complete theirgrowth, this doubt now seems to us not well-founded, more especially asthese movements are in many instances so strongly pronounced. We may heremention another point of similarity between sleeping leaves and cotyledons, namely, that some of the latter (for instance, those of Cassia and Githago)are easily affected by the absence of light; and they then either close, orif closed do not open; whereas others (as with the cotyledons of Oxalis)are very little affected by light. In the next chapter it will be shownthat the nyctitropic movements both of cotyledons and leaves consist of amodified form of circumnutation. 

As in the Leguminosae and Oxalidae, the leaves and the cotyledons of thesame species generally sleep, the idea at first naturally occurred to us, that the sleep of the cotyledons was merely an early development of a habitproper to a more advanced stage of life. But no such explanation can beadmitted, although there seems to be some connection, as might have beenexpected, between the two sets of cases. For the leaves of many plantssleep, whilst their cotyledons do not do so--of which fact Desmodium gyransoffers a good instance, as likewise do three species of Nicotiana observedby us; also Sida rhombifolia, Abutilon Darwinii, and Chenopodium album. Onthe other[page 315]hand, the cotyledons of some plants sleep and not the leaves, as with thespecies of Beta, Brassica, Geranium, Apium, Solanum, and Mirabilis, namedin our list. Still more striking is the fact that, in the same genus, theleaves of several or of all the species may sleep, but the cotyledons ofonly some of them, as occurs with Trifolium, Lotus, Gossypium, andpartially with Oxalis. Again, when both the cotyledons and the leaves ofthe same plant sleep, their movements may be of a widely dissimilar nature:thus with Cassia the cotyledons rise vertically up at night, whilst theirleaves sink down and twist round so as to turn their lower surfacesoutwards. With seedlings of Oxalis Valdiviana, having 2 or 3 well-developedleaves, it was a curious spectacle to behold at night each leaflet foldedinwards and hanging perpendicularly downwards, whilst at the same time andon the same plant the cotyledons stood vertically upwards. 

These several facts, showing the independence of the nocturnal movements ofthe leaves and cotyledons on the same plant, and on plants belonging to thesame genus, lead to the belief that the cotyledons have acquired theirpower of movement for some special purpose. Other facts lead to the sameconclusion, such as the presence of pulvini, by the aid of which thenocturnal movement is continued during some weeks. In Oxalis the cotyledonsof some species move vertically upwards, and of others vertically downwardsat night; but this great difference within the same natural genus is not sosurprising as it may at first appear, seeing that the cotyledons of all thespecies are continually oscillating up and down during the day, so that asmall cause might determine whether they should rise or sink at night. Again, the peculiar nocturnal movement of the left-hand coty-[page 316]ledon of Trifolium strictum, in combination with that of the first trueleaf. Lastly, the wide distribution in the dicotyledonous series of plantswith cotyledons which sleep. Reflecting on these several facts, ourconclusion seems justified, that the nyctitropic movements of cotyledons, by which the blade is made to stand either vertically or almost verticallyupwards or downwards at night, has been acquired, at least in most cases, for some special purpose; nor can we doubt that this purpose is theprotection of the upper surface of the blade, and perhaps of the centralbud or plumule, from radiation at night. [page 317]

CHAPTER VII. 

MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES. 

Conditions necessary for these movements--List of Genera and Families, which include sleeping plants--Description of the movements in the severalGenera--Oxalis: leaflets folded at night--Averrhoa: rapid movements of theleaflets--Porlieria: leaflets close when plant kept very dry--Tropaeolum:leaves do not sleep unless well illuminated during day--Lupinus: variousmodes of sleeping--Melilotus: singular movements of terminal leaflet--Trifolium--Desmodium: rudimentary lateral leaflets, movements of, notdeveloped on young plants, state of their pulvini--Cassia: complexmovements of the leaflets--Bauhinia: leaves folded at night--Mimosa pudica:compounded movements of leaves, effect of darkness--Mimosa albida, reducedleaflets of--Schrankia: downward movement of the pinnae--Marsilea: the onlycryptogam known to sleep--Concluding remarks and summary--Nyctitropismconsists of modified circumnutation, regulated by the alternations of lightand darkness--Shape of first true leaves. 

WE now come to the nyctitropic or sleep movements of leaves. It should beremembered that we confine this term to leaves which place their blades atnight either in a vertical position or not more than 30o from thevertical, --that is, at least 60o above or beneath the horizon. In some fewcases this is effected by the rotation of the blade, the petiole not beingeither raised or lowered to any considerable extent. The limit of 30o fromthe vertical is obviously an arbitrary one, and has been selected forreasons previously assigned, namely, that when the blade approaches theperpendicular as nearly as this, only half as much of the surface isexposed at night to the[page 318]zenith and to free radiation as when the blade is horizontal. Nevertheless, in a few instances, leaves which seem to be prevented by their structurefrom moving to so great an extent as 60o above or beneath the horizon, havebeen included amongst sleeping plants. 

It should be premised that the nyctitropic movements of leaves are easilyaffected by the conditions to which the plants have been subjected. If theground is kept too dry, the movements are much delayed or fail: accordingto Dassen, * even if the air is very dry the leaves of Impatiens and Malvaare rendered motionless. Carl Kraus has also lately insisted** on the greatinfluence which the quantity of water absorbed has on the periodicmovements of leaves; and he believes that this cause chiefly determines thevariable amount of sinking of the leaves of Polygonum convolvulus at night;and if so, their movements are not in our sense strictly nyctitropic. Plants in order to sleep must have been exposed to a proper temperature:Erythrina crista-galli, out of doors and nailed against a wall, seemed infairly good health, but the leaflets did not sleep, whilst those on anotherplant kept in a warm greenhouse were all vertically dependent at night. Ina kitchen-garden the leaflets of Phaseolus vulgaris did not sleep duringthe early part of the summer. Ch. Royer says, *** referring I suppose to thenative plants in France, that they do not sleep when the temperature isbelow 5o C. Or 41o F. In the case of several sleeping plants, viz. , speciesof

* Dassen, 'Tijdschrift vor. Naturlijke Gesch. En Physiologie, ' 1837, vol. Iv. P. 106. See also Ch. Royer on the importance of a proper state ofturgescence of the cells, in 'Annal. Des Sc. Nat. Bot. ' (5th series), ix. 1868, p. 345. 

** 'Beiträge zur Kentniss der Bewegungen, ' etc. , in 'Flora, ' 1879, pp. 42, 43, 67, etc. 

*** 'Annal. Des Sc. Nat. Bot. ' (5th Series), ix. 1868, p. 366. [page 319]

Tropaeolum, Lupinus, Ipomoea, Abutilon, Siegesbeckia, and probably othergenera, it is indispensable that the leaves should be well illuminatedduring the day in order that they may assume at night a vertical position;and it was probably owing to this cause that seedlings of Chenopodium albumand Siegesbeckia orientalis, raised by us during the middle of the winter, though kept at a proper temperature, did not sleep. Lastly, violentagitation by a strong wind, during a few minutes, of the leaves of Marantaarundinacea (which previously had not been disturbed in the hot-house), prevented their sleeping during the two next nights. 

We will now give our observations on sleeping plants, made in the mannerdescribed in the Introduction. The stem of the plant was always secured(when not stated to the contrary) close to the base of the leaf, themovements of which were being observed, so as to prevent the stem fromcircumnutating. As the tracings were made on a vertical glass in front ofthe plant, it was obviously impossible to trace its course as soon as theleaf became in the evening greatly inclined either upwards or downwards; itmust therefore be understood that the broken lines in the diagrams, whichrepresent the evening and nocturnal courses, ought always to be prolongedto a much greater distance, either upwards or downwards, than appears inthem. The conclusions which may be deduced from our observations will begiven near the end of this chapter. 

In the following list all the genera which include sleeping plants aregiven, as far as known to us. The same arrangement is followed as in formercases, and the number of the Family is appended. This list possesses someinterest, as it shows that the habit of[page 320]sleeping is common to some few plants throughout the whole vascular series. The greater number of the genera in the list have been observed byourselves with more or less care; but several are given on the authority ofothers (whose names are appended in the list), and about these we havenothing more to say. No doubt the list is very imperfect, and severalgenera might have been added from the 'Somnus Plantarum' by Linnaeus; butwe could not judge in some of his cases, whether the blades occupied atnight a nearly vertical position. He refers to some plants as sleeping, forinstance, Lathyrus odoratus and Vicia faba, in which we could observe nomovement deserving to be called sleep, and as no one can doubt the accuracyof Linnaeus, we are left in doubt. 

[List of Genera, including species the leaves of which sleep. 

CLASS I. DICOTYLEDONS. 

Sub-class I. ANGIOSPERMS. 

Genus Family. 

Githago Caryophylleae (26). Stellaria (Batalin). "Portulaca (Ch. Royer). Portulaceae (27). Sida Malvaceae (36). Abutilon. "Malva (Linnaeus and Pfeffer). "Hibiscus (Linnaeus). "Anoda. "Gossypium. "Ayenia (Linnaeus). Sterculaceae (37). Triumfetta (Linnaeus). Tiliaceae (38). Linum (Batalin). Lineae (39). Oxalis. Oxalidae (41). Averrhoa. "Porlieria. Zygophylleae (45). Guiacum. "Impatiens (Linnaeus, Pfeffer, Batalin). Balsamineae (48). Tropaeolum. Tropaeoleae (49). Crotolaria (Thiselton Dyer). Leguminosae (75) Tribe II. Lupinus. " "Cytisus. " "Trigonella. " Tr. III. Medicago. "Melilotus. " "Trifolium. " "Securigera. " Tr. IV. Lotus. " "Psoralea. " Tr. V. Amorpha (Cuchartre). " "Daelea. " "Indigofera. " "Tephrosia. " "Wistaria. " "Robinia. " "Sphaerophysa. " "Colutea. " "Astragalus. " "Glycyrrhiza. " "Coronilla. " Tr. VI. Hedysarum. " "[page 321]

List of Genera (continued). 

CLASS I. DICOTYLEDONS. 

Sub-class I. ANGIOSPERMS. 

Genus Family. Onobrychis. Leguminosae (75) Tr. VI. Smithia. " "Arachis. " "Desmodium. " "Urania. " "Vicia. " Tr. VII. Centrosema. " Tr. VIII. Amphicarpaea. " "Glycine. " "Erythrina. " "Apios. " "Phaseolus. " "Sophora. " Tr. X. Caesalpinia. " Tr. XIII. Haematoxylon. " "Gleditschia (Duchartre). " "Poinciana. " "Cassia. " Tr. XIV. Bauhinia. " Tr. XV. Tamarindus. " Tr. XVI. Adenanthera. " Tr. XX. Prosopis. " "Neptunia. " "Mimosa. " "Schrankia. " "Acacia. " Tr. XXII. Albizzia. " Tr. XXIII. Melaleuca (Bouché). Myrtaceae (94). 

Sub-class I. ANGIOSPERMS (continued). 

Genus Family. Aenothera (Linnaeus). Omagrarieae (100). Passiflora. Passifloracea (105). Siegesbeckia. Compositae (122). Ipomoea. Convolvulacea (151). Nicotiana. Solaneae (157). Mirabilis. Nyctagineae (177). Polygonum (Batalin). Polygoneae (179). Amaranthus. Amaranthaceae (180). Chenopodium. Chenopodieae (181). Pimelia (Bouché). Thymeteae (188). Euphorbia. Euphorbiaceae (202)Phyllanthus (Pfeffer). "

Sub-class II. GYMNOSPERMS. Aies (Chatin). 

CLASS II. MONOCOTYLEDONS. 

Thalia. Cannaceae (21). Maranta. "Colocasia. Aroideae (30). Strephium. Gramineae (55). 

CLASS III. ACOTYLEDONS. 

Marsilea. Marsileaceae (4). 

Githago segetum (Caryophylleae). --The first leaves produced by youngseedlings, rise up and close together at night. On a rather older seedling, two young leaves stood at noon at 55o above the horizon, and at night at86o, so each had risen 31o. The angle, however, was less in some cases. Similar observations were occasionally made on young leaves (for the olderones moved very little) produced by nearly full-grown plants. Batalin says('Flora, ' Oct. 1st, 1873, p. 437) that the young leaves of Stellaria closeup so completely at night that they form together great buds. 

Sida (Malvaceae). --the nyctitropic movements of the leaves in this genusare remarkable in some respects. Batalin informs[page 322]us (see also 'Flora, ' Oct. 1st, 1873, p. 437) that those of S. Napaea fallat night, but to what angle he cannot remember. The leaves of S. Rhombifolia and retusa, on the other hand, rise up vertically, and arepressed against the stem. We have therefore here within the same genus, directly opposite movements. Again, the leaves of S. Rhombifolia arefurnished with a pulvinus, formed of a mass of small cells destitute ofchlorophyll, and with their longer axes perpendicular to the axis of thepetiole. As measured along this latter line, these cells are only 1/5th ofthe length of those of the petiole; but instead of being abruptly separatedfrom them (as is usual with the pulvinus in most plants), they graduateinto the larger cells of the petiole. On the other hand, S. Napaea, according to Batalin, does not possess a pulvinus; and he informs us that agradation may be traced in the several species of the genus between thesetwo states of the petiole. Sida rhombifolia presents another peculiarity, of which we have seen no other instance with leaves that sleep: for thoseon very young plants, though they rise somewhat in the evening, do not goto sleep, as we observed

Fig. 126. Sida rhombifolia: circumnutation and nyctitropic (or sleep)movements of a leaf on a young plant, 9 ½ inches high; filament fixed tomidrib of nearly full-grown leaf, 2 3/8 inches in length; movement tracedunder a sky-light. Apex of leaf 5 5/8 inches from the vertical glass, sodiagram not greatly enlarged. [page 323]

on several occasions; whilst those on rather older plants sleep in aconspicuous manner. For instance a leaf (. 85 of an inch in length) on avery young seedling 2 inches high, stood at noon 9o above the horizon, andat 10 P. M. At 28o, so it had risen only 19o; another leaf (1. 4 inch inlength) on a seedling of the same height, stood at the same two periods at7o and 32o, and therefore had risen 25o. These leaves, which moved solittle, had a fairly well-developed pulvinus. After an interval of someweeks, when the same seedlings were 2 ½ and 3 inches in height, some of theyoung leaves stood up at night quite vertically, and others were highlyinclined; and so it was with bushes which were fully grown and wereflowering. 

The movement of a leaf was traced from 9. 15 A. M. On May 28th to 8. 30 A. M. On the 30th. The temperature was too low (15o - 16o C. ), and theillumination hardly sufficient; consequently the leaves did not becomequite so highly inclined at night, as they had done previously and as theydid subsequently in the hot-house: but the movements did not appearotherwise disturbed. On the first day the leaf sank till 5. 15 P. M. ; it thenrose rapidly and greatly till 10. 5 P. M. , and only a little higher duringthe rest of the night (Fig. 126). Early on the next day (29th) it fell in aslightly zigzag line rapidly until 9 A. M. , by which time it had reachednearly the same place as on the previous morning. During the remainder ofthe day it fell slowly, and zigzagged laterally. The evening rise beganafter 4 P. M. In the same manner as before, and on the second morning itagain fell rapidly. The ascending and descending lines do not coincide, asmay be seen in the diagram. On the 30th a new tracing was made (not heregiven) on a rather enlarged scale, as the apex of the leaf now stood 9inches from the vertical glass. In order to observe more carefully thecourse pursued at the time when the diurnal fall changes into the nocturnalrise, dots were made every half-hour between 4 P. M. And 10. 30 P. M. Thisrendered the lateral zigzagging movement during the evening moreconspicuous than in the diagram given, but it was of the same nature asthere shown. The impression forced on our minds was that the leaf wasexpending superfluous movement, so that the great nocturnal rise might notoccur at too early an hour. 

Abutilon Darwinii (Malvaceae). --The leaves on some very young plants stoodalmost horizontally during the day, and hung down vertically at night. Veryfine plants kept in a[page 324]large hall, lighted only from the roof, did not sleep at night for in orderto do so the leaves must be well illuminated during the day. The cotyledonsdo not sleep. Linnaeus says that the leaves of his Sida abutilon sinkperpendicularly down at night, though the petioles rise. Prof. Pfefferinforms us that the leaves of a Malva, allied to M. Sylvestris, risegreatly at night; and this genus, as well as that of Hibiscus, are includedby Linnaeus in his list of sleeping plants. 

Anoda Wrightii (Malvaceae). --The leaves, produced by very young plants, when grown to a moderate size, sink at night either almost vertically downor to an angle of about 45o beneath the horizon; for there is aconsiderable degree of variability in the amount of sinking at night, whichdepends in part on the degree to which they have been illuminated duringthe day. But the leaves, whilst quite young, do not sink down at night, andthis is a very unusual circumstance. The summit of the petiole, where itjoins the blade, is developed into a pulvinus, and this is present in veryyoung leaves which do not sleep; though it is not so well defined as inolder leaves. 

Gossypium (var. Nankin cotton, Malvaceae). --Some young leaves, between 1and 2 inches in length, borne by two seedlings 6 and 7 ½ inches in height, stood horizontally, or were raised a little above the horizon at noon onJuly 8th and 9th; but by 10 P. M. They had sunk down to between 68o and 90obeneath the horizon. When the same plants had grown to double the aboveheight, their leaves stood at night almost or quite vertically dependent. The leaves on some large plants of G. Maritimum and Brazilense, which werekept in a very badly lighted hot-house, only occasionally sank muchdownwards at night, and hardly enough to be called sleep. 

Oxalis (Oxalidae). --In most of the species in this large genus the threeleaflets sink vertically down at night; but as their sub-petioles are shortthe blades could not assume this position from the want of space, unlessthey were in some manner rendered narrower; and this is effected by theirbecoming more or less folded (Fig. 127). The angle formed by the two halvesof the same leaflet was found to vary in different individuals of severalspecies between 92o and 150o; in three of the best folded leaflets of O. Fragrans it was 76o, 74o, and 54o. The angle is often different in the threeleaflets of the same leaf. As the leaflets sink down at night and becomefolded, their lower surfaces are brought near together (see B), or eveninto[page 325]close contact; and from this circumstance it might be thought that theobject of the folding was the protection of their lower surfaces. If thishad been the case, it would have formed a strongly marked exception to therule, that when there is any difference in the degree of protection fromradiation of the two surfaces of the leaves, it is always the upper surfacewhich is the best protected. But that the folding of the leaflets, andconsequent mutual approximation of their lower surfaces, serves merely toallow them to sink down vertically, may be

Fig. 127. Oxalis acetosella: A, leaf seen from vertically above; B, diagramof leaf asleep, also seen from vertically above. 

inferred from the fact that when the leaflets do not radiate from thesummit of a common petiole, or, again, when there is plenty of room fromthe sub-petioles not being very short, the leaflets sink down withoutbecoming folded. This occurs with the leaflets of O. Sensitiva, Plumierii, and bupleurifolia. 

There is no use in giving a long list of the many species which sleep inthe above described manner. This holds good with species having ratherfleshy leaves, like those of O. Carnosa, or large leaves like those of O. Ortegesii, or four leaflets like those of O. Variabilis. There are, however, some species which show no signs of sleep, viz. , O. Pentaphylla, enneaphylla, hirta, and rubella. We will now describe the nature of themovements in some of the species. 

Oxalis acetosella. --The movement of a leaflet, together with that of themain petiole, are shown in the following diagram (Fig. 128), traced between11 A. M. On October 4th and 7. 45 A. M. On the 5th. After 5. 30 P. M. On the 4ththe leaflet sank rapidly, and at 7 P. M. Depended vertically. For some timebefore it assumed this latter position, its movements could, of course, nolonger be traced on the vertical glass, and the broken line in the diagramought to be extended much further[page 326]down in this and all other cases. By 6. 45 A. M. On the following morning ithad risen considerably, and continued to rise for the next hour; but, judging from other observations, it would soon have begun to fall again. Between 11 A. M. And 5. 30 P. M. The leaflet moved at least four times up andfour times down before the great nocturnal fall commenced; it reached itshighest point at noon. Similar observations were made on two otherleaflets, with nearly the same results. Sachs and Pfeffer have alsodescribed briefly* the autonomous movements of the leaves of this plant. 

Fig 128. Oxalis acetosella: circumnutation and nyctitropic movements of anearly full-grown leaf, with filament attached to the midrib of one of theleaflets; traced on vertical glass during 20 h. 45m. 

On another occasion the petiole of a leaf was secured to a little stickclose beneath the leaflets, and a filament tipped with a bead ofsealing-wax was affixed to the mid-rib of one of them, and a mark wasplaced close behind. At 7 P. M. , when the leaflets were asleep, the filamentdepended vertically down, and the movements of the bead were then tracedtill 10. 40 P. M. , as shown in the following diagram (Fig. 129). We here seethat the leaflet moved a little from side to side, as well as a little upand down, whilst asleep. 

* Sachs in 'Flora, ' 1863, p. 470, etc; Pfeffer, 'Die Period. Bewegungen, 'etc. , 1875, p. 53. [page 327]

Fig 129. Oxalis acetosella: circumnutation of leaflet when asleep; tracedon vertical glass during 3 h. 40 m. 

Oxalis Valdiviana. --The leaves resemble those of the last species, and themovements of two leaflets (the main petioles of both having been secured)were traced during two days; but the tracings are not given, as theyresembled that of O. Acetosella, with the exception that the up and downoscillations were not so frequent during the day, and there was morelateral movement, so that broader ellipses were described. The leaves awokeearly in the morning, for by 6. 45 A. M. On June 12th and 13th they had notonly risen to their full height, but had already begun to fall, that is, they were circumnutating. We have seen in the last chapter that thecotyledons, instead of sinking, rise up vertically at night. 

Oxalis Ortegesii. --The large leaves of this plant sleep like those of theprevious species. The main petioles are long, and that of a young leaf rose20o between noon and 10 P. M. , whilst the petiole of an older leaf rose only13o. Owing to this rising of the petioles, and the vertical sinking of thelarge leaflets, the leaves become crowded together at night, and the wholeplant then exposes a much smaller surface to radiation than during the day. 

Oxalis Plumierii. --In this species the three leaflets do not surround thesummit of the petiole, but the terminal leaflet projects in the line of thepetiole, with a lateral leaflet on each side. They all sleep by bendingvertically downwards, but do not become at all folded. The petiole israther long, and, one having been secured to a stick, the movement of theterminal leaflet was traced during 45 h. On a vertical glass. It moved in avery simple manner, sinking rapidly after 5 P. M. , and rising rapidly earlynext morning. During the middle of the day it moved slowly and a littlelaterally. Consequently the ascending and descending lines did notcoincide, and a single great ellipse was formed each day. There was noother evidence of circumnutation, and this fact is of interest, as we shallhereafter see. 

Oxalis sensitiva. --The leaflets, as in the last species, bend verticallydown at night, without becoming folded. The much elongated main petiolerises considerably in the evening, but in[page 328]some very young plants the rise did not commence until late at night. Wehave seen that the cotyledons, instead of sinking like the leaflets, riseup vertically at night. 

Oxalis bupleurifolia. --This species is rendered remarkable by the petiolesbeing foliaceous, like the phyllodes of many Acacias. The leaflets aresmall, of a paler green and more tender consistence than the foliaceouspetioles. The leaflet which was observed was . 55 inch in length, and wasborne by a petiole 2 inches long and . 3 inch broad. It may be suspectedthat the leaflets are on the road to abortion or obliteration, as hasactually occurred with those of another Brazilian species, O. Rusciformis. Nevertheless, in the present species the nyctitropic movements areperfectly performed. The foliaceous petiole was first observed during 48h. , and found to be in continued circumnutation, as shown in theaccompanying figure (Fig. 130). It rose during the day and early part ofthe night, and fell during the remainder of the night and early morning;but the movement was not sufficient to be called sleep. The ascending anddescending lines did not coincide, so that an ellipse was formed each day. There was but little zigzagging; if the filament had been fixedlongitudinally, we should probably have seen that there was more lateralmovement than appears in the diagram. 

Fig. 130. Oxalis bupleurifolia: circumnutation of foliaceous petiole, filament fixed obliquely across end of petiole; movements traced onvertical glass from 9 A. M. June 26th to 8. 50 A. M. 28th. Apex of leaflet 4 ½inches from the glass, so movement not much magnified. Plant 9 inches high, illuminated from above. Temp. 23 1/2o - 24 1/2o C. 

A terminal leaflet on another leaf was next observed (the petiole beingsecured), and its movements are shown in Fig. 131. During the day theleaflets are extended horizontally, and at night depend vertically; and asthe petiole rises during the day the leaflets have to bend down in theevening[page 329]more than 90o, so as to assume at night their vertical position. On thefirst day the leaflet simply moved up and down; on the

Fig. 131. Oxalis bupleurifolia: circumnutation and nyctitropic movement ofterminal leaflet, with filament affixed along the midrib; traced on avertical glass from 9 A. M. On June 26th to 8. 45 A. M. 28th. Conditions thesame as in the last case. 

second day it plainly circumnutated between 8 A. M. And 4. 30 P. M. , afterwhich hour the great evening fall commenced. [page 330]

Averrhoa bilimbi (Oxalidae). --It has long been known, * firstly, that theleaflets in this genus sleep; secondly, that they move spontaneously duringthe day; and thirdly, that they are sensitive to a touch; but in none ofthese respects do they differ essentially from the species of Oxalis. Theydiffer, however, as Mr. R. I. Lynch** has lately shown, in theirspontaneous movements being strongly marked. In the case of A. Bilimbi, itis a wonderful spectacle to behold on a warm sunny day the leaflets oneafter the other sinking rapidly downwards, and again ascending slowly. Their movements rival those of Desmodium gyrans. At night the leaflets hangvertically down; and now

Fig. 132. Averrhoa bilimbi: leaf asleep; drawing reduced. 

they are motionless, but this may be due to the opposite ones being pressedtogether (Fig. 132). The main petiole is in constant movement during theday, but no careful observations were made on it. The following diagramsare graphic representations of the variations in the angle, which a givenleaflet makes with the vertical. The observations were made as follows. Theplant growing in a pot was kept in a high temperature, the petiole of theleaf to be observed pointing straight at the observer, being separated fromhim by a vertical pane of glass. The petiole was secured so that the basaljoint, or pulvinus, of one of the lateral leaflets was at the centre of agraduated arc placed close behind the leaflet. A fine glass filament wasfixed to the leaf, so as to project like a continuation of the

* Dr. Bruce, 'Philosophical Trans. , ' 1785, p. 356. 

** 'Journal Linn. Soc. , ' vol. Xvi. 1877, p. 231. [page 331]

midrib. This filament acted as an index; and as the leaf rose and fell, rotating about its basal joint, its angular movement

Fig. 133. Averrhoa bilimbi: angular movements of a leaflet during itsevening descent, when going to sleep. Temp. 78o - 81o F. 

could be recorded by reading off at short intervals of time the position ofthe glass filament on the graduated arc. In order[page 332]to avoid errors of parallax, all readings were made by looking through asmall ring painted on the vertical glass, in a line with the joint of theleaflet and the centre of the graduated arc. In the following diagrams theordinates represent the angles which the leaflet made with the vertical atsuccessive instants. * It follows that a fall in the curve represents anactual dropping of the leaf, and that the zero line represents a verticallydependent position. Fig. 133 represents the nature of the movements whichoccur in the evening, as soon as the leaflets begin to assume theirnocturnal position. At 4. 55 P. M. The leaflet formed an angle of 85o withthe vertical, or was only 5o below the horizontal; but in order that thediagram might get into our page, the leaflet is represented falling from75o instead of 85o. Shortly after 6 P. M. It hung vertically down, and hadattained its nocturnal position. Between 6. 10 and 6. 35 P. M. It performed anumber of minute oscillations of about 2o each, occupying periods of 4 or 5m. The complete state of rest of the leaflet which ultimately followed isnot shown in the diagram. It is manifest that each oscillation consists ofa gradual rise, followed by a sudden fall. Each time the leaflet fell, itapproached nearer to the nocturnal position than it did on the previousfall. The amplitude of the oscillations diminished, while the periods ofoscillation became shorter. 

In bright sunshine the leaflets assume a highly inclined dependentposition. A leaflet in diffused light was observed rising for 25 m. A blindwas then pulled up so that the plant was brightly illuminated (BR in Fig. 134), and within a minute it began to fall, and ultimately fell 47o, asshown in the diagram. This descent was performed by six descending steps, precisely similar to those by which the nocturnal fall is effected. Theplant was then again shaded (SH), and a long slow rise occurred untilanother series of falls commenced at BR', when the sun was again admitted. In this experiment cool air was allowed to enter by the windows beingopened at the same time that the blinds were pulled up, so that in spite ofthe sun shining on the plant the temperature was not raised. 

The effect of an increase of temperature in diffused light is

* In all the diagrams 1 mm. In the horizontal direction represents oneminute of time. Each mm. In the vertical direction represents one degree ofangular movement. In Figs. 133 and 134 the temperature is represented(along the ordinates) in the scale of 1 mm. To each 0. 1 degree C. In Fig. 135 each mm. Equals 0. 2o F. [page 333]

shown in Fig. 135. The temperature began to rise at 11. 35 A. M. (inconsequence of the fire being lighted), but by 12. 42 a marked fall hadoccurred. It may be seen in the diagram that when the temperature washighest there were rapid oscillations

Fig. 134. Averrhoa bilimbi: angular movements of leaflet during a changefrom bright illumination to shade; temperature (broken line) remainingnearly the same. 

of small amplitude, the mean position of the leaflet being at the timenearer the vertical. When the temperature began to fall, the oscillationsbecame slower and larger, and the mean position of the leaf againapproached the horizontal. The rate of oscillation was sometimes quickerthan is represented in the above diagram. Thus, when the temperature wasbetween 31o and[page 334]

Fig. 135. Averrhoa bilimbi: angular movement of leaflet during a change oftemperature; light remaining the same. The broken line shows the change oftemperature. [page 335]

32o C. , 14 oscillations of a few degrees occurred in 19 m. On the otherhand, an oscillation may be much slower; thus a leaflet was observed(temperature 25o C. ) to rise during 40 m. Before it fell and completed itsoscillation. 

Fig. 136. Porlieria hygrometrica: circumnutation and nyctitropic movementsof petiole of leaf, traced from 9. 35 A. M. July 7th to about midnight on the8th. Apex of leaf 7 ½ inches from the vertical glass. Temp. 19 1/2o - 201/2o C. 

Porlieria hygrometrica (Zygophylleae). --The leaves of this plant (Chilianform) are from 1 to 1 ½ inch in length, and bear as many as 16 or 17 smallleaflets on each side, which do not stand opposite one another. They arearticulated to the petiole, and the petiole to the branch by a pulvinus. Wemust premise that apparently two forms are confounded under the same name:the leaves on a bush from Chili, which was sent to us from Kew, bore manyleaflets, whilst those on plants in the Botanic Garden at Würzburg boreonly 8 or 9 pairs; and the whole character of the bushes appeared somewhatdifferent. We shall also see that they differ in a remarkable physiologicalpeculiarity. On the Chilian plant the petioles of the younger leaves onupright branches, stood horizontally during the day, and at night sank downvertically so as to depend parallel and close to the branch beneath. Thepetioles of rather older leaves did not become at night verticallydepressed, but only highly inclined. In one instance we found a branchwhich had grown perpendicularly downwards, and the petioles on it moved inthe same direction relatively to the branch as just stated, and thereforemoved upwards. On horizontal branches the younger petioles likewise move atnight in the same direction as before, that is, towards the branch, and areconsequently then extended horizontally; but it is remarkable that theolder petioles on the[page 336]same branch, though moving a little in the same direction, also benddownwards; they thus occupy a somewhat different position, relatively tothe centre of the earth and to the branch, from that of the petioles on theupright branches. With respect to the leaflets, they move at night towardsthe apex of the petiole until their midribs stand nearly parallel to it;and they then lie neatly imbricated one over the other. Thus half of theupper surface of each leaflet is in close contact with half of the lowersurface of the one next in advance; and all the leaflets, excepting thebasal ones, have the whole of their upper surfaces and half of their lowersurfaces well protected. Those on the opposite sides of the same petiole donot come into close contact at night, as occurs with the leaflets of somany Leguminosae but are separated by an open furrow; nor could theyexactly coincide, as they stand alternately with respect to one another. 

The circumnutation of the petiole of a leaf 3/4 of an inch in length, on anupright branch, was observed during 36h. , and is shown in the precedingdiagram (Fig. 136). On the first morning, the leaf fell a little and thenrose until 1 P. M. , and this was probably due to its being now illuminatedthrough a skylight from above; it then circumnutated on a very small scaleround the same spot until about 4 P. M. , when the great evening fallcommenced. During the latter part of the night or very early on the nextmorning the leaf rose again. On the second day it fell during the morningtill 1 P. M. , and this no doubt is its normal habit. From 1 to 4 P. M. Itrose in a zigzag line, and soon afterwards the great evening fallcommenced. It thus completed a double oscillation during the 24 h. 

The specific name given to this plant by Ruiz and Pavon, indicates that inits native arid home it is affected in some manner by the dryness ordampness of the atmosphere. * In the Botanic Garden at Würzburg, there was aplant in a pot out of doors which was daily watered, and another in theopen ground which was never watered. After some hot and dry weather therewas a great difference in the state of the leaflets on these two plants;those on the unwatered plant in the open ground remaining half, * 'Systema Veg. Florae Peruvianae et Chilensis, ' tom. I. P. 95, 1798. Wecannot understand the account given by the authors of the behaviour of thisplant in its native home. There is much about its power of foretellingchanges in the weather; and it appears as if the brightness of the skylargely determined the opening and closing of the leaflets. [page 337]

or even quite, closed during the day. But twigs cut from this bush, withtheir ends standing in water, or wholly immersed in it, or kept in damp airunder a bell-glass, opened their leaves though exposed to a blazing sun;whilst those on the plant in the ground remained closed. The leaves on thissame plant, after some heavy rain, remained open for two days; they thenbecame half closed during two days, and after an additional day were quiteclosed. This plant was now copiously watered, and on the following morningthe leaflets were fully expanded. The other plant growing in a pot, afterhaving been exposed to heavy rain, was placed before a window in theLaboratory, with its leaflets open, and they remained so during the daytimefor 48 h. ; but after an additional day were half closed. The plant was thenwatered, and the leaflets on the two following days remained open. On thethird day they were again half closed, but on being again watered remainedopen during the two next days. From these several facts we may concludethat the plant soon feels the want of water; and that as soon as thisoccurs, it partially or quite closes its leaflets, which in their thenimbricated condition expose a small surface to evaporation. It is thereforeprobable that this sleep-like movement, which occurs only when the groundis dry, is an adaptation against the loss of moisture. 

A bush about 4 feet in height, a native of Chili, which was thickly coveredwith leaves, behaved very differently, for during the day it never closedits leaflets. On July 6th the earth in the small pot in which it grewappeared extremely dry, and it was given a very little water. After 21 and22 days (on the 27th and 28th), during the whole of which time the plantdid not receive a drop of water, the leaves began to droop, but they showedno signs of closing during the day. It appeared almost incredible that anyplant, except a fleshy one, could have kept alive in soil so dry, whichresembled the dust on a road. On the 29th, when the bush was shaken, someleaves fell off, and the remaining ones were unable to sleep at night. Itwas therefore moderately watered, as well as syringed, late in the evening. On the next morning (30th) the bush looked as fresh as ever, and at nightthe leaves went to sleep. It may be added that a small branch while growingon the bush was enclosed, by means of a curtain of bladder, during 13 daysin a large bottle half full of quicklime, so that the air within must havebeen intensely dry; yet the leaves on this branch did not suffer in the[page 338]least, and did not close at all during the hottest days. Another trial wasmade with the same bush on August 2nd and 6th (the soil appearing at thislatter date extremely dry), for it was exposed out of doors during thewhole day to the wind, but the leaflets showed no signs of closing. TheChilian form therefore differs widely from the one at Würzburg, in notclosing its leaflets when suffering from the want of water; and it can livefor a surprisingly long time without water. 

Tropaeolum majus (?) (cultivated var. ) (Tropaeoleae). --Several plants inpots stood in the greenhouse, and the blades of the leaves which faced thefront-lights were during the day highly inclined and at night vertical;whilst the leaves on the back of the pots, though of course illuminatedthrough the roof, did not become vertical at night. We thought, at first, that this difference in their positions was in some manner due toheliotropism, for the leaves are highly heliotropic. The true explanation, however, is that unless they are well illuminated during at least a part ofthe day they do not sleep at night; and a little difference in the degreeof illumination determines whether or not they shall become vertical atnight. We have observed no other so well-marked a case as this, of theinfluence of previous illumination on nyctitropic movements. The leavespresent also another peculiarity in their habit of rising or awaking in themorning, being more strongly fixed or inherited than that of sinking orsleeping at night. The movements are caused by the bending of an upper partof the petiole, between ½ and 1 inch in length; but the part close to theblade, for about 1/4 of an inch in length, does not bend and always remainsat right angles to the blade. The bending portion does not present anyexternal or internal difference in structure from the rest of the petiole. We will now give the experiments on which the above conclusions arefounded. 

A large pot with several plants was brought on the morning of Sept. 3rd outof the greenhouse and placed before a north-east window, in the sameposition as before with respect to the light, as far as that was possible. On the front of the plants, 24 leaves were marked with thread, some ofwhich had their blades horizontal, but the greater number were inclined atabout 45o, beneath the horizon; at night all these, without exception, became vertical. Early on the following morning (4th) they reassumed theirformer positions, and at night again became vertical. On the 5th theshutters were opened at 6. 15 A. M. , and[page 339]by 8. 18 A. M. , after the leaves had been illuminated for 2 h. 3 m. And hadacquired their diurnal position, they were placed in a dark cupboard. Theywere looked at twice during the day and thrice in the evening, the lasttime at 10. 30 P. M. , and not one had become vertical. At 8 A. M. On thefollowing morning (6th) they still retained the same diurnal position, andwere now replaced before the north-east window. At night all the leaveswhich had faced the light had their petioles curved and their bladesvertical; whereas none of the leaves on the back of the plants, althoughthey had been moderately illuminated by the diffused light of the room, were vertical. They were now at night placed in the same dark cupboard; at9 A. M. On the next morning (7th) all those which had been asleep hadreassumed their diurnal position. The pot was then placed for 3 h. In thesunshine, so as to stimulate the plants; at noon they were placed beforethe same north-east window, and at night the leaves slept in the usualmanner and awoke on the following morning. At noon on this day (8th) theplants, after having been left before the north-east window for 5 h. 45 m. And thus illuminated (though not brightly, as the sky was cloudy during thewhole time), were replaced in the dark cupboard, and at 3 P. M. The positionof the leaves was very little, if at all, altered, so that they are notquickly affected by darkness; but by 10. 15 P. M. All the leaves which hadfaced the north-east sky during the 5 h. 45 m. Of illumination stoodvertical, whereas those on the back of the plant retained their diurnalposition. On the following morning (9th) the leaves awoke as on the twoformer occasions in the dark, and they were kept in the dark during thewhole day; at night a very few of them became vertical, and this was theone instance in which we observed any inherited tendency or habit in thisplant to sleep at the proper time. That it was real sleep was shown bythese same leaves reassuming their diurnal position on the followingmorning (10th) whilst still kept in the dark. 

The pot was then (9. 45 A. M. 10th) replaced, after having been kept for 36h. In darkness, before the north-east window; and at night the blades ofall the leaves (excepting a few on the back of the plants) becameconspicuously vertical. At 6. 45 A. M. (11th) after the plants had been illuminated on the same sideas before during only 25 m. , the pot was turned round, so that the leaveswhich had faced the light now faced the interior of the room, and not oneof these went to sleep at night;[page 340]whilst some, but not many, of those which had formerly stood facing theback of the room and which had never before been well illuminated or goneto sleep, now assumed a vertical position at night. On the next day (12th)the plant was turned round into its original position, so that the sameleaves faced the light as formerly, and these now went to sleep in theusual manner. We will only add that with some young seedlings kept in thegreenhouse, the blades of the first pair of true leaves (the cotyledonsbeing hypogean) stood during the day almost horizontally and at nightalmost vertically. 

A few observations were subsequently made on the circumnutation of threeleaves, whilst facing a north-east window; but the tracings are not given, as the leaves moved somewhat towards the light. It was, however, manifestthat they rose and fell more than once during the daytime, the ascendingand descending lines being in parts extremely zigzag. The nocturnal fallcommenced about 7 P. M. , and the leaves had risen considerably by 6. 45 A. M. On the following morning. 

Leguminosae. --This Family includes many more genera with sleeping speciesthan all the other families put together. The number of the tribes to whicheach genus belongs, according to Bentham and Hooker's arrangement, has beenadded. 

Crotolaria (sp. ?) (Tribe 2). --This plant is monophyllous, and we areinformed by Mr. T. Thiselton Dyer that the leaves rise up vertically atnight and press against the stem. 

Lupinus (Tribe 2). --The palmate or digitate leaves of the species in thislarge genus sleep in three different manners. One of the simplest, is thatall the leaflets become steeply inclined downwards at night, having beenduring the day extended horizontally. This is shown in the accompanyingfigures (Fig. 137), of a leaf of L. Pilosus, as seen during the day fromvertically above, and of another leaf asleep with the leaflets inclineddownwards. As in this position they are crowded together, and as they donot become folded like those in the genus Oxalis, they cannot occupy avertically dependent position; but they are often inclined at an angle of50o beneath the horizon. In this species, whilst the leaflets are sinking, the petioles rise up, in two instances when the angles were measured to theextent of 23o. The leaflets of L. Sub-carnosus and arboreus, which werehorizontal during the day, sank down at night in nearly the same manner;the former to an angle of 38o and the latter of 36o, beneath the horizon;but their petioles[page 341]did not move in any plainly perceptible degree. It is, however, quitepossible, as we shall presently see, that if a large number of plants ofthe three foregoing and of the following species

Fig. 137. Lupinus pilosus: A, leaf seen from vertically above in daytime;B, leaf asleep, seen laterally at night. 

were to be observed at all seasons, some of the leaves would be found tosleep in a different manner. 

In the two following species the leaflets, instead of moving downwards, rise at night. With L. Hartwegii some stood at noon at a mean angle of 36oabove the horizon, and at night at 51o, thus forming together a hollow conewith moderately steep sides. The petiole of one leaf rose 14o and of asecond 11o at night. With L. Luteus a leaflet rose from 47o at noon to 65oabove the horizon at night, and another on a distinct leaf rose from 45o to69o. The petioles, however, sink at night to a small extent, viz. , in threeinstances by 2o, 6o, and 9o 30 seconds. Owing to this movement of thepetioles, the outer and longer leaflets have to bend up a little more thanthe shorter and inner ones, in order that all should stand symmetrically atnight. We shall presently see that some leaves on the same individualplants of L. Luteus sleep in a very different manner. 

We now come to a remarkable position of the leaves when asleep, which iscommon to several species of Lupines. On the same leaf the shorterleaflets, which generally face the centre of the plant, sink at night, whilst the longer ones on the opposite side rise; the intermediate andlateral ones merely twisting on their own axes. But there is somevariability with respect to which leaflets rise or fall. As might have beenexpected from such diverse and complicated movements, the[page 342]base of each leaflet is developed (at least in the case of L. Luteus) intoa pulvinus. The result is that all the leaflets on the same leaf stand atnight more or less highly inclined, or even quite vertically, forming inthis latter case a vertical star. This occurs with the leaves of a speciespurchased under the name of

Fig. 138. Lupinus pubescens: A, leaf viewed laterally during the day; B, same leaf at night; C, another leaf with the leaflet forming a verticalstar at night. Figures reduced. 

L. Pubescens; and in the accompanying figures we see at A (Fig. 138) theleaves in their diurnal position; and at B the same plant at night with thetwo upper leaves having their leaflets almost vertical. At C another leaf, viewed laterally, is shown with the leaflets quite vertical. It is chieflyor exclusively the youngest leaves which form at night vertical stars. Butthere[page 343]is much variability in the position of the leaves at night on the sameplant; some remaining with their leaflets almost horizontal, others formingmore or less highly inclined or vertical stars, and some with all theirleaflets sloping downwards, as in our first class of cases. It is also aremarkable fact, that although all the plants produced from the same lot ofseeds were identical in appearance, yet some individuals at night had theleaflets of all their leaves arranged so as to form more or less highlyinclined stars; others had them all sloping downwards and never forming astar; and others, again, retained them either in a horizontal position orraised them a little. 

We have as yet referred only to the different positions of the leaflets ofL. Pubescens at night; but the petioles likewise differ in their movements. That of a young leaf which formed a highly inclined star at night, stood atnoon at 42o above the horizon, and during the night at 72o, so had risen30o. The petiole of another leaf, the leaflets of which occupied a similarposition at night, rose only 6o. On the other hand, the petiole of a leafwith all its leaflets sloping down at night, fell at this time 4o. Thepetioles of two rather older leaves were subsequently observed; both ofwhich stood during the day at exactly the same angle, viz. , 50o above thehorizon, and one of these rose 7o - 8o, and the other fell 3o - 4o at night. We meet with cases like that of L. Pubescens with some other species. On asingle plant of L. Mutabilis some leaves, which stood horizontally duringthe day, formed highly inclined stars at night, and the petiole of one rose7o. Other leaves which likewise stood horizontally during the day, had atnight all their leaflets sloping downwards at 46o beneath the horizon, buttheir petioles had hardly moved. Again, L. Luteus offered a still moreremarkable case, for on two leaves, the leaflets which stood at noon atabout 45o above the horizon, rose at night to 65o and 69o, so that theyformed a hollow cone with steep sides. Four leaves on the same plant, whichhad their leaflets horizontal at noon, formed vertical stars at night; andthree other leaves equally horizontal at noon, had all their leafletssloping downwards at night. So that the leaves on this one plant assumed atnight three different positions. Though we cannot account for this fact, wecan see that such a stock might readily give birth to species having widelydifferent nyctitropic habits. 

Little more need be said about the sleep of the species of Lupinus;several, namely, L. Polyphyllus, nanus, Menziesii, speciosus, [page 344]and albifrons, though observed out of doors and in the greenhouse, did notchange the position of their leaves sufficiently at night to be said tosleep. From observations made on two sleeping species, it appears that, aswith Tropaeolum majus, the leaves must be well illuminated during the dayin order to sleep at night. For several plants, kept all day in asitting-room with north-east windows, did not sleep at night; but when thepots were placed on the following day out of doors, and were brought in atnight, they slept in the usual manner. The trial was repeated on thefollowing day and night with the same result. 

Some observations were made on the circumnutation of the leaves of L. Luteus and arboreus. It will suffice to say that the leaflets of the latterexhibited a double oscillation in the course of 24 h. ; for they fell fromthe early morning until 10. 15 A. M. , then rose and zigzagged greatly till 4P. M. , after which hour the great nocturnal fall commenced. By 8 A. M. On thefollowing morning the leaflets had risen to their proper height. We haveseen in the fourth chapter, that the leaves of Lupinus speciosus, which donot sleep, circumnutate to an extraordinary extent, making many ellipses inthe course of the day. 

Cytisus (Tribe 2), Trigonella and Medicago (Tribe 3). --Only

Fig. 139. Medicago marina: A, leaves during the day; B, leaves asleep atnight. 

a few observations were made on these three genera. The petioles on a youngplant, about a foot in height, of Cytisus fragrans rose at night, on oneoccasion 23o and on another 33o. The three leaflets also bend upwards, andat the same time[page 345]approach each other, so that the base of the central leaflet overlaps thebases of the two lateral leaflets. They bend up so much that they pressagainst the stem; and on looking down on one of these young plants fromvertically above, the lower surfaces of the leaflets are visible; and thustheir upper surfaces, in accordance with the general rule, are bestprotected from radiation. Whilst the leaves on these young plants were thusbehaving, those on an old bush in full flower did not sleep at night. 

Trigonella Cretica resembles a Melilotus in its sleep, which will beimmediately described. According to M. Royer, * the leaves of Medicagomaculata rise up at night, and "se renversent un peu de manière à presenterobliquement au ciel leur face inférieure. " A drawing is here given (Fig. 139) of the leaves of M. Marina awake and asleep; and this would almostserve for Cytisus fragrans in the same two states. 

Melilotus (Tribe 3). --The species in this genus sleep in a remarkablemanner. The three leaflets of each leaf twist through an angle of 90o, sothat their blades stand vertically at night with one lateral edge presentedto the zenith (Fig. 140). We shall best understand the other and morecomplicated movements, if we imagine ourselves always to hold the leaf withthe tip of the terminal leaflet pointed to the north. The leaflets inbecoming vertical at night could of course twist so that their uppersurfaces should face to either side; but the two lateral leaflets alwaystwist so that this surface tends to face the north, but as they move at thesame time towards the terminal leaflet, the upper surface of the one facesabout N. N. W. , and that of the other N. N. E. The terminal leaflet behavesdifferently, for it twists to either side, the upper surface facingsometimes east and sometimes west, but rather more commonly west than east. The terminal leaflet also moves in another and more remarkable manner, forwhilst its blade is twisting and becoming vertical, the whole leaflet bendsto one side, and invariably to the side towards which the upper surface isdirected; so that if this surface faces the west the whole leaflet bends tothe west, until it comes into contact with the upper and vertical surfaceof the western lateral leaflet. Thus the upper surface of the terminal andof one of the two lateral leaflets is well protected. 

The fact of the terminal leaflet twisting indifferently to either

* 'Annales des Sc. Nat. Bot. ' (5th series), ix. 1868, p. 368. [page 346]

side and afterwards bending to the same side, seemed to us so remarkable, that we endeavoured to discover the cause. We imagined that at thecommencement of the movement it might be determined by one of the twohalves of the leaflet being a little heavier than the other. Therefore bitsof wood were gummed on one side of several leaflets, but this produced noeffect; and they continued to twist in the same direction as

Fig. 140. Melilotus officinalis: A, leaf during the daytime. B, anotherleaf asleep. C, a leaf asleep as viewed from vertically above; but in thiscase the terminal leaflet did not happen to be in such close contact withthe lateral one, as is usual. 

they had previously done. In order to discover whether the same leaflettwisted permanently in the same direction, black threads were tied to 20leaves, the terminal leaflets of which twisted so that their upper surfacesfaced west, and 14 white threads to leaflets which twisted to the east. These were observed occasionally during 14 days, and they all continued, with a single exception, to twist and bend in the same direction; for[page 347]one leaflet, which had originally faced east, was observed after 9 days toface west. The seat of both the twisting and bending movement is in thepulvinus of the sub-petioles. 

We believe that the leaflets, especially the two lateral ones, inperforming the above described complicated movements generally bend alittle downwards; but we are not sure of this, for, as far as the mainpetiole is concerned, its nocturnal movement is largely determined by theposition which the leaf happens to occupy during the day. Thus one mainpetiole was observed to rise at night 59o, whilst three others rose only 7oand 9o. The petioles and sub-petioles are continually circumnutating duringthe whole 24 h. , as we shall presently see. 

The leaves of the following 15 species, M. Officinalis, suaveolens, parviflora, alba, infesta, dentata, gracilis, sulcata, elegans, coerulea, petitpierreana, macrorrhiza, Italica, secundiflora, and Taurica, sleep innearly the same manner as just described; but the bending to one side ofthe terminal leaflet is apt to fail unless the plants are growingvigorously. With M. Petitpierreana and secundiflora the terminal leafletwas rarely seen to bend to one side. In young plants of M. Italica it bentin the usual manner, but with old plants in full flower, growing in thesame pot and observed at the same hour, viz. , 8. 30 P. M. , none of theterminal leaflets on several scores of leaves had bent to one side, thoughthey stood vertically; nor had the two lateral leaflets, though standingvertically, moved towards the terminal one. At 10. 30 P. M. , and again onehour after midnight, the terminal leaflets had become very slightly bent toone side, and the lateral leaflets had moved a very little towards theterminal one, so that the position of the leaflets even at this late hourwas far from the ordinary one. Again, with M. Taurica the terminal leafletswere never seen to bend towards either of the two lateral leaflets, thoughthese, whilst becoming vertical, had bent towards the terminal one. Thesub-petiole of the terminal leaflet in this species is of unusual length, and if the leaflet had bent to one side, its upper surface could have comeinto contact only with the apex of either lateral leaflet; and this, perhaps, is the meaning of the loss of the lateral movement. 

The cotyledons do not sleep at night. The first leaf consists of a singleorbicular leaflet, which twists at night so that the blade standsvertically. It is a remarkable fact that with M. Taurica, and in a somewhatless degree with M. Macrorrhiza and petitpierreana, all the many small andyoung leaves produced during[page 348]the early spring from shoots on some cut-down plants in the greenhouse, slept in a totally different manner from the normal one; for the threeleaflets, instead of twisting on their own axes so as to present theirlateral edges to the zenith, turned upwards and stood vertically with theirapices pointing to the zenith. They thus assumed nearly the same positionas in the allied genus Trifolium; and on the same principle thatembryological characters reveal the lines of descent in the animal kingdom, so the movements of the small leaves in the above three species ofMelilotus, perhaps indicate that this genus is descended from a form whichwas closely allied to and slept like a Trifolium. Moreover, there is onespecies, M. Messanensis, the leaves of which, on full-grown plants between2 and 3 feet in height, sleep like the foregoing small leaves and likethose of a Trifolium. We were so much surprised at this latter case that, until the flowers and fruit were examined, we thought that the seeds ofsome Trifolium had been sown by mistake instead of those of a Melilotus. Itappears therefore probable that M. Messanensis has either retained orrecovered a primordial habit. 

The circumnutation of a leaf of M. Officinalis was traced, the stem beingleft free; and the apex of the terminal leaflet described three laterallyextended ellipses, between 8 A. M. And 4 P. M. ; after the latter hour thenocturnal twisting movement commenced. It was afterwards ascertained thatthe above movement was compounded of the circumnutation of the stem on asmall scale, of the main petiole which moved most, and of the sub-petioleof the terminal leaflet. The main petiole of a leaf having been secured toa stick, close to the base of the sub-petiole of the terminal leaflet, thelatter described two small ellipses between 10. 30 A. M. , and 2 P. M. At 7. 15P. M. , after this same leaflet (as well as another) had twisted themselvesinto their vertical nocturnal position, they began to rise slowly, andcontinued to do so until 10. 35 P. M. , after which hour they were no longerobserved. 

As M. Messanensis sleeps in an anomalous manner, unlike that of any otherspecies in the genus, the circumnutation of a terminal leaflet, with thestem secured, was traced during two days. On each morning the leaflet fell, until about noon, and then began to rise very slowly; but on the first daythe rising movement was interrupted between 1 and 3 P. M. By the formationof a laterally extended ellipse, and on the second day, at the same time, by two smaller ellipses. The rising movement then[page 349]recommenced, and became rapid late in the evening, when the leaflet wasbeginning to go to sleep. The awaking or sinking movement had alreadycommenced by 6. 45 A. M. On both mornings. 

Trifolium (Tribe 3). --The nyctitropic movements of 11 species wereobserved, and were found to be closely similar. If we select a leaf of T. Repens having an upright petiole, and with the three leaflets expandedhorizontally, the two lateral leaflets will be seen in the evening to twistand approach each other, until their upper surfaces come into contact. Atthe same time they bend downwards in a plane at right angles to that oftheir former position, until their midribs form an angle of about 45o withthe upper part of the petiole. This peculiar change of position requires aconsiderable amount of torsion in the pulvinus. The terminal leaflet merelyrises up without any twist-

Fig. 141. Trifolium repens: A, leaf during the day; B, leaf asleep atnight. 

ing and bends over until it rests on and forms a roof over the edges of thenow vertical and united lateral leaflets. Thus the terminal leaflet alwayspasses through an angle of at least 90o, generally of 130o or 140o, and notrarely--as was often observed with T. Subterraneum--of 180o. In this lattercase the terminal leaflet stands at night horizontally (as in Fig. 141), with its lower surface fully exposed to the zenith. Besides the differencein the angles, at which the terminal leaflets stand at night in theindividuals of the same species, the degree to which the lateral leafletsapproach each other often likewise differs. 

We have seen that the cotyledons of some species and not of others rise upvertically at night. The first true leaf is generally unifoliate andorbicular; it always rises, and either stands vertically at night or morecommonly bends a little over so as to expose the lower surface obliquely tothe zenith, in the same manner as does the terminal leaflet of the matureleaf. But it does not twist itself like the corresponding first simple leafof Melilotus. [page 350]With T. Pannonicum the first true leaf was generally unifoliate, butsometimes trifoliate, or again partially lobed and in an intermediatecondition. 

Circumnutation. --Sachs described in 1863* the spontaneous up and downmovements of the leaflets of T. Incarnatum, when kept in darkness. Pfeffermade many observations on the similar movements in T. Pratense. ** He statesthat the terminal leaflet of this species, observed at different times, passed through angles of from 30o to 120o in the course of from 1 ½ to 4 h. We observed the movements of T. Subterraneum, resupinatum, and repens. 

Trifolium subterraneum. --A petiole was secured close to the base of thethree leaflets, and the movement of the terminal leaflet was traced during26 ½ h. , as shown in the figure on the next page. 

Between 6. 45 A. M. And 6 P. M. The apex moved 3 times up and 3 times down, completing 3 ellipses in 11 h. 15 m. The ascending and descending linesstand nearer to one another than is usual with most plants, yet there wassome lateral motion. At 6 P. M. The great nocturnal rise commenced, and onthe next morning the sinking of the leaflet was continued until 8. 30 A. M. , after which hour it circumnutated in the manner just described. In thefigure the great nocturnal rise and the morning fall are greatlyabbreviated, from the want of space, and are merely represented by a shortcurved line. The leaflet stood horizontally when at a point a littlebeneath the middle of the diagram; so that during the daytime it oscillatedalmost equally above and beneath a horizontal position. At 8. 30 A. M. Itstood 48o beneath the horizon, and by 11. 30 A. M. It had risen 50o above thehorizon; so that it passed through 98o in 3 h. By the aid of the tracing weascertained that the distance travelled in the 3 h. By the apex of thisleaflet was 1. 03 inch. If we look at the figure, and prolong upwards in ourmind's eye the short curved broken line, which represents the nocturnalcourse, we see that the latter movement is merely an exaggeration orprolongation of one of the diurnal ellipses. The same leaflet had beenobserved on the previous day, and the course then pursued was almostidentically the same as that here described. 

* 'Flora, ' 1863, p. 497. 

** 'Die Period. Bewegungen, ' 1875, pp. 35, 52. [page 351]

Fig. 142. Trifolium subterraneum: circumnutation and nyctitropic movementof terminal leaflet (. 68 inch in length), traced from 6. 45 A. M. July 4th to9. 15 A. M. 5th. Apex of leaf 3 7/8 inches from the vertical glass, andmovement, as here shown, magnified 5 1/4 times, reduced to one-half oforiginal scale. Plant illuminated from above; temp. 16o - 17o C. 

Trifolium resupinatum. --A plant left entirely free was placed before anorth-east window, in such a position that a terminal leaflet projected atright angles to the source of the light, the sky being uniformly cloudedall day. The movements of this leaflet were traced during two days, and onboth were closely similar. Those executed on the second day are shown inFig. 143. The obliquity of the several lines is due partly to the manner inwhich the leaflet was viewed, and partly to its having moved a littletowards the light. From 7. 50 A. M. To 8. 40 A. M. The leaflet fell, that is, the awakening movement was continued. It then rose and moved a littlelaterally towards the light. At 12. 30 it retrograded, and at 2. 30 resumedits original course, having thus completed a small ellipse during themiddle of the day. In the evening it rose rapidly, and by 8 A. M. On thefollowing morning had returned to exactly the same spot as on the previousmorning. The line representing the nocturnal course ought to be extendedmuch higher up, and is here abbreviated into a short, [page 352]curved, broken line. The terminal leaflet, therefore, of this speciesdescribed during the daytime only a single additional ellipse, instead oftwo additional ones, as in the case of T. Subterraneum. But we shouldremember that it was shown in the fourth chapter that the stemcircumnutates, as no doubt does the main petiole and the sub-petioles; sothat the movement represented in Fig. 143 is a compounded one. We tried toobserve the movements of a leaf kept during the day in darkness, but itbegan to go to sleep after 2 h. 15 m. , and this was well pronounced after 4h. 30 m. 

Fig 143. Trifolium resupinatum: circumnutation and nyctitropic movements ofthe terminal leaflet during 24 hours. 

Trifolium repens. --A stem was secured close to the base of a moderately oldleaf, and the movement of the terminal leaflet was observed during twodays. This case is interesting solely from the simplicity of the movements, in contrast with those of the two preceding species. On the first day theleaflet fell between 8 A. M. And 3 P. M. , and on the second between 7 A. M. And 1 P. M. On both days the descending course was somewhat zigzag, and thisevidently represents the circumnutating movement of the two previousspecies during the middle of the day. After 1 P. M. , Oct. 1st (Fig. 144), the leaflet began to rise, but the movement was slow on both days, bothbefore and after this hour, until 4 P. M. The rapid evening and nocturnalrise then commenced. Thus in this species the course during 24 h. Consistsof a single great ellipse; in T. Resupinatum of two ellipses, one of whichincludes the nocturnal movement and is much elongated; and in T. Subterraneum of three ellipses, of which the nocturnal one is likewise ofgreat length. 

Securigera coronilla (Tribe 4). --The leaflets, which stand opposite oneanother and are numerous, rise up at night, come into close contact, andbend backwards at a moderate angle towards the base of the petiole. [page 353]

Fig. 144. Trifolium repens: circumnutation and nyctitropic movements of anearly full-grown terminal leaflet, traced on a vertical glass from 7 A. M. Sept. 30th to 8 A. M. Oct. 1st. Nocturnal course, represented by curvedbroken line, much abbreviated. 

Lotus (Tribe 4). --The nyctitropic movements of 10 species in this genuswere observed, and found to be the same. The main petiole rises a little atnight, and the three leaflets rise till they become vertical, and at thesame time approach each other. This was conspicuous with L. Jacoboeus, inwhich the leaflets are almost linear. In most of the species the leafletsrise so much as to press against the stem, and not rarely they becomeinclined a little inwards with their lower surfaces exposed obliquely tothe zenith. This was clearly the case with L. Major, as its petioles areunusually long, and the leaflets are thus enabled to bend further inwards. The young leaves on the summits of the stems close up at night so much, asoften to resemble large buds. The stipule-like leaflets, which are often oflarge size, rise up like the other leaflets, and press against the stem(Fig. 145). All the leaflets of L. Gebelii, and probably of the otherspecies, are provided at their bases with distinct pulvini, of a yellowishcolour, and formed of very small cells. The circumnutation of a terminalleaflet of L. Peregrinus (with the stem secured) was traced during twodays, but the movement was so simple that it is not worth while to give thediagram. The leaflet fell slowly from the early morning till about 1 P. M. It then rose gradually at first, but rapidly late in the evening. Itoccasionally stood still for about 20 m. During the day, and sometimeszigzagged a little. The movement of one of the basal, stipule-like leafletswas likewise traced in the same manner and at the same time, and its coursewas closely similar to that of the terminal leaflet. 

In Tribe 5 of Bentham and Hooker, the sleep-movements of species in 12genera have been observed by ourselves and[page 354]others, but only in Robinia with any care. Psoralea acaulis raises itsthree leaflets at night; whilst Amorpha fruticosa, * Dalea alopecuroides, and Indigofera tinctoria depress them. Ducharte** states that Tephrosiacaribaea is the sole example of "folioles couchées le long du pétiole etvers la base;" but a

Fig. 145. Lotus Creticus: A, stem with leaves awake during the day; B, withleaves asleep at night. SS, stipule-like leaflets. 

similar movement occurs, as we have already seen, and shall again see inother cases. Wistaria Sinensis, according to Royer, *** "abaisse lesfolioles qui par une disposition bizarre sont inclinées dans la mêmefeuille, les supérieures vers le

* Ducharte, 'Eléments de Botanique', 1867, p. 349. 

** Ibid. , p. 347. 

*** 'Ann. Des Sciences Nats. Bot. ' (5th series), ix. 1868. [page 355]

sommet, les inférieures vers la base du petiole commun;" but the leafletson a young plant observed by us in the greenhouse merely sank verticallydownwards at night. The leaflets are raised in Sphaerophysa salsola, Colutea arborea, and Astragalus uliginosus, but are depressed, according toLinnaeus, in Glycyrrhiza. The leaflets of Robinia pseudo-acacia likewisesink vertically down at night, but the petioles rise a little, viz. , in onecase 3o, and in another 4o. The circumnutating movements of a terminalleaflet on a rather old leaf were traced during two days, and were simple. The leaflet fell slowly, in a slightly zigzag line, from 8 A. M. To 5 P. M. , and then more rapidly; by 7 A. M. On the following morning it had risen toits diurnal position. There was only one peculiarity in the movement, namely, that on both days there was a distinct though small oscillation upand down between 8. 30 and 10 A. M. , and this would probably have been morestrongly pronounced if the leaf had been younger. 

Coronilla rosea (Tribe 6). --the leaves bear 9 or 10 pairs of oppositeleaflets, which during the day stand horizontally, with

Fig. 146. Coronilla rosea: leaf asleep. 

their midribs at right angles to the petiole. At night they rise up so thatthe opposite leaflets come nearly into contact, and those on the youngerleaves into close contact. At the same time they bend back towards the baseof the petiole, until their midribs form with it angles of from 40o to 50oin a vertical plane, as here figured (Fig. 146). The leaflets, however, sometimes bend so much back that their midribs become parallel to and lieon the petiole. They thus occupy a reversed position to what they do inseveral Leguminosae, for instance, in Mimosa[page 356]pudica; but, from standing further apart, they do not overlap one anothernearly so much as in this latter plant. The main petiole is curved slightlydownwards during the day, but straightens itself at night. In three casesit rose from 3o above the horizon at noon, to 9o at 10 P. M. ; from 11o to33o; and from 5o to 33o--the amount of angular movement in this latter caseamounting to 28o. In several other species of Coronilla the leaflets showedonly feeble movements of a similar kind. 

Hedysarum coronarium (Tribe 6). --The small lateral leaflets on plantsgrowing out of doors rose up vertically at night, but the large terminalone became only moderately inclined. The petioles apparently did not riseat all. 

Smithia Pfundii (Tribe 6). --The leaflets rise up vertically, and the mainpetiole also rises considerably. 

Arachis hypogoea (Tribe 6). --The shape of a leaf, with its two pairs ofleaflets, is shown at A (Fig. 147); and a leaf asleep, Fig. 147. Arachis hypogoea: A, leaf during the day, seen from verticallyabove; B, leaf asleep, seen laterally, copied from a photograph. Figuresmuch reduced. 

traced from a photograph (made by the aid of aluminium light), is given atB. The two terminal leaflets twist round at night until their blades standvertically, and approach each other until they meet, at the same timemoving a little upwards and backwards. The two lateral leaflets meet eachother in this same manner, but move to a greater extent forwards, that is, in a contrary direction to the two terminal leaflets, which they partiallyembrace. Thus all four leaflets form together a single packet, with theiredges directed to the zenith, and with their lower surfaces turnedoutwards. On a plant which was not growing vigorously the closed leafletsseemed too heavy for the[page 357]petioles to support them in a vertical position, so that each night themain petiole became twisted, and all the packets were extendedhorizontally, with the lower surfaces of the leaflets on one side directedto the zenith in a most anomalous manner. This fact is mentioned solely asa caution, as it surprised us greatly, until we discovered that it was ananomaly. The petioles are inclined upwards during the day, but sink atnight, so as to stand at about right angles with the stem. The amount ofsinking was measured only on one occasion, and found to be 39o. A petiolewas secured to a stick at the base of the two terminal leaflets, and thecircumnutating movement of one of these leaflets was traced from 6. 40 A. M. To 10. 40 P. M. , the plant being illuminated from above. The temperature was17o - 17 1/2o C. , and therefore rather too low. During the 16 h. Theleaflet moved thrice up and thrice down, and as the ascending anddescending lines did not coincide, three ellipses were formed. 

Fig. 148. Desmodium gyrans: leaf seen from above, reduced to one-halfnatural size. The minute stipules unusually large. 

Desmodium gyrans (Tribe 6). --A large and full-grown leaf of this plant, sofamous for the spontaneous movements of the two little lateral leaflets, ishere represented (Fig. 148). The large terminal leaflet sleeps by sinkingvertically down, whilst the petiole rises up. The cotyledons do not sleep, but the first-formed leaf sleeps equally well as the older ones. Theappearance presented by a sleeping branch and one in the day-time, copiedfrom two photographs, are shown at A and B (Fig. 149), and we see how atnight the leaves are crowded together, as if for mutual protection, by therising of the petioles. The petioles of the younger leaves near the summitsof the shoots rise up at night, so as to stand vertical and parallel to thestem; whilst those on the sides were found in four cases to have risenrespectively 46 1/2o, 36o, 20o, and 19. 5o above the inclined positions whichthey had occupied during the day. For instance, in the first of these fourcases the petiole stood in the day at 23o, and at night at 69 1/2o abovethe horizon. In the evening the rising of the petioles is almost completedbefore the leaflets sink perpendicularly downwards. [page 358]

Circumnutation. --The circumnutating movements of four young shoots wereobserved during 5 h. 15 m. ; and in this time each completed an oval figureof small size. The main petiole also circumnutates rapidly, for in thecourse of 31 m. (temp. 91o F. ) it changed its course by as much as arectangle six times, describing a figure which apparently represented twoellipses. 

Fig. 149. Desmodium gyrans: A, stem during the day; B, stem with leavesasleep. Figures reduced. 

The movement of the terminal leaflet by means of its sub-petiole orpulvinus is quite as rapid, or even more so, than that of the main petiole, and has much greater amplitude. Pfeffer has seen* these leaflets movethrough an angle of 8o in the course of from 10 to 30 seconds. 

A fine, nearly full-grown leaf on a young plant, 8 inches in height, withthe stem secured to a stick at the base of the leaf, was observed from 8. 30A. M. June 22nd to 8 A. M. June 24th. 

* 'Die Period. Beweg. , ' p. 35. [page 359]

In the diagram given on the next page (Fig. 150), the two curved brokenlines at the base, which represent the nocturnal courses, ought to beprolonged far downwards. On the first day the leaflet moved thrice down andthrice up, and to a considerable distance laterally; the course was alsoremarkably crooked. The dots were generally made every hour; if they hadbeen made every few minutes all the lines would have been zigzag to anextraordinary degree, with here and there a loop formed. We may infer thatthis would have been the case, because five dots were made in the course of31 m. (between 12. 34 and 1. 5 P. M. ), and we see in the upper part of thediagram how crooked the course here is; if only the first and last dots hadbeen joined we should have had a straight line. Exactly the same fact maybe seen in the lines representing the course between 2. 24 P. M. And 3 P. M. , when six intermediate dots were made; and again at 4. 46 and 4. 50. But theresult was widely different after 6 P. M. , --that is, after the greatnocturnal descent had commenced; for though nine dots were then made in thecourse of 32 m. , when these were joined (see Figure) the line thus formedwas almost straight. The leaflets, therefore, begin to descend in theafternoon by zigzag lines, but as soon as the descent becomes rapid theirwhole energy is expended in thus moving, and their course becomesrectilinear. After the leaflets are completely asleep they move very littleor not at all. 

Had the above plant been subjected to a higher temperature than 67o - 70oF. , the movements of the terminal leaflet would probably have been evenmore rapid and wider in extent than those shown in the diagram; for a plantwas kept for some time in the hot-house at from 92o - 93o F. , and in thecourse of 35 m. The apex of a leaflet twice descended and once ascended, travelling over a space of 1. 2 inch in a vertical direction and of . 82 inchin a horizontal direction. Whilst thus moving the leaflet also rotated onits own axis (and this was a point to which no attention had been beforepaid), for the plane of the blade differed by 41o after an interval of onlya few minutes. Occasionally the leaflet stood still for a short time. Therewas no jerking movement, which is so characteristic of the little lateralleaflets. A sudden and considerable fall of temperature causes the terminalleaflet to sink downwards; thus a cut-off leaf was immersed in water at 95oF. , which was slowly raised to 103o F. , and afterwards allowed to sink to70o F. , and the sub-petiole of the terminal leaflet then curved downwards. The water was afterwards[page 360]

Fig. 150. Desmodium gyrans: circumnutation and nyctitropic movement of leaf(3 3/4 inches in length, petiole included) during 48 h. Filament affixed tomidrib of terminal leaflet; its apex 6 inches from the vertical glass. Diagram reduced to one-third of original scale. Plant illuminated fromabove. Temp. 19o - 20o C. [page 361]

raised to 120o F. , and the sub-petiole straightened itself. Similarexperiments with leaves in water were twice repeated, with nearly the sameresult. It should be added, that water raised to even 122o F. Does not soonkill a leaf. A plant was placed in darkness at 8. 37 A. M. , and at 2 P. M. (i. E. After 5 h. 23 m. ), though the leaflets had sunk considerably, theyhad by no means acquired their nocturnal vertically dependent position. Pfeffer, on the other hand, says* that this occurred with him in from 3/4h. To 2 h. ; perhaps the difference in our results may be due to the planton which we experimented being a very young and vigorous seedling. 

The Movements of the little Lateral Leaflets . --These have been so oftendescribed, that we will endeavour to be as brief as possible in giving afew new facts and conclusions. The leaflets sometimes quickly change theirposition by as much as nearly 180o; and their sub-petioles can then be seento become greatly curved. They rotate on their own axes, so that theirupper surfaces are directed to all points of the compass. The figuredescribed by the apex is an irregular oval or ellipse. They sometimesremain stationary for a period. In these several respects there is nodifference, except in rapidity and extent, between their movements and thelesser ones performed by the large terminal leaflet whilst making its greatoscillations. The movements of the little leaflets are much influenced, asis well known, by temperature. This was clearly shown by immersing leaveswith motionless leaflets in cold water, which was slowly raised to 103o F. , and the leaflets then moved quickly, describing about a dozen littleirregular circles in 40 m. By this time the water had become much cooler, and the movements became slower or almost ceased; it was then raised to100o F. , and the leaflets again began to move quickly. On another occasiona tuft of fine leaves was immersed in water at 53o F. , and the leafletswere of course motionless. The water was raised to 99o, and the leafletssoon began to move; it was raised to 105o, and the movements became muchmore rapid; each little circle or oval being completed in from 1 m. 30 s. To 1 m. 45 s. There was, however, no jerking, and this fact may perhaps beattributed to the resistance of the water. 

Sachs states that the leaflets do not move until the surrounding air is ashigh as 71o - 72o F. , and this agrees with our

* 'Die Period. Beweg. , ' p. 39. [page 362]

experience on full-grown, or nearly full-grown, plants. But the leaflets ofyoung seedlings exhibit a jerking movement at much lower temperatures. Aseedling was kept (April 16th) in a room for half the day where thetemperature was steady at 64o F. , and the one leaflet which it bore wascontinually jerking, but not so rapidly as in the hot-house. The pot wastaken in the evening into a bed-room where the temperature remained at 62oduring nearly the whole night; at 10 and 11 P. M. And at 1 A. M. The leafletwas still jerking rapidly; at 3. 30 A. M. It was not seen to jerk, but wasobserved during only a short time. It was, however, now inclined at a muchlower angle than that occupied at 1 A. M. At 6. 30 A. M. (temp. 61o F. ) itsinclination was still less than before, and again less at 6. 45 A. M. ; by7. 40 A. M. It had risen, and at 8. 30 A. M. Was again seen to jerk. Thisleaflet, therefore, was moving during the whole night, and the movement wasby jerks up to 1 A. M. (and possibly later) and again at 8. 30 A. M. , thoughthe temperature was only 61o to 62o F. We must therefore conclude that thelateral leaflets produced by young plants differ somewhat in constitutionfrom those on older plants. 

In the large genus Desmodium by far the greater number of the species aretrifoliate; but some are unifoliate, and even the same plant may bear uni-and trifoliate leaves. In most of the species the lateral leaflets are onlya little smaller than the terminal one. Therefore the lateral leaflets ofD. Gyrans (see Fig. 148) must be considered as almost rudimentary. They arealso rudimentary in function, if this expression may be used; for theycertainly do not sleep like the full-sized terminal leaflets. It is, however, possible that the sinking down of the leaflets between 1 A. M. And6. 45 A. M. , as above described, may represent sleep. It is well known thatthe leaflets go on jerking during the early part of the night; but mygardener observed (Oct. 13th) a plant in the hot-house between 5 and 5. 30A. M. , the temperature having been kept up to 82o F. , and found that all theleaflets were inclined, but he saw no jerking movement until 6. 55 A. M. , bywhich time the terminal leaflet had risen and was awake. Two daysafterwards (Oct. 15th) the same plant was observed by him at 4. 47 A. M. (temp. 77o F. ), and he found that the large terminal leaflets were awake, though not quite horizontal; and the only cause which we could assign forthis anomalous wakefulness was that the plant had been kept forexperimental purposes during[page 363]the previous day at an unusually high temperature; the little lateralleaflets were also jerking at this hour, but whether there was anyconnection between this latter fact and the sub-horizontal position of theterminal leaflets we do not know. Anyhow, it is certain that the lateralleaflets do not sleep like the terminal leaflets; and in so far they may besaid to be in a functionally rudimentary condition. They are in a similarcondition in relation to irritability; for if a plant be shaken orsyringed, the terminal leaflets sink down to about 45o beneath the horizon;but we could never detect any effect thus produced on the lateral leaflets;yet we are not prepared to assert positively that rubbing or pricking thepulvinus produces no effect. 

As in the case of most rudimentary organs, the leaflets are variable insize; they often depart from their normal position and do not standopposite one another; and one of the two is frequently absent. This absenceappeared in some, but not in all the cases, to be due to the leaflet havingbecome completely confluent with the main petiole, as might be inferredfrom the presence of a slight ridge along its upper margin, and from thecourse of the vessels. In one instance there was a vestige of the leaflet, in the shape of a minute point, at the further end of the ridge. Thefrequent, sudden and complete disappearance of one or both of therudimentary leaflets is a rather singular fact; but it is a much moresurprising one that the leaves which are first developed on seedling plantsare not provided with them. Thus, on one seedling the seventh leaf abovethe cotyledons was the first which bore any lateral leaflets, and then onlya single one. On another seedling, the eleventh leaf first bore a leaflet;of the nine succeeding leaves five bore a single lateral leaflet, and fourbore none at all; at last a leaf, the twenty-first above the cotyledons, was provided with two rudimentary lateral leaflets. From a widespreadanalogy in the animal kingdom, it might have been expected that theserudimentary leaflets would have been better developed and more regularlypresent on very young than on older plants. But bearing in mind, firstly, that long-lost characters sometimes reappear late in life, and secondly, that the species of Desmodium are generally trifoliate, but that some areunifoliate, the suspicion arises that D. Gyrans is descended from aunifoliate species, and that this was descended from a trifoliate one; forin this case both the absence of the little lateral leaflets on very youngseedlings, and their sub-[page 364]sequent appearance, may be attributed to reversion to more or less distantprogenitors. *

No one supposes that the rapid movements of the lateral leaflets of 'D. Gyrans' are of any use to the plant; and why they should behave in thismanner is quite unknown. We imagined that their power of movement mightstand in some relation with their rudimentary condition, and thereforeobserved the almost rudimentary leaflets of Mimosa albida vel sensitiva (ofwhich a drawing will hereafter be given, Fig. 159); but they exhibited noextraordinary movements, and at night they went to sleep like thefull-sized leaflets. There is, however, this remarkable difference in thetwo cases; in Desmodium the pulvinus of the rudimentary leaflets has notbeen reduced in length, in correspondence with the reduction of the blade, to the same extent as has occurred in the Mimosa; and it is on the lengthand degree of curvature of the pulvinus that the amount of movement of theblade depends. Thus the average length of the pulvinus in the largeterminal leaflets of Desmodium is 3 mm. , whilst that of the rudimentaryleaflets is 2. 86 mm. ; so that they differ only a little in length. But indiameter they differ much, that of the pulvinus of the little leafletsbeing only 0. 3 mm. To 0. 4 mm. ; whilst that of the terminal leaflets is 1. 33mm. If we now turn to the Mimosa, we find that the average length of thepulvinus of the almost rudimentary leaflets is only 0. 466 mm. , or rathermore than a quarter of the length of the pulvinus of the full-sizedleaflets, namely, 1. 66 mm. In this small reduction in length of thepulvinus of the rudimentary leaflets of Desmodium, we apparently have theproximate cause of their great and rapid circumnutating movement, incontrast with that of the almost rudimentary leaflets of the Mimosa. Thesmall size and weight of the blade, and the little resistance opposed bythe air to its movement, no doubt also come into play; for we have seenthat these leaflets if immersed in water, when the resistance would be muchgreater, were prevented from jerking forwards. Why, during the reduction ofthe lateral leaflets of Desmodium, or during their reappearance--if theyowe their origin to reversion--the pulvinus should have been so much lessaffected than the blade, whilst with the

* Desmodium vespertilionis is closely allied to D. Gyrans, and it seemsonly occasionally to bear rudimentary lateral leaflets. Duchartre, 'Eléments de Botanique, ' 1867, p. 353. [page 365]

Mimosa the pulvinus has been greatly reduced, we do not know. Nevertheless, it deserves notice that the reduction of the leaflets in these two generahas apparently been effected by a different process and for a differentend; for with the Mimosa the reduction of the inner and basal leaflets wasnecessary from the want of space; but no such necessity exists withDesmodium, and the reduction of its lateral leaflets seems to have been dueto the principle of compensation, in consequence of the great size of theterminal leaflet. Uraria (Tribe 6) and Centrosema (Tribe 8). --The leaflets of Uraria lagopusand the leaves of a Centrosema from Brazil both sink vertically down atnight. In the latter plant the petiole at the same time rose 16 1/2o. 

Amphicarpoea monoica (Tribe 8). --The leaflets sink down vertically atnight, and the petioles likewise fall considerably. 

Fig. 151. Amphicarpoea monoica: circumnutation and nyctitropic movement ofleaf during 48 h. ; its apex 9 inches from the vertical glass. Figurereduced to one-third of original scale. Plant illuminated from above; temp17 1/2o - 18 1/2o C. 

A petiole, which was carefully observed, stood during the day 25o above thehorizon and at night 32o below it; it therefore fell 57o. A filament wasfixed transversely across the terminal leaflet of a fine young leaf (2 1/4inches in length including the[page 366]petiole), and the movement of the whole leaf was traced on a verticalglass. This was a bad plan in some respects, because the rotation of theleaflet, independently of its rising or falling, raised and depressed thefilament; but it was the best plan for our special purpose of observingwhether the leaf moved much after it had gone to sleep. The plant hadtwined closely round a thin stick, so that the circumnutation of the stemwas prevented. The movement of the leaf was traced during 48 h. , from 9A. M. July 10th to 9 A. M. July 12th. In the figure given (Fig. 151) we seehow complicated its course was on both days: during the second day itchanged its course greatly 13 times. The leaflets began to go to sleep alittle after 6 P. M. , and by 7. 15 P. M. Hung vertically down and werecompletely asleep; but on both nights they continued to move from 7. 15 P. M. To 10. 40 and 10. 50 P. M. , quite as much as during the day; and this was thepoint which we wished to ascertain. We see in the figure that the greatsinking movement late in the evening does not differ essentially from thecircumnutation during the day. 

Glycine hispida (Tribe 8). --The three leaflets sink vertically down atnight. 

Erythrina (Tribe 8). --Five species were observed, and the leaflets of allsank vertically down at night; with E. Caffra and with a second unnamedspecies, the petioles at the same time rose slightly. The movements of theterminal leaflet of E. Crista-galli (with the main petiole secured to astick) were traced from 6. 40 A. M. June 8th, to 8 A. M. On the 10th. In orderto observe the nyctitropic movements of this plant, it is necessary that itshould have grown in a warm greenhouse, for out of doors in our climate itdoes not sleep. We see in the tracing (Fig. 152) that the leafletoscillated twice up and down between early morning and noon; it then fellgreatly, afterwards rising till 3 P. M. At this latter hour the greatnocturnal fall commenced. On the second day (of which the tracing is notgiven) there was exactly the same double oscillation before noon, but onlya very small one in the afternoon. On the third morning the leaflet movedlaterally, which was due to its beginning to assume an oblique position, asseems invariably to occur with the leaflets of this species as they growold. On both nights after the leaflets were asleep and hung verticallydown, they continued to move a little both up and down, and from side toside. 

Erythrina caffra. --A filament was fixed transversely across[page 367]

a terminal leaflet, as we wished to observe its movements when asleep. Theplant was placed in the morning of June 10th under a skylight, where thelight was not bright; and we do not know whether it was owing to this causeor to the plant having been disturbed, but the leaflet hung vertically downall day; nevertheless it circumnutated in this position, describing afigure which represented two irregular ellipses. On the next day itcircumnutated in a greater degree, describing four irregular ellipses, andby 3 P. M. Had risen into a horizontal position. By 7. 15 P. M. It was asleepand vertically dependent, but continued to circumnutate as long asobserved, until 11 P. M. 

Fig. 152. Erythrina crista-galli: circumnutation and nyctitropic movementof terminal leaflet, 3 3/4 inches in length, traced during 25 h. ; apex ofleaf 3 ½ inches from the vertical glass. Figure reduced to one-half oforiginal scale. Plant illuminated from above; temp. 17 1/2o - 18 1/2o C. 

Erythrina corallodendron. --The movements of a terminal leaflet were traced. During the second day it oscillated four times up and four times downbetween 8 A. M. And 4 P. M. , after which hour the great nocturnal fallcommenced. On the third day the movement was equally great in amplitude, but was remarkably simple, for the leaflet rose in an almost perfectlystraight line from 6. 50 A. M. To 3 P. M. , and then sank down in an equallystraight line until vertically dependent and asleep. [page 368]

Apios tuberosa (Tribe 8). --The leaflets sink vertically down at night. 

Phaseolus vulgaris (Tribe 8). --The leaflets likewise sink vertically downat night. In the greenhouse the petiole of a young leaf rose 16o, and thatof an older leaf 10o at night. With plants growing out of doors theleaflets apparently do not sleep until somewhat late in the season, for onthe nights of July 11th and 12th none of them were asleep; whereas on thenight of August 15th the same plants had most of their leaflets verticallydependent and asleep. With Ph. Caracalla and Hernandesii, the primaryunifoliate leaves and the leaflets of the secondary trifoliate leaves sinkvertically down at night. This holds good with the secondary trifoliateleaves of Ph. Roxburghii, but it is remarkable that the primary unifoliateleaves which are much elongated, rise at night from about 20o to about 60oabove the horizon. With older seedlings, however, having the secondaryleaves just developed, the primary leaves stand in the middle of the dayhorizontally, or are deflected a little beneath the horizon. In one suchcase the primary leaves rose from 26o beneath the horizon at noon, to 20oabove it at 10 P. M. ; whilst at this same hour the leaflets of the secondaryleaves were vertically dependent. Here, then, we have the extraordinarycase of the primary and secondary leaves on the same plant moving at thesame time in opposite directions. 

We have now seen that the leaflets in the six genera of Phaseoleae observedby us (with the exception of the primary leaves of Phaseolus Roxburghii)all sleep in the same manner, namely, by sinking vertically down. Themovements of the petioles were observed in only three of these genera. Theyrose in Centrosema and Phaseolus, and sunk in Amphicarpaea. 

Sophora chrysophylla (Tribe 10). --The leaflets rise at night, and are atthe same time directed towards the apex of the leaf, as in Mimosa pudica. 

Caesalpinia, Hoematoxylon, Gleditschia, Poinciana. --The leaflets of twospecies of Caesalpinia (Tribe 13) rose at night. With HaematoxylonCampechianum (Tribe 13) the leaflets move forwards at night, so that theirmidribs stand parallel to the petiole, and their now vertical lowersurfaces are turned outwards (Fig. 153). The petiole sinks a little. InGleditschia, if we understand correctly Duchartre's description, and inPoin-[page 369]ciana Gilliesii (both belonging to Tribe 13), the leaves behave in the samemanner. 

Fig. 153. Haematoxylon Campechianum: A, branch during daytime; B, branchwith leaves asleep, reduced to two-thirds of natural scale. 

Cassia (Tribe 14). --The nyctitropic movements of the leaves in many speciesin this genus are closely alike, and are highly complex. They were firstbriefly described by Linnaeus, and since by Duchartre. Our observationswere made chiefly on C. Floribunda* and corymbosa, but several otherspecies were casually observed. The horizontally extended leaflets sinkdown vertically at night; but not simply, as in so many other genera, foreach leaflet rotates on its own axis, so that its lower surface facesoutwards. The upper surfaces of the opposite leaflets are thus brought intocontact with one another beneath the petiole, and are well protected (Fig. 154). The rotation and other movements are effected by means of awell-developed pulvinus at the base of each leaflet, as could be plainlyseen when a straight narrow black line had been painted along it during theday. The two terminal leaflets in the daytime include rather less than aright angle; but their divergence increases greatly whilst they

* I am informed by Mr. Dyer that Mr. Bentham believes that C. Floribunda (acommon greenhouse bush) is a hybrid raised in France, and that it comesvery near to C. Laevigata. It is no doubt the same as the form described byLindley ('Bot. Reg. , ' Tab. 1422) as C. Herbertiana. [page 370]

sink downwards and rotate, so that they stand laterally at night, as may beseen in the figure. Moreover, they move somewhat backwards, so as to pointtowards the base of the petiole. 

Fig. 154. Cassia corymbosa: A, plant during day; B, same plant at night. Both figures copied from photographs. 

In one instance we found that the midrib of a terminal leaflet formed atnight an angle of 36o, with a line dropped[page 371]perpendicularly from the end of the petiole. The second pair of leafletslikewise moves a little backwards, but less than the terminal pair; and thethird pair moves vertically downwards, or even a little forwards. Thus allthe leaflets, in those species which bear only 3 or 4 pairs, tend to form asingle packet, with their upper surfaces in contact, and their lowersurfaces turned outwards. Lastly, the main petiole rises at night, but withleaves of different ages to very different degrees, namely some rosethrough an angle of only 12o, and others as much as 41o. 

Cassia calliantha. --The leaves bear a large number of leaflets, which moveat night in nearly the same manner as just described; but the petiolesapparently do not rise, and one which was carefully observed certainly fell3o. Cassia pubescens. --The chief difference in the nyctitropic

Fig. 155. Cassia pubescens: A, upper part of plant during the day; B, sameplant at night. Figures reduced from photographs. 

movements of this species, compared with those of the former species, consists in the leaflets not rotating nearly so much;[page 372]therefore their lower surfaces face but little outwards at night. Thepetioles, which during the day are inclined only a little above thehorizon, rise at night in a remarkable manner, and stand nearly or quitevertically. This, together with the dependent position of the leaflets, makes the whole plant wonderfully compact at night. In the two foregoingfigures, copied from photographs, the same plant is represented awake andasleep (Fig. 155), and we see how different is its appearance. 

Cassia mimosoides. --At night the numerous leaflets on each leaf rotate ontheir axes, and their tips move towards the apex of the leaf; they thusbecome imbricated with their lower surfaces directed upwards, and withtheir midribs almost parallel to the petiole. Consequently, this speciesdiffers from all the others seen by us, with the exception of the followingone, in the leaflets not sinking down at night. A petiole, the movement ofwhich was measured, rose 8o at night. 

Cassia Barclayana. --The leaflets of this Australian species are numerous, very narrow, and almost linear. At night they rise up a little, and alsomove towards the apex of the leaf. For instance, two opposite leafletswhich diverged from one another during the day at an angle of 104o, diverted at night only 72o; so that each had risen 16o above its diurnalposition. The petiole of a young leaf rose at night 34o, and that of anolder leaf 19o. Owing to the slight movement of the leaflets and theconsiderable movement of the petiole, the bush presents a differentappearance at night to what it does by day; yet the leaves can hardly besaid to sleep. 

The circumnutating movements of the leaves of C. Floribunda, calliantha, and pubescens were observed, each during three or four days; they wereessentially alike, those of the last-named species being the simplest. Thepetiole of C. Floribunda was secured to a stick at the base of the twoterminal leaflets, and a filament was fixed along the midrib of one ofthem. Its movements were traced from 1 P. M. On August 13th to 8. 30 A. M. 17th; but those during the last 2 h. Are alone given in Fig. 156. From 8A. M. On each day (by which hour the leaf had assumed its diurnal position)to 2 or 3 P. M. , it either zigzagged or circumnutated over nearly the samesmall space; at between 2 and 3 P. M. The great evening fall commenced. Thelines representing this fall and the early morning rise are oblique, owingto the peculiar manner in which the leaflets sleep, as already described. After the leaflet was asleep at 6 P. M. , and whilst the glass filament hung[page 373]perpendicularly down, the movement of its apex was traced until 10. 30 P. M. ;and during this whole time it swayed from side to side, completing morethan one ellipse. 

Fig 156. Cassia floribunda: circumnutation and nyctitropic movement of aterminal leaflet (1 5/6 inch in length) traced from 8. 30 A. M. To same houron following morning. Apex of leaflet 5 ½ inches from the vertical glass. Main petiole 3 3/4 inches long. Temp. 16o - 17 1/2o C. Figure reduced toone-half of the original scale. 

Bauhinia (Tribe 15). --The nyctitropic movements of four species were alike, and were highly peculiar. A plant raised from seed sent us from SouthBrazil by Fritz Müller, was more especially observed. The leaves are largeand deeply notched at their ends. At night the two halves rise up and closecompletely together, like the opposite leaflets of many Leguminosae. Withvery young plants the petioles rise considerably at the same time; one, which was inclined at noon 45o above the horizon, at night stood at 75o; itthus rose 30o; another rose 34o. Whilst the two halves of the leaf areclosing, the midrib at first sinks vertically downwards and afterwardsbends backwards, so as to pass close along one side of its own upwardlyinclined petiole; the midrib being thus directed towards the stem or axisof the plant. The angle which the midrib formed with the horizon wasmeasured in one case at different hours: at noon it stood horizontally;late in the evening it depended vertically; then rose to the opposite side, and at 10. 15 P. M. Stood at only 27o beneath the horizon, being directedtowards the stem. It had thus travelled through 153o. [page 374]Owing to this movement--to the leaves being folded--and to the petiolesrising, the whole plant is as much more compact at night than during theday, as a fastigiate Lombardy poplar is compared with any other species ofpoplar. It is remarkable that when our plants had grown a little older, viz. , to a height of 2 or 3 feet, the petioles did not rise at night, andthe midribs of the folded leaves were no longer bent back along one side ofthe petiole. We have noticed in some other genera that the petioles of veryyoung plants rise much more at night than do those of older plants. 

Tamarindus Indica (Tribe 16). --The leaflets approach or meet each other atnight, and are all directed towards the apex of the leaf. They thus becomeimbricated with their midribs parallel to the petiole. The movement isclosely similar to that of Haematoxylon (see Fig. 153), but more strikingfrom the greater number of the leaflets. 

Adenanthera, Prosopis, and Neptunia (Tribe 20). --With Adenanthera pavoniathe leaflets turn edgeways and sink at night. In Prosopis they turnupwards. With Neptunia oleracea the leaflets on the opposite sides of thesame pinna come into contact at night and are directed forwards. The pinnaethemselves move downwards, and at the same time backwards or towards thestem of the plant. The main petiole rises. 

Mimosa pudica (Tribe 20). --This plant has been the subject of innumerableobservations; but there are some points in relation to our subject whichhave not been sufficiently attended to. At night, as is well known, theopposite leaflets come into contact and point towards the apex of the leaf;they thus become neatly imbricated with their upper surfaces protected. Thefour pinnae also approach each other closely, and the whole leaf is thusrendered very compact. The main petiole sinks downwards during the day tilllate in the evening, and rises until very early in the morning. The stem iscontinually circumnutating at a rapid rate, though not to a wide extent. Some very young plants, kept in darkness, were observed during two days, and although subjected to a rather low temperature of 57o - 59o F. , thestem of one described four small ellipses in the course of 12 h. We shallimmediately see that the main petiole is likewise continuallycircumnutating, as is each separate pinna and each separate leaflet. Therefore, if the movement of the apex of any one leaflet were to betraced, the course described would be compounded of the movements of fourseparate parts. [page 375]A filament had been fixed on the previous evening, longitudinally to themain petiole of a nearly full-grown, highly-sensitive leaf (four inches inlength), the stem having been secured to a stick at its base; and a tracingwas made on a vertical glass in the hot-house under a high temperature. Inthe figure given (Fig. 157), the first dot was made at 8. 30 A. M. August2nd, and the last at 7 P. M. On the 3rd. During 12 h. On the first day thepetiole moved thrice downwards and twice upwards. Within the same length oftime on the second day, it moved five times downwards and four timesupwards. As the ascending and descending lines do not coincide, the petiolemanifestly circumnutates; the great evening fall and nocturnal rise beingan exaggeration of one of the circumnutations. It should, however, beobserved that the petiole fell much lower down in the evenings than couldbe seen on the vertical glass or is represented in the diagram. After 7P. M. On the 3rd (when the last dot in Fig. 157 was made) the pot wascarried into a bed-room, and the petiole was found at 12. 50 A. M. (i. E. After midnight) standing almost upright, and much more highly inclined thanit was at 10. 40 P. M. When observed again at 4 A. M. It had begun to fall, and continued falling till 6. 15 A. M. , after which hour it zigzagged andagain circumnutated. Similar observations were made on another petiole, with nearly the same result. 

Fig. 157 Mimosa pudica: circumnutation and nyctitropic movement of mainpetiole, traced during 34 h. 30 m. 

On two other occasions the movement of the main petiole[page 376]was observed every two or three minutes, the plants being kept at a ratherhigh temperature, viz. , on the first occasion at 77o - 81o F. , and thefilament then described 2 ½ ellipses in 69 m. On the second occasion, whenthe temperature was 81o - 86o F. , it made rather more than 3 ellipses in 67m. Therefore, Fig. 157, though now sufficiently complex, would have beenincomparably more so, if dots had been made on the glass every 2 or 3minutes, instead of every hour or half-hour. Although the main petiole iscontinually and rapidly describing small ellipses during the day, yet afterthe great nocturnal rising movement has commenced, if dots are made every 2or 3 minutes, as was done for an hour between 9. 30 and 10. 30 P. M. (temp. 84o F. ), and the dots are then joined, an almost absolutely straight lineis the result. 

To show that the movement of the petiole is in all probability due to thevarying turgescence of the pulvinus, and not to growth (in accordance withthe conclusions of Pfeffer), a very old leaf, with some of its leafletsyellowish and hardly at all sensitive, was selected for observation, andthe plant was kept at the highly favourable temp. Of 80o F. The petiolefell from 8 A. M. Till 10. 15 A. M. , it then rose a little in a somewhatzigzag line, often remaining stationary, till 5 P. M. , when the greatevening fall commenced, which was continued till at least 10 P. M. By 7 A. M. On the following morning it had risen to the same level as on the previousmorning, and then descended in a zigzag line. But from 10. 30 A. M. Till 4. 15P. M. It remained almost motionless, all power of movement being now lost. The petiole, therefore, of this very old leaf, which must have long ceasedgrowing, moved periodically; but instead of circumnutating several timesduring the day, it moved only twice down and twice up in the course of 24h. , with the ascending and descending lines not coincident. 

It has already been stated that the pinnae move independently of the mainpetiole. The petiole of a leaf was fixed to a cork support, close to thepoint whence the four pinnae diverge, with a short fine filament cementedlongitudinally to one of the two terminal pinnae, and a graduatedsemicircle was placed close beneath it. By looking vertically down, itsangular or lateral movements could be measured with accuracy. Between noonand 4. 15 P. M. The pinna changed its position to one side by only 7o; butnot continuously in the same direction, as it moved four times to one side, and three times to the opposite side, [page 377]in one instance to the extent of 16o. This pinna, therefore circumnutated. Later in the evening the four pinnae approach each other, and the one whichwas observed moved inwards 59o between noon and 6. 45 P. M. Ten observationswere made in the course of 2 h. 20 m. (at average intervals of 14 m. ), between 4. 25 and 6. 45 P. M. ; and there was now, when the leaf was going tosleep, no swaying from side to side, but a steady inward movement. Heretherefore there is in the evening the same conversion of a circumnutatinginto a steady movement in one direction, as in the case of the mainpetiole. 

It has also been stated that each separate leaflet circumnutates. A pinnawas cemented with shellac on the summit of a little stick driven firmlyinto the ground, immediately beneath a pair of leaflets, to the midribs ofboth of which excessively fine glass filaments were attached. Thistreatment did not injure the leaflets, for they went to sleep in the usualmanner, and long retained their sensitiveness. The movements of one of themwere traced during 49 h. , as shown in Fig. 158. On the first day theleaflet sank down till 11. 30 A. M. , and then rose till late in the eveningin a zigzag line, indicating circumnutation. On the second day, when moreaccustomed to its new state, it oscillated twice up and twice down duringthe 24 h. This plant was subjected to a rather low temperature, viz. , 62o -64o F. ; had it been kept warmer, no doubt the movements of the leafletwould have been much more rapid and complicated. It may be seen in thediagram that the ascending and descending lines do not coincide; but thelarge amount of lateral movement in the evening is the result of theleaflets bending towards the apex of the leaf when going to sleep. Anotherleaflet was casually observed, and found to be continually circumnutatingduring the same length of time. 

The circumnutation of the leaves is not destroyed by their being subjectedto moderately long continued darkness; but the proper periodicity of theirmovements is lost. Some very young seedlings were kept during two days inthe dark (temp. 57o - 59o F. ) except when the circumnutation of their stemswas occasionally observed; and on the evening of the second day theleaflets did not fully and properly go to sleep. The pot was then placedfor three days in a dark cupboard, under nearly the same temperature, andat the close of this period the leaflets showed no signs of sleeping, andwere only slightly sensitive to a touch. On the following day the stem wascemented to a[page 378]stick, and the movements of two leaves were traced on a vertical glassduring 72 h. The plants were still kept in the dark, excepting that at eachobservation, which lasted 3 or 4 minutes, Fig 158. Mimosa pudica: circumnutation and nyctitropic movement of aleaflet (with pinna secured), traced on a vertical glass, from 8 A. M. Sept. 14th to 9 A. M. 16th. 

they were illuminated by two candles. On the third day the leaflets stillexhibited a vestige of sensitiveness when forcibly pressed, but in theevening they showed no signs of sleep. Nevertheless, their petiolescontinued to circumnutate distinctly, [page 379]although the proper order of their movements in relation to the day andnight was wholly lost. Thus, one leaf descended during the first two nights(i. E. Between 10 P. M. And 7 A. M. Next morning) instead of ascending, and onthe third night it moved chiefly in a lateral direction. The second leafbehaved in an equally abnormal manner, moving laterally during the firstnight, descending greatly during the second, and ascending to an unusualheight during the third night. 

With plants kept at a high temperature and exposed to the light, the mostrapid circumnutating movement of the apex of a leaf which was observed, amounted to 1/500 of an inch in one second; and this would have equalled1/8 of an inch in a minute, had not the leaf occasionally stood still. Theactual distance travelled by the apex (as ascertained by a measure placedclose to the leaf) was on one occasion nearly 3/4 of an inch in a verticaldirection in 15 m. ; and on another occasion 5/8 of an inch in 60 m. ; butthere was also some lateral movement. 

Mimosa albida. *--The leaves of this plant, one of which is here figured(Fig. 159) reduced to 2/3 of the natural size, present some

Fig. 159. Mimosa albida: leaf seen from vertically above. 

interesting peculiarities. It consists of a long petiole bearing only twopinnae (here represented as rather more divergent than is usual), each withtwo pairs of leaflets. But the inner

* Mr. Thiselton Dyer informs us that this Peruvian plant (which was sent tous from Kew) is considered by Mr. Bentham ('Trans. Linn. Soc. , ' vol. Xxx. P. 390) to be "the species or variety which most commonly represents the M. Sensitiva of our gardens. "[page 380]

basal leaflets are greatly reduced in size, owing probably to the want ofspace for their full development, so that they may be considered as almostrudimentary. They vary somewhat in size, and both occasionally disappear, or only one. Nevertheless, they are not in the least rudimentary infunction, for they are sensitive, extremely heliotropic, circumnutate atnearly the same rate as the fully developed leaflets, and assume whenasleep exactly the same position. With M. Pudica the inner leaflets at thebase and between the pinnae are likewise much shortened and obliquelytruncated; this fact was well seen in some seedlings of M. Pudica, in whichthe third leaf above the cotyledons bore only two pinnae, each with only 3or 4 pairs of leaflets, of which the inner basal one was less than half aslong as its fellow; so that the whole leaf resembled pretty closely that ofM. Albida. In this latter species the main petiole terminates in a littlepoint, and on each side of this there is a pair of minute, flattened, lancet-shaped projections, hairy on their margins, which drop off anddisappear soon after the leaf is fully developed. There can hardly be adoubt that these little projections are the last and fugaciousrepresentatives of an additional pair of leaflets to each pinna; for theouter one is twice as broad as the inner one, and a little longer, viz. 7/100 of an inch, whilst the inner one is only 5/100 - 6/100 long. Now ifthe basal pair of leaflets of the existing leaves were to becomerudimentary, we should expect that the rudiments would still exhibit sometrace of their present great inequality of size. The conclusion that thepinnae of the parent-form of M. Albida possessed at least three pairs ofleaflets, instead of, as at present, only two, is supported by thestructure of the first true leaf; for this consists of a simple petiole, often bearing three pairs of leaflets. This latter fact, as well as thepresence of the rudiments, both lead to the conclusion that M. Albida isdescended from a form the leaves of which bore more than two pairs ofleaflets. The second leaf above the cotyledons resembles in all respectsthe leaves on fully developed plants. 

When the leaves go to sleep, each leaflet twists half round, so as topresent its edge to the zenith, and comes into close contact with itsfellow. The pinnae also approach each other closely, so that the fourterminal leaflets come together. The large basal leaflets (with the littlerudimentary ones in contact with them) move inwards and forwards, so as toembrace the outside of the united terminal leaflets, and thus all eightleaflets[page 381](the rudimentary ones included) form together a single vertical packet. Thetwo pinnae at the same time that they approach each other sink downwards, and thus instead of extending horizontally in the same line with the mainpetiole, as during the day, they depend at night at about 45o, or even at agreater angle, beneath the horizon. The movement of the main petiole seemsto be variable; we have seen it in the evening 27o lower than during theday; but sometimes in nearly the same position. Nevertheless, a sinkingmovement in the evening and a rising one during the night is probably thenormal course, for this was well-marked in the petiole of the first-formedtrue leaf. 

The circumnutation of the main petiole of a young leaf was traced during 23/4 days, and was considerable in extent, but less complex than that of M. Pudica. The movement was much more lateral than is usual withcircumnutating leaves, and this was the sole peculiarity which itpresented. The apex of one of the terminal leaflets was seen under themicroscope to travel 1/50 of an inch in 3 minutes. 

Mimosa marginata. --The opposite leaflets rise up and approach each other atnight, but do not come into close contact, except in the case of very youngleaflets on vigorous shoots. Full-grown leaflets circumnutate during theday slowly and on a small scale. 

Schrankia uncinata (Tribe 20). --A leaf consists of two or three pairs ofpinnae, each bearing many small leaflets. These, when the plant is asleep, are directed forwards and become imbricated. The angle between the twoterminal pinnae was diminished at night, in one case by 15o; and they sankalmost vertically downwards. The hinder pairs of pinnae likewise sinkdownwards, but do not converge, that is, move towards the apex of the leaf. The main petiole does not become depressed, at least during the evening. Inthis latter respect, as well as in the sinking of the pinnae, there is amarked difference between the nyctitropic movements of the present plantand of Mimosa pudica. It should, however, be added that our specimen wasnot in a very vigorous condition. The pinnae of Schrankia aculeata alsosink at night. 

Acacia Farnesiana (Tribe 22). --The different appearance presented by a bushof this plant when asleep and awake is wonderful. The same leaf in the twostates is shown in the following figure (Fig. 160). The leaflets movetowards the apex of the pinna and become imbricated, and the pinnae thenlook like bits of dangling string. The following remarks and measurements[page 382]do not fully apply to the small leaf here figured. The pinnae move forwardsand at the same time sink downwards, whilst the main petiole risesconsiderably. With respect to the degree of movement: the two terminalpinnae of one specimen formed together an angle of 100o during the day, andat night of only 38o, so each had moved 31o forwards. The penultimatepinnae during the day formed together an angle of 180o, that is, they stoodin a straight line opposite one another, and at night each had moved 65oforwards. The basal pair of pinnae were directed

Fig. 160. Acacia Farnesiana: A, leaf during the day; B, the same leaf atnight. 

during the day, each about 21o backwards, and at night 38o forwards, soeach had moved 59o forwards. But the pinnae at the same time sink greatly, and sometimes hang almost perpendicularly downwards. The main petiole, onthe other hand, rises much: by 8. 30 P. M. One stood 34o higher than at noon, and by 6. 40 A. M. On the following morning it was still higher by 10o;shortly after this hour the diurnal sinking movement commenced. The courseof a nearly full-grown leaf was traced during 14 h. ; it was stronglyzigzag, and apparently[page 383]represented five ellipses, with their longer axes differently directed. 

Albizzia lophantha (Tribe 23). --The leaflets at night come into contactwith one another, and are directed towards the apex of the pinna. Thepinnae approach one another, but remain in the same plane as during theday; and in this respect they differ much from those of the above Schrankiaand Acacia. The main petiole rises but little. The first-formed leaf abovethe cotyledons bore 11 leaflets on each side, and these slept like those onthe subsequently formed leaves; but the petiole of this first leaf wascurved downwards during the day and at night straightened itself, so thatthe chord of its arc then stood 16o higher than in the day-time. 

Melaleuca ericaefolia (Myrtaceae). --According to Bouché ('Bot. Zeit. , '1874, p. 359) the leaves sleep at night, in nearly the same manner as thoseof certain species of Pimelia. 

Oenothera mollissima (Onagrarieae). --According to Linnaeus ('SomnusPlantarum'), the leaves rise up vertically at night. 

Passiflora gracilis (Passifloracae). --The young leaves sleep by theirblades hanging vertically downwards, and the whole length of the petiolethen becomes somewhat curved downwards. Externally no trace of a pulvinuscan be seen. The petiole of the uppermost leaf on a young shoot stood at10. 45 A. M. At 33o above the horizon; and at 10. 30 P. M. , when the blade wasvertically dependent, at only 15o, so the petiole had fallen 18o. That ofthe next older leaf fell only 7o. From some unknown cause the leaves do notalways sleep properly. The stem of a plant, which had stood for some timebefore a north-east window, was secured to a stick at the base of a youngleaf, the blade of which was inclined at 40o below the horizon. From itsposition the leaf had to be viewed obliquely, consequently the verticallyascending and descending movements appeared when traced oblique. On thefirst day (Oct. 12th) the leaf descended in a zigzag line until late in theevening; and by 8. 15 A. M. On the 13th had risen to nearly the same level ason the previous morning. A new tracing was now begun (Fig. 161). The leafcontinued to rise until 8. 50 A. M. , then moved a little to the right, andafterwards descended. Between 11 A. M. And 5 P. M. It circumnutated, andafter the latter hour the great nocturnal fall commenced. At 7. 15 P. M. Itdepended vertically. The dotted line ought to have been prolonged muchlower down in the figure. By 6. 50 A. M. On the following morning (14th) the[page 384]leaf had risen greatly, and continued to rise till 7. 50 A. M. , after whichhour it redescended. It should be observed that the lines traced on thissecond morning would have coincided with and confused those previouslytraced, had not the pot been slided a very little to the left. In theevening (14th) a mark was placed behind the filament attached to the apexof the leaf, and its movement was carefully traced from 5 P. M. To 10. 15P. M. 

Fig. 161. Passiflora gracilis: circumnutation and nyctitropic movement ofleaf, traced on vertical glass, from 8. 20 A. M. Oct. 13th to 10 A. M. 14th. Figure reduced to two-thirds of original scale. 

Between 5 and 7. 15 P. M. The leaf descended in a straight line, and at thelatter hour it appeared vertically dependent. But between 7. 15 and 10. 15P. M. The line consisted of a succession of steps, the cause of which wecould not understand; it was, however, manifest that the movement was nolonger a simple descending one. 

Siegesbeckia orientalis (Compositae). --Some seedlings were raised in themiddle of winter and kept in the hot-house; they flowered, but did not growwell, and their leaves never showed any signs of sleep. The leaves on otherseedlings raised in May were horizontal at noon (June 22nd), and dependedat a consi-[page 385]derable angle beneath the horizon at 10 P. M. In the case of four youngishleaves which were from 2 to 2 ½ inches in length, these angles were foundto be 50o, 56o, 60o, and 65o. At the end of August when the plants had grownto a height of 10 to 11 inches, the younger leaves were so much curveddownwards at night that they might truly be said to be asleep. This is one

Fig. 162. Nicotiana glauca: shoots with leaves expanded during the day, andasleep at night. Figures copied from photographs, and reduced. 

of the species which must be well illuminated during the day in order tosleep, for on two occasions when plants were kept all day in a room withnorth-east windows, the leaves did not sleep at night. The same causeprobably accounts for the leaves on our seedlings raised in the dead of thewinter not sleeping. Professor Pfeffer informs us that the leaves ofanother species (S. Jorullensis ?) hang vertically down at night. [page 386]

Ipomoea caerulea and purpurea (Convolvulaceae). --The leaves on very youngplants, a foot or two in height, are depressed at night to between 68o and80o beneath the horizon; and some hang quite vertically downwards. On thefollowing morning they again rise into a horizontal position. The petiolesbecome at night downwardly curved, either through their entire length or inthe upper part alone; and this apparently causes the depression of theblade. It seems necessary that the leaves should be well illuminated duringthe day in order to sleep, for those which stood on the back of a plantbefore a north-east window did not sleep. 

Nicotiana tabacum (var. Virginian) and glauca (Solaneae). --The young leavesof both these species sleep by bending vertically upwards. Figures of twoshoots of N. Glauca, awake and asleep (Fig. 162), are given on p. 385: oneof the shoots, from which the photographs were taken, was accidentally bentto one side. 

Fig. 163. Nicotiana tabacum: circumnutation and nyctitropic movement of aleaf (5 inches in length), traced on a vertical glass, from 3 P. M. July10th to 8. 10 A. M. 13th. Apex of leaf 4 inches from glass. Temp. 17 1/2o -18 1/2o C. Figure reduced to one-half original scale. 

At the base of the petiole of N. Tabacum, on the outside, there is a massof cells, which are rather smaller than elsewhere, and[page 387]have their longer axes differently directed from the cells of theparenchyma, and may therefore be considered as forming a sort of pulvinus. A young plant of N. Tabacum was selected, and the circumnutation of thefifth leaf above the cotyledons was observed during three days. On thefirst morning (July 10th) the leaf fell from 9 to 10 A. M. , which is itsnormal course, but rose during the remainder of the day; and this no doubtwas due to its being illuminated exclusively from above; for properly theevening rise does not commence until 3 or 4 P. M. In the figure as given onp. 386 (Fig. 163) the first dot was made at 3 P. M. ; and the tracing wascontinued for the following 65 h. When the leaf pointed to the dot nextabove that marked 3 P. M. It stood horizontally. The tracing is remarkableonly from its simplicity and the straightness of the lines. The leaf eachday described a single great ellipse; for it should be observed that theascending and descending lines do not coincide. On the evening of the 11ththe leaf did not descend quite so low as usual, and it now zigzagged alittle. The diurnal sinking movement had already commenced each morning by7 A. M. The broken lines at the top of the figure, representing thenocturnal vertical position of the leaf, ought to be prolonged much higherup. 

Mirabilis longiflora and jalapa (Nyctagineae). --The first pair of leavesabove the cotyledons, produced by seedlings of both these species, wereconsiderably divergent during the day, and at night stood up vertically inclose contact with one another. The two upper leaves on an older seedlingwere almost horizontal by day, and at night stood up vertically, but werenot in close contact, owing to the resistance offered by the central bud. 

Polygonum aviculare (Polygoneae). --Professor Batalin informs us that theyoung leaves rise up vertically at night. This is likewise the case, according to Linnaeus, with several species of Amaranthus (Amaranthaceae);and we observed a sleep movement of this kind in one member of the genus. Again, with Chenopodium album (Chenopodieae), the upper young leaves ofsome seedlings, about 4 inches in height, were horizontal or sub-horizontalduring the day, and at 10 P. M. On March 7th were quite, or almost quite, vertical. Other seedlings raised in the greenhouse during the winter (Jan. 28th) were observed day and night, and no difference could be perceived inthe position of their leaves. According to Bouché ('Bot. Zeitung, ' 1874, p. 359) the leaves of Pimelia linoides and spectabilis (Thymeleae) sleep atnight. [page 388]

Euphorbia jacquiniaeflora (Euphorbiaceae). --Mr. Lynch called our attentionto the fact that the young leaves of this plant sleep by dependingvertically. The third leaf from the summit (March 11th) was inclined duringthe day 30o beneath the horizon, and at night hung vertically down, as didsome of the still younger leaves. It rose up to its former level on thefollowing morning. The fourth and fifth leaves from the summit stoodhorizontally during the day, and sank down at night only 38o. The sixthleaf did not sensibly alter its position. The sinking movement is due tothe downward curvature of the petiole, no part of which exhibits anystructure like that of a pulvinus. Early on the morning of June 7th afilament was fixed longitudinally to a young leaf (the third from thesummit, and 2 5/8 inches in length), and its movements were traced on avertical glass during 72 h. , the plant being illuminated from above througha skylight. Each day the leaf fell in a nearly straight line from 7 A. M. To5 P. M. , after which hour it was so much inclined downwards that themovement could no longer be traced; and during the latter part of eachnight, or early in the morning, the leaf rose. It therefore circumnutatedin a very simple manner, making a single large ellipse every 24 h. , for theascending and descending lines did not coincide. On each successive morningit stood at a less height than on the previous one, and this was probablydue partly to the increasing age of the leaf, and partly to theillumination being insufficient; for although the leaves are very slightlyheliotropic, yet, according to Mr. Lynch's and our own observations, theirinclination during the day is determined by the intensity of the light. Onthe third day, by which time the extent of the descending movement had muchdecreased, the line traced was plainly much more zigzag than on anyprevious day, and it appeared as if some of its powers of movement werethus expended. At 10 P. M. On June 7th, when the leaf depended vertically, its movements were observed by a mark being placed behind it, and the endof the attached filament was seen to oscillate slowly and slightly fromside to side, as well as upwards and downwards. 

Phyllanthus Niruri (Euphorbiaceae). --The leaflets of this plant sleep, asdescribed by Pfeffer, * in a remarkable manner, apparently like those ofCassia, for they sink downwards at night and twist round, so that theirlower surfaces are turned

* 'Die Period. Beweg. , ' p. 159. [page 389]

outwards. They are furnished as might have been expected from this complexkind of movement, with a pulvinus. 

GYMNOSPERMS. 

Pinus Nordmanniana (Coniferae). --M. Chatin states* that the leaves, whichare horizontal during the day, rise up at night, so as to assume a positionalmost perpendicular to the branch from which they arise; we presume thathe here refers to a horizontal branch. He adds: "En même temps, cemouvement d'érection est accompangé d'un mouvement de torsion imprimé à lapartie basilaire de la feuille, et pouvant souvent parcourir un arc de 90degrés. " As the lower surfaces of the leaves are white, whilst the upperare dark green, the tree presents a widely different appearance by day andnight. The leaves on a small tree in a pot did not exhibit with us anynyctitropic movements. We have seen in a former chapter that the leaves ofPinus pinaster and Austriaca are continually circumnutating. 

MONOCOTYLEDONS. 

Thalia dealbata (Cannaceae). --the leaves of this plant sleep by turningvertically upwards; they are furnished with a well-developed pulvinus. Itis the only instance known to us of a very large leaf sleeping. The bladeof a young leaf, which was as yet only 13 1/4 inches in length and 6 ½ inbreadth, formed at noon an angle with its tall petiole of 121o, and atnight stood vertically in a line with it, and so had risen 59o. The actualdistance travelled by the apex (as measured by an orthogonic tracing) ofanother large leaf, between 7. 30 A. M. And 10 P. M. , was 10 ½ inches. Thecircumnutation of two young and dwarfed leaves, arising amongst the tallerleaves at the base of the plant, was traced on a vertical glass during twodays. On the first day the apex of one, and on the second day the apex ofthe other leaf, described between 6. 40 A. M. And 4 P. M. Two ellipses, thelonger axes of which were extended in very different directions from thelines representing the great diurnal sinking and nocturnal rising movement. 

Maranta arundinacea (Cannaceae). --The blades of the leaves, which arefurnished with a pulvinus, stand horizontally during

* 'Comptes Rendus, ' Jan. 1876, p. 171. [page 390]

the day or between 10o and 20o above the horizon, and at night verticallyupwards. They therefore rise between 70o and 90o at night. The plant wasplaced at noon in the dark in the hot-house, and on the following day themovements of the leaves were traced. Between 8. 40 and 10. 30 A. M. They rose, and then fell greatly till 1. 37 P. M. But by 3 P. M. They had again risen alittle, and continued to rise during the rest of the afternoon and night;on the following morning they stood at the same level as on the previousday. Darkness, therefore, during a day and a half does not interfere withthe periodicity of their movements. On a warm but stormy evening, the plantwhilst being brought into the house, had its leaves violently shaken, andat night not one went to sleep. On the next morning the plant was takenback to the hot-house, and again at night the leaves did not sleep; but onthe ensuing night they rose in the usual manner between 70o and 80o. Thisfact is analogous with what we have observed with climbing plants, namely, that much agitation checks for a time their power of circumnutation; butthe effect in this instance was much more strongly marked and prolonged. 

Colocasia antiquorum (Caladium esculentum, Hort. ) (Aroideae). --The leavesof this plant sleep by their blades sinking in the evening, so as to standhighly inclined, or even quite vertically with their tips pointing to theground. They are not provided with a pulvinus. The blade of one stood atnoon 1 degree beneath the horizon; at 4. 20 P. M. , 20o; at 6 P. M. 43o; at7. 20 P. M. , 69o; and at 8. 30 P. M. , 68o; so it had now begun to rise; at10. 15 P. M. It stood at 65o, and on the following early morning at 11obeneath the horizon. The circumnutation of another young leaf (with itspetiole only 3 1/4 inches, and the blade 4 inches in length), was traced ona vertical glass during 48 h. ; it was dimly illuminated through a skylight, and this seemed to disturb the proper periodicity of its movements. Nevertheless, the leaf fell greatly during both afternoons, till either7. 10 P. M. Or 9 P. M. , when it rose a little and moved laterally. By an earlyhour on both mornings, it had assumed its diurnal position. The well-markedlateral movement for a short time in the early part of the night, was theonly interesting fact which it presented, as this caused the ascending anddescending lines not to coincide, in accordance with the general rule withcircumnutating organs. The movements of the leaves of this plant are thusof the most simple kind; and the tracing is not worth giving. We have seenthat in another genus of the Aroideae, namely, Pistia, the leaves[page 391]rise so much at night that they may almost be said to sleep. 

Strephium floribundum* (Gramineae). --The oval leaves are provided with apulvinus, and are extended horizontally or declined a little beneath thehorizon during the day. Those on the upright culms simply rise upvertically at night, so that their tips are directed towards the zenith. (Fig. 164. )

Fig. 164. Strephium floribundum: culms with leaves during the day, and whenasleep at night. Figures reduced. 

Horizontally extended leaves arising from much inclined or almosthorizontal culms, move at night so that their tips point towards the apexof the culm, with one lateral margin directed towards the zenith; and inorder to assume this position the leaves have to twist on their own axesthrough an angle of nearly 90o. Thus the surface of the blade always standsvertically, whatever may be the position of the midrib or of the leaf as awhole. 

The circumnutation of a young leaf (2. 3 inches in length) was traced during48 h. (Fig. 165). The movement was remarkably simple; the leaf descendedfrom before 6. 40 A. M. Until 2 or 2. 50 P. M. , and then rose so as to standvertically at about 6 P. M. , descending again late in the night or in thevery early morning. 

* A. Brongniart first observed that the leaves of this plant and ofMarsilea sleep: see 'Bull. De la Soc. Bot. De France, ' tom. Vii. 1860, p. 470. [page 392]

On the second day the descending line zigzagged slightly. As usual, theascending and descending lines did not coincide. On another occasion, whenthe temperature was a little higher, viz. , 24o - 26 1/2o C. , a leaf wasobserved 17 times between 8. 50 A. M. And 12. 16 P. M. ; it changed its courseby as much as a rectangle six times in this interval of 3 h. 26 m. , anddescribed two irregular triangles and a half. The leaf, therefore, on thisoccasion circumnutated rapidly and in a complex manner. 

Fig. 165. Strephium floribundum: circumnutation and nyctitropic movement ofa leaf, traced from 9 A. M. June 26th to 8. 45 A. M. 27th; filament fixedalong the midrib. Apex of leaf 8 1/4 inches from the vertical glass; plantilluminated from above. Temp. 23 1/2o - 24 1/2o C. 

ACOTYLEDONS. 

Marsilea quadrifoliata (Marsileaceae). --The shape of a leaf, expandedhorizontally during the day, is shown at A (Fig. 166). Each leaflet isprovided with a well-developed pulvinus. When the leaves sleep, the twoterminal leaflets rise up, twist half round and come into contact with oneanother (B), and are afterwards embraced by the two lower leaflets (C); sothat the four leaflets with their lower surfaces turned outwards form avertical packet. The curvature of the summit of the petiole of the leaffigured asleep, is merely accidental. The plant was brought into a room, where the temperature was only a little above 60o F. , and the movement ofone of the leaflets (the petiole having been secured) was traced[page 393]during 24 h. (Fig. 167). The leaf fell from the early morning till 1. 50P. M. , and then rose till 6 P. M. , when it was asleep. A

Fig. 166. Marsilea quadrifoliata: A, leaf during the day, seen fromvertically above; B, leaf beginning to go to sleep, seen laterally; C, thesame asleep. Figures reduced to one-half of natural scale. 

vertically dependent glass filament was now fixed to one of the terminaland inner leaflets; and part of the tracing in Fig. 167, after 6 P. M. , shows that it continued to sink, making one zigzag, until 10. 40 P. M. At6. 45 A. M. On the following morning, the leaf was awaking, and the filamentpointed above the vertical glass, Fig. 167. Marsilea quadrifoliata: circumnutation and nyctitropic movementof leaflet traced on vertical glass, during nearly 24 h. Figure reduced totwo-thirds of original scale. Plant kept at rather too low a temperature. 

but by 8. 25 A. M. It occupied the position shown in the figure. The diagramdiffers greatly in appearance from most of those previously given; and thisis due to the leaflet twisting and moving laterally as it approaches andcomes into contact with[page 394]its fellow. The movement of another leaflet, when asleep, was tracedbetween 6 P. M. And 10. 35 P. M. , and it clearly circumnutated, for itcontinued for two hours to sink, then rose, and then sank still lower thanit was at 6 P. M. It may be seen in the preceding figure (167) that theleaflet, when the plant was subjected to a rather low temperature in thehouse, descended and ascended during the middle of the day in a somewhatzigzag line; but when kept in the hot-house from 9 A. M. To 3 P. M. At a highbut varying temperature (viz. , between 72o and 83o F. ) a leaflet (with thepetiole secured) circumnutated rapidly, for it made three large verticalellipses in the course of the six hours. According to Brongniart, Marsileapubescens sleeps like the present species. These plants are the solecryptogamic ones known to sleep. ]

Summary and Concluding Remarks on the Nyctitropic or Sleep-movements ofLeaves. --That these movements are in some manner of high importance to theplants which exhibit them, few will dispute who have observed how complexthey sometimes are. Thus with Cassia, the leaflets which are horizontalduring the day not only bend at night vertically downwards with theterminal pair directed considerably backwards, but they also rotate ontheir own axes, so that their lower surfaces are turned outwards. Theterminal leaflet of Melilotus likewise rotates, by which movement one ofits lateral edges is directed upwards, and at the same time it moves eitherto the left or to the right, until its upper surface comes into contactwith that of the lateral leaflet on the same side, which has likewiserotated on its own axis. With Arachis, all four leaflets form togetherduring the night a single vertical packet; and to the effect this the twoanterior leaflets have to move upwards and the two posterior ones forwards, besides all twisting on their own axes. In the genus Sida the leaves ofsome species move at night through an angle of 90o upwards, and of others[page 395]through the same angle downwards. We have seen a similar difference in thenyctitropic movements of the cotyledons in the genus Oxalis. In Lupinus, again, the leaflets move either upwards or downwards; and in some species, for instance L. Luteus, those on one side of the star-shaped leaf move up, and those on the opposite side move down; the intermediate ones rotating ontheir axes; and by these varied movements, the whole leaf forms at night avertical star instead of a horizontal one, as during the day. Some leavesand leaflets, besides moving either upwards or downwards, become more orless folded at night, as in Bauhinia and in some species of Oxalis. Thepositions, indeed, which leaves occupy when asleep are almost infinitelydiversified; they may point either vertically upwards or downwards, or, inthe case of leaflets, towards the apex or towards the base of the leaf, orin any intermediate position. They often rotate at least as much as 90o ontheir own axes. The leaves which arise from upright and from horizontal ormuch inclined branches on the same plant, move in some few cases in adifferent manner, as with Porlieria and Strephium. The whole appearance ofmany plants is wonderfully changed at night, as may be seen with Oxalis, and still more plainly with Mimosa. A bush of Acacia Farnesiana appears atnight as if covered with little dangling bits of string instead of leaves. Excluding a few genera not seen by ourselves, about which we are in doubt, and excluding a few others the leaflets of which rotate at night, and donot rise or sink much, there are 37 genera in which the leaves or leafletsrise, often moving at the same time towards the apex or towards the base ofthe leaf, and 32 genera in which they sink at night. 

The nyctitropic movements of leaves, leaflets, and[page 396]petioles are effected in two different ways; firstly, by alternatelyincreased growth on their opposite sides, preceded by increased turgescenceof the cells; and secondly by means of a pulvinus or aggregate of smallcells, generally destitute of chlorophyll, which become alternately moreturgescent on nearly opposite sides; and this turgescence is not followedby growth except during the early age of the plant. A pulvinus seems to beformed (as formerly shown) by a group of cells ceasing to grow at a veryearly age, and therefore does not differ essentially from the surroundingtissues. The cotyledons of some species of Trifolium are provided with apulvinus, and others are destitute of one, and so it is with the leaves inthe genus Sida. We see also in this same genus gradations in the state ofthe development of the pulvinus; and in Nicotiana we have what may probablybe considered as the commencing development of one. The nature of themovement is closely similar, whether a pulvinus is absent or present, as isevident from many of the diagrams given in this chapter. It deserves noticethat when a pulvinus is present, the ascending and descending lines hardlyever coincide, so that ellipses are habitually described by the leaves thusprovided, whether they are young or so old as to have quite ceased growing. This fact of ellipses being described, shows that the alternately increasedturgescence of the cells does not occur on exactly opposite sides of thepulvinus, any more than the increased growth which causes the movements ofleaves not furnished with pulvini. When a pulvinus is present, thenyctitropic movements are continued for a very much longer period than whensuch do not exist. This has been amply proved in the case of cotyledons, and Pfeffer has given observations to the same effect with respect[page 379[97]]to leaves. We have seen that a leaf of Mimosa pudica continued to move inthe ordinary manner, though somewhat more simply, until it withered anddied. It may be added that some leaflets of Trifolium pratense were pinnedopen during 10 days, and on the first evening after being released theyrose up and slept in the usual manner. Besides the long continuance of themovements when effected by the aid of a pulvinus (and this appears to bethe final cause of its development), a twisting movement at night, asPfeffer has remarked, is almost confined to leaves thus provided. 

It is a very general rule that the first true leaf, though it may differsomewhat in shape from the leaves on the mature plant, yet sleeps likethem; and this occurs quite independently of the fact whether or not thecotyledons themselves sleep, or whether they sleep in the same manner. Butwith Phaseolus Roxburghii the first unifoliate leaves rise at night almostsufficiently to be said to sleep, whilst the leaflets of the secondarytrifoliate leaves sink vertically at night. On young plants of Sidarhombaefolia, only a few inches in height, the leaves did not sleep, thoughon rather older plants they rose up vertically at night. On the other hand, the leaves on very young plants of Cytisus fragrans slept in a conspicuousmanner, whilst on old and vigorous bushes kept in the greenhouse, theleaves did not exhibit any plain nyctitropic movement. In the genus Lotusthe basal stipule-like leaflets rise up vertically at night, and areprovided with pulvini. 

As already remarked, when leaves or leaflets change their position greatlyat night and by complicated movements, it can hardly be doubted that thesemust be in some manner beneficial to the plant. If so, we[page 398]must extend the same conclusion to a large number of sleeping plants; forthe most complicated and the simplest nyctitropic movements are connectedtogether by the finest gradations. But owing to the causes specified in thebeginning of this chapter, it is impossible in some few cases to determinewhether or not certain movements should be called nyctitropic. Generally, the position which the leaves occupy at night indicates with sufficientclearness, that the benefit thus derived, is the protection of their uppersurfaces from radiation into the open sky, and in many cases the mutualprotection of all the parts from cold by their being brought into closeapproximation. It should be remembered that it was proved in the lastchapter, that leaves compelled to remain extended horizontally at night, suffered much more from radiation than those which were allowed to assumetheir normal vertical position. 

The fact of the leaves of several plants not sleeping unless they have beenwell illuminated during the day, made us for a time doubt whether theprotection of their upper surfaces from radiation was in all cases thefinal cause of their well-pronounced nyctitropic movements. But we have noreason to suppose that the illumination from the open sky, during even themost clouded day, is insufficient for this purpose; and we should bear inmind that leaves which are shaded from being seated low down on the plant, and which sometimes do not sleep, are likewise protected at night from fullradiation. Nevertheless, we do not wish to deny that there may exist casesin which leaves change their position considerably at night, without theirderiving any benefit from such movements. 

Although with sleeping plants the blades almost[page 399]always assume at night a vertical, or nearly vertical position, it is apoint of complete indifference whether the apex, or the base, or one of thelateral edges, is directed to the zenith. It is a rule of wide generality, that whenever there is any difference in the degree of exposure toradiation between the upper and the lower surfaces of leaves and leaflets, it is the upper which is the least exposed, as may be seen in Lotus, Cytisus, Trifolium, and other genera. In several species of Lupinus theleaflets do not, and apparently from their structure cannot, placethemselves vertically at night, and consequently their upper surfaces, though highly inclined, are more exposed than the lower; and here we havean exception to our rule. But in other species of this genus the leafletssucceed in placing themselves vertically; this, however, is effected by avery unusual movement, namely, by the leaflets on the opposite sides of thesame leaf moving in opposite directions. 

It is again a very common rule that when leaflets come into close contactwith one another, they do so by their upper surfaces, which are thus bestprotected. In some cases this may be the direct result of their risingvertically; but it is obviously for the protection of the upper surfacesthat the leaflets of Cassia rotate in so wonderful a manner whilst sinkingdownwards; and that the terminal leaflet of Melilotus rotates and moves toone side until it meets the lateral leaflet on the same side. When oppositeleaves or leaflets sink vertically down without any twisting, their lowersurfaces approach each other and sometimes come into contact; but this isthe direct and inevitable result of their position. With many species ofOxalis the lower surfaces of the adjoining leaflets are pressed together, and are thus better protected[page 400]than the upper surfaces; but this depends merely on each leaflet becomingfolded at night so as to be able to sink vertically downwards. The torsionor rotation of leaves and leaflets, which occurs in so many cases, apparently always serves to bring their upper surfaces into closeapproximation with one another, or with other parts of the plant, for theirmutual protection. We see this best in such cases as those of Arachis, Mimosa albida, and Marsilea, in which all the leaflets form together atnight a single vertical packet. If with Mimosa pudica the opposite leafletshad merely moved upwards, their upper surfaces would have come into contactand been well protected; but as it is, they all successively move towardsthe apex of the leaf; and thus not only their upper surfaces are protected, but the successive pairs become imbricated and mutually protect one anotheras well as the petioles. This imbrication of the leaflets of sleepingplants is a common phenomenon. 

The nyctitropic movement of the blade is generally effected by thecurvature of the uppermost part of the petiole, which has often beenmodified into a pulvinus; or the whole petiole, when short, may be thusmodified. But the blade itself sometimes curves or moves, of which factBauhinia offers a striking instance, as the two halves rise up and comeinto close contact at night. Or the blade and the upper part of the petiolemay both move. Moreover, the petiole as a whole commonly either rises orsinks at night. This movement is sometimes large: thus the petioles ofCassia pubescens stand only a little above the horizon during the day, andat night rise up almost, or quite, perpendicularly. The petioles of theyounger leaves of Desmodium gyrans also rise up vertically at night. On theother hand, with Amphi-[page 401]carpaea, the petioles of some leaves sank down as much as 57o at night;with Arachis they sank 39o, and then stood at right angles to the stem. Generally, when the rising or sinking of several petioles on the same plantwas measured, the amount differed greatly. This is largely determined bythe age of the leaf: for instance, the petiole of a moderately old leaf ofDesmodium gyrans rose only 46o, whilst the young ones rose up vertically;that of a young leaf of Cassia floribunda rose 41o, whilst that of an olderleaf rose only 12o. It is a more singular fact that the age of the plantsometimes influences greatly the amount of movement; thus with some youngseedlings of a Bauhinia the petioles rose at night 30o and 34o, whereasthose on these same plants, when grown to a height of 2 or 3 feet, hardlymoved at all. The position of the leaves on the plant as determined by thelight, seems also to influence the amount of movement of the petiole; forno other cause was apparent why the petioles of some leaves of Melilotusofficinalis rose as much as 59o, and others only 7o and 9o at night. 

In the case of many plants, the petioles move at night in one direction andthe leaflets in a directly opposite one. Thus, in three genera ofPhaseoleae the leaflets moved vertically downwards at night, and thepetioles rose in two of them, whilst in the third they sank. Species in thesame genus often differ widely in the movements of their petioles. Even onthe same plant of Lupinus pubescens some of the petioles rose 30o, othersonly 6o, and others sank 4o at night. The leaflets of Cassia Barclayanamoved so little at night that they could not be said to sleep, yet thepetioles of some young leaves rose as much as 34o. These several factsapparently indicate that the movements[page 402]of the petioles are not performed for any special purpose; though aconclusion of this kind is generally rash. When the leaflets sinkvertically down at night and the petioles rise, as often occurs, it iscertain that the upward movement of the latter does not aid the leaflets inplacing themselves in their proper position at night, for they have to movethrough a greater angular space than would otherwise have been necessary. 

Notwithstanding what has just been said, it may be strongly suspected thatin some cases the rising of the petioles, when considerable, doesbeneficially serve the plant by greatly reducing the surface exposed toradiation at night. If the reader will compare the two drawings (Fig. 155, p. 371) of Cassia pubescens, copied from photographs, he will see that thediameter of the plant at night is about one-third of what it is by day, andtherefore the surface exposed to radiation is nearly nine times less. Asimilar conclusion may be deduced from the drawings (Fig. 149, p. 358) of abranch awake and asleep of Desmodium gyrans. So it was in a very strikingmanner with young plants of Bauhinia, and with Oxalis Ortegesii. 

We are led to an analogous conclusion with respect to the movements of thesecondary petioles of certain pinnate leaves. The pinnae of Mimosa pudicaconverge at night; and thus the imbricated and closed leaflets on eachseparate pinna are all brought close together into a single bundle, andmutually protect one another, with a somewhat smaller surface exposed toradiation. With Albizzia lophantha the pinnae close together in the samemanner. Although the pinnae of Acacia Farnesiana do not converge much, theysink downwards. Those of Neptunia oleracea likewise[page 403]move downwards, as well as backwards, towards the base of the leaf, whilstthe main petiole rises. With Schrankia, again, the pinnae are depressed atnight. Now in these three latter cases, though the pinnae do not mutuallyprotect one another at night, yet after having sunk down they expose, asdoes a dependent sleeping leaf, much less surface to the zenith and toradiation than if they had remained horizontal. 

Any one who had never observed continuously a sleeping plant, wouldnaturally suppose that the leaves moved only in the evening when going tosleep, and in the morning when awaking; but he would be quite mistaken, forwe have found no exception to the rule that leaves which sleep continue tomove during the whole twenty-four hours; they move, however, more quicklywhen going to sleep and when awaking than at other times. That they are notstationary during the day is shown by all the diagrams given, and by themany more which were traced. It is troublesome to observe the movements ofleaves in the middle of the night, but this was done in a few cases; andtracings were made during the early part of the night of the movements inthe case of Oxalis, Amphicarpaea, two species of Erythrina, a Cassia, Passiflora, Euphorbia and Marsilea; and the leaves after they had gone tosleep, were found to be in constant movement. When, however, oppositeleaflets come into close contact with one another or with the stem atnight, they are, as we believe, mechanically prevented from moving, butthis point was not sufficiently investigated. 

When the movements of sleeping leaves are traced during twenty-four hours, the ascending and descending lines do not coincide, except occasionally andby accident for a short space; so that with many plants a[page 404]single large ellipse is described during each twenty-four hours. Suchellipses are generally narrow and vertically directed, for the amount oflateral movement is small. That there is some lateral movement is shown bythe ascending and descending lines not coinciding, and occasionally, aswith Desmodium gyrans and Thalia dealbata, it was strongly marked. In thecase of Melilotus the ellipses described by the terminal leaflet during theday are laterally extended, instead of vertically, as is usual; and thisfact evidently stands in relation with the terminal leaflet movinglaterally when it goes to sleep. With the majority of sleeping plants theleaves oscillate more than once up and down in the twenty-four hours; sothat frequently two ellipses, one of moderate size, and one of very largesize which includes the nocturnal movement, are described within thetwenty-four hours. For instance, a leaf which stands vertically up duringthe night will sink in the morning, then rise considerably, again sink inthe afternoon, and in the evening reascend and assume its verticalnocturnal position. It will thus describe, in the course of the twenty-fourhours, two ellipses of unequal sizes. Other plants describe within the sametime, three, four, or five ellipses. Occasionally the longer axes of theseveral ellipses extend in different directions, of which Acacia Farnesianaoffered a good instance. The following cases will give an idea of the rateof movement: Oxalis acetosella completed two ellipses at the rate of 1 h. 25 m. For each; Marsilea quadrifoliata, at the rate of 2 h. ; Trifoliumsubterraneum, one in 3 h. 30 m. ; and Arachis hypogaea, in 4 h. 50 m. Butthe number of ellipses described within a given time depends largely on thestate of the plant and on the conditions to which it is exposed. It oftenhappens that a single ellipse may be described during one[page 405]day, and two on the next. Erythrina corallodendron made four ellipses onthe first day of observation and only a single one on the third, apparentlyowing to having been kept not sufficiently illuminated and perhaps not warmenough. But there seems likewise to be an innate tendency in differentspecies of the same genus to make a different number of ellipses in thetwenty-four hours: the leaflets of Trifolium repens made only one; those ofT. Resupinatum two, and those of T. Subterraneum three in this time. Again, the leaflets of Oxalis Plumierii made a single ellipse; those of O. Bupleurifolia, two; those of O. Valdiviana, two or three; and those of O. Acetosella, at least five in the twenty-four hours. 

The line followed by the apex of a leaf or leaflet, whilst describing oneor more ellipses during the day, is often zigzag, either throughout itswhole course or only during the morning or evening: Robinia offered aninstance of zigzagging confined to the morning, and a similar movement inthe evening is shown in the diagram (Fig. 126) given under Sida. The amountof the zigzag movement depends largely on the plant being placed underhighly favourable conditions. But even under such favourable conditions, ifthe dots which mark the position of the apex are made at considerableintervals of time, and the dots are then joined, the course pursued willstill appear comparatively simple, although the number of the ellipses willbe increased; but if dots are made every two or three minutes and these arejoined, the result often is that all the lines are strongly zigzag, manysmall loops, triangles, and other figures being also formed. This fact isshown in two parts of the diagram (Fig. 150) of the movements of Desmodiumgyrans. Strephium floribundum, observed under a high temperature, [page 406]made several little triangles at the rate of 43 m. For each. Mimosa pudica, similarly observed, described three little ellipses in 67 m. ; and the apexof a leaflet crossed 1/500 of an inch in a second, or 0. 12 inch in aminute. The leaflets of Averrhoa made a countless number of littleoscillations when the temperature was high and the sun shining. The zigzagmovement may in all cases be considered as an attempt to form small loops, which are drawn out by a prevailing movement in some one direction. Therapid gyrations of the little lateral leaflets of Desmodium belong to thesame class of movements, somewhat exaggerated in rapidity and amplitude. The jerking movements, with a small advance and still smaller retreat, apparently not exactly in the same line, of the hypocotyl of the cabbageand of the leaves of Dionaea, as seen under the microscope, all probablycome under this same head. We may suspect that we here see the energy whichis freed during the incessant chemical changes in progress in the tissues, converted into motion. Finally, it should be noted that leaflets andprobably some leaves, whilst describing their ellipses, often rotateslightly on their axes; so that the plane of the leaf is directed first toone and then to another side. This was plainly seen to be the case with thelarge terminal leaflets of Desmodium, Erythrina and Amphicarpaea, and isprobably common to all leaflets provided with a pulvinus. 

With respect to the periodicity of the movements of sleeping leaves, Pfeffer* has so clearly shown that this depends on the daily alternationsof light and darkness, that nothing farther need be said on this

* 'Die Periodischen Bewegungen der Blattorgane, ' 1875, p. 30, et passim. [page 407]

head. But we may recall the behaviour of Mimosa in the North, where the sundoes not set, and the complete inversion of the daily movements byartificial light and darkness. It has also been shown by us, that althoughleaves subjected to darkness for a moderately long time continue tocircumnutate, yet the periodicity of their movements is soon greatlydisturbed, or quite annulled. The presence of light or its absence cannotbe supposed to be the direct cause of the movements, for these arewonderfully diversified even with the leaflets of the same leaf, althoughall have of course been similarly exposed. The movements depend on innatecauses, and are of an adaptive nature. The alternations of light anddarkness merely give notice to the leaves that the period has arrived forthem to move in a certain manner. We may infer from the fact of severalplants (Tropaeolum, Lupinus, etc. ) not sleeping unless they have been wellilluminated during the day, that it is not the actual decrease of light inthe evening, but the contrast between the amount at this hour and duringthe early part of the day, which excites the leaves to modify theirordinary mode of circumnutation. 

As the leaves of most plants assume their proper diurnal position in themorning, although light be excluded, and as the leaves of some plantscontinue to move in the normal manner in darkness during at least a wholeday, we may conclude that the periodicity of their movements is to acertain extent inherited. * The strength of such inheritance differs

* Pfeffer denies such inheritance; he attributes ('Die Period. Bewegungen, 'pp. 30-56) the periodicity when prolonged for a day or two in darkness, to"Nachwirkung, " or the after-effects of light and darkness. But we areunable to follow his train of reasoning. There does not seem to be any morereason for[[page 408]]attributing such movements to this cause than, for instance, the inheritedhabit of winter and summer wheat to grow best at different seasons; forthis habit is lost after a few years, like the movements of leaves indarkness after a few days. No doubt some effect must be produced on theseeds by the long-continued cultivation of the parent-plants underdifferent climates, but no one probably would call this the "Nachwirkung"of the climates. [page 408]much in different species, and seems never to be rigid; for plants havebeen introduced from all parts of the world into our gardens andgreenhouses; and if their movements had been at all strictly fixed inrelation to the alternations of day and night, they would have slept inthis country at very different hours, which is not the case. Moreover, ithas been observed that sleeping plants in their native homes change theirtimes of sleep with the changing seasons. *

We may now turn to the systematic list. This contains the names of all thesleeping plants known to us, though the list undoubtedly is very imperfect. It may be premised that, as a general rule, all the species in the samegenus sleep in nearly the same manner. But there are some exceptions; inseveral large genera including many sleeping species (for instance, Oxalis), some do not sleep. One species of Melilotus sleeps like aTrifolium, and therefore very differently from its congeners; so does onespecies of Cassia. In the genus Sida, the leaves either rise or fall atnight; and with Lupinus they sleep in three different methods. Returning tothe list, the first point which strikes us, is that there are many moregenera amongst the Leguminosae (and in almost every one of the Leguminoustribes) than in all the other families put together; and we are tempted toconnect this fact with the great

* Pfeffer, ibid. , p. 46. [page 409]

mobility of the stems and leaves in this family, as shown by the largenumber of climbing species which it contains. Next to the Leguminosae comethe Malvaceae, together with some closely allied families. But by far themost important point in the list, is that we meet with sleeping plants in28 families, in all the great divisions of the Phanerogamic series, and inone Cryptogam. Now, although it is probable that with the Leguminosae thetendency to sleep may have been inherited from one or a few progenitors, and possibly so in the cohorts of the Malvales and Chenopodiales, yet it ismanifest that the tendency must have been acquired by the several genera inthe other families, quite independently of one another. Hence the questionnaturally arises, how has this been possible? and the answer, we cannotdoubt is that leaves owe their nyctitropic movements to their habit ofcircumnutating, --a habit common to all plants, and everywhere ready for anybeneficial development or modification. 

It has been shown in the previous chapters that the leaves and cotyledonsof all plants are continually moving up and down, generally to a slight butsometimes to a considerable extent, and that they describe either one orseveral ellipses in the course of twenty-four hours; they are also so faraffected by the alternations of day and night that they generally, or atleast often, move periodically to a small extent; and here we have a basisfor the development of the greater nyctitropic movements. That themovements of leaves and cotyledons which do not sleep come within the classof circumnutating movements cannot be doubted, for they are closely similarto those of hypocotyls, epicotyls, the stems of mature plants, and ofvarious other organs. Now, if we take the simplest[page 410]case of a sleeping leaf, we see that it makes a single ellipse in thetwenty-four hours, which resembles one described by a non-sleeping leaf inevery respect, except that it is much larger. In both cases the coursepursued is often zigzag. As all non-sleeping leaves are incessantlycircumnutating, we must conclude that a part at least of the upward anddownward movement of one that sleeps, is due to ordinary circumnutation;and it seems altogether gratuitous to rank the remainder of the movementunder a wholly different head. With a multitude of climbing plants theellipses which they describe have been greatly increased for anotherpurpose, namely, catching hold of a support. With these climbing plants, the various circumnutating organs have been so far modified in relation tolight that, differently from all ordinary plants, they do not bend towardsit. With sleeping plants the rate and amplitude of the movements of theleaves have been so far modified in relation to light, that they move in acertain direction with the waning light of the evening and with theincreasing light of the morning more rapidly, and to a greater extent, thanat other hours. 

But the leaves and cotyledons of many non-sleeping plants move in a muchmore complex manner than in the cases just alluded to, for they describetwo, three, or more ellipses in the course of a day. Now, if a plant ofthis kind were converted into one that slept, one side of one of theseveral ellipses which each leaf daily describes, would have to be greatlyincreased in length in the evening, until the leaf stood vertically, whenit would go on circumnutating about the same spot. On the followingmorning, the side of another ellipse would have to be similarly increasedin length so as to bring the leaf back again into its diurnal position, when it would again circumnutate[page 411]until the evening. If the reader will look, for instance, at the diagram(Fig. 142, p. 351), representing the nyctitropic movements of the terminalleaflet of Trifolium subterraneum, remembering that the curved broken linesat the top ought to be prolonged much higher up, he will see that the greatrise in the evening and the great fall in the morning together form a largeellipse like one of those described during the daytime, differing only insize. Or, he may look at the diagram (Fig. 103, p. 236) of the 3 ½ ellipsesdescribed in the course of 6 h. 35 m. By a leaf of Lupinus speciosus, whichis one of the species in this genus that does not sleep; and he will seethat by merely prolonging upwards the line which was already rising late inthe evening, and bringing it down again next morning, the diagram wouldrepresent the movements of a sleeping plant. 

With those sleeping plants which describe several ellipses in the daytime, and which travel in a strongly zigzag line, often making in their courseminute loops, triangles, etc. , if as soon as one of the ellipses begins inthe evening to be greatly increased in size, dots are made every 2 or 3minutes and these are joined, the line then described is almost strictlyrectilinear, in strong contrast with the lines made during the daytime. This was observed with Desmodium gyrans and Mimosa pudica. With this latterplant, moreover, the pinnae converge in the evening by a steady movement, whereas during the day they are continually converging and diverging to aslight extent. In all such cases it was scarcely possible to observe thedifference in the movement during the day and evening, without beingconvinced that in the evening the plant saves the expenditure of force bynot moving laterally, and that its whole energy is now expended[page 412]in gaining quickly its proper nocturnal position by a direct course. Inseveral other cases, for instance, when a leaf after describing during theday one or more fairly regular ellipses, zigzags much in the evening, itappears as if energy was being expended, so that the great evening rise orfall might coincide with the period of the day proper for this movement. 

The most complex of all the movements performed by sleeping plants, is thatwhen leaves or leaflets, after describing in the daytime several verticallydirected ellipses, rotate greatly on their axes in the evening, by whichtwisting movement they occupy a wholly different position at night to whatthey do during the day. For instance, the terminal leaflets of Cassia notonly move vertically downwards in the evening, but twist round, so thattheir lower surfaces face outwards. Such movements are wholly, or almostwholly, confined to leaflets provided with a pulvinus. But this torsion isnot a new kind of movement introduced solely for the purpose of sleep; forit has been shown that some leaflets whilst describing their ordinaryellipses during the daytime rotate slightly, causing their blades to facefirst to one side and then to another. Although we can see how the slightperiodical movements of leaves in a vertical plane could be easilyconverted into the greater yet simple nyctitropic movements, we do not atpresent know by what graduated steps the more complex movements, effectedby the torsion of the pulvini, have been acquired. A probable explanationcould be given in each case only after a close investigation of themovements in all the allied forms. 

From the facts and considerations now advanced we may conclude thatnyctitropism, or the sleep of leaves[page 413]and cotyledons, is merely a modification of their ordinary circumnutatingmovement, regulated in its period and amplitude by the alternations oflight and darkness. The object gained is the protection of the uppersurfaces of the leaves from radiation at night, often combined with themutual protection of the several parts by their close approximation. Insuch cases as those of the leaflets of Cassia--of the terminal leaflets ofMelilotus--of all the leaflets of Arachis, Marsilea, etc. --we have ordinarycircumnutation modified to the extreme extent known to us in any of theseveral great classes of modified circumnutation. On this view of theorigin of nyctitropism we can understand how it is that a few plants, widely distributed throughout the Vascular series, have been able toacquire the habit of placing the blades of their leaves vertically atnight, that is, of sleeping, --a fact otherwise inexplicable. 

The leaves of some plants move during the day in a manner, which hasimproperly been called diurnal sleep; for when the sun shines brightly onthem, they direct their edges towards it. To such cases we shall recur inthe following chapter on Heliotropism. It has been shown that the leafletsof one form of Porlieria hygrometrica keep closed during the day, as longas the plant is scantily supplied with water, in the same manner as whenasleep; and this apparently serves to check evaporation. There is only oneother analogous case known to us, namely, that of certain Gramineae, whichfold inwards the sides of their narrow leaves, when these are exposed tothe sun and to a dry atmosphere, as described by Duval-Jouve. * We have alsoobserved the same phenomenon in Elymus arenareus. 

* 'Annal. Des Sc. Nat. (Bot. ), ' 1875, tom. I. Pp. 326-329. [page 414]

There is another movement, which since the time of Linnaeus has generallybeen called sleep, namely, that of the petals of the many flowers whichclose at night. These movements have been ably investigated by Pfeffer, whohas shown (as was first observed by Hofmeister) that they are caused orregulated more by temperature than by the alternations of light anddarkness. Although they cannot fail to protect the organs of reproductionfrom radiation at night, this does not seem to be their chief function, butrather the protection of the organs from cold winds, and especially fromrain, during the day. The latter seems probable, as Kerner* has shown thata widely different kind of movement, namely, the bending down of the upperpart of the peduncle, serves in many cases the same end. The closure of theflowers will also exclude nocturnal insects which may be ill-adapted fortheir fertilisation, and the well-adapted kinds at periods when thetemperature is not favourable for fertilisation. Whether these movements ofthe petals consist, as is probable, of modified circumnutation we do notknow. 

Embryology of Leaves. --A few facts have been incidentally given in thischapter on what may be called the embryology of leaves. With most plantsthe first leaf which is developed after the cotyledons, resembles closelythe leaves produced by the mature plant, but this is not always the case. The first leaves produced by some species of Drosera, for instance by D. Capensis, differ widely in shape from those borne by the mature plant, andresemble closely the leaves of D. Rotundifolia, as was shown to us by Prof. Williamson of Manchester. The first true leaf of

* 'Die Schutzmittel des Pollens, ' 1873, pp. 30-39. [page 415]

the gorse, or Ulex, is not narrow and spinose like the older leaves. On theother hand, with many Leguminous plants, for instance, Cassia, Acacialophantha, etc. , the first leaf has essentially the same character as theolder leaves, excepting that it bears fewer leaflets. In Trifolium thefirst leaf generally bears only a single leaflet instead of three, and thisdiffers somewhat in shape from the corresponding leaflet on the olderleaves. Now, with Trifolium Pannonicum the first true leaf on someseedlings was unifoliate, and on others completely trifoliate; and betweenthese two extreme states there were all sorts of gradations, some seedlingsbearing a single leaflet more or less deeply notched on one or both sides, and some bearing a single additional and perfect lateral leaflet. Here, then, we have the rare opportunity of seeing a structure proper to a moreadvanced age, in the act of gradually encroaching on and replacing anearlier or embryological condition. 

The genus Melilotus is closely allied to Trifolium, and the first leafbears only a single leaflet, which at night rotates on its axis so as topresent one lateral edge to the zenith. Hence it sleeps like the terminalleaflet of a mature plant, as was observed in 15 species, and wholly unlikethe corresponding leaflet of Trifolium, which simply bends upwards. It istherefore a curious fact that in one of these 15 species, viz. , M. Taurica(and in a lesser degree in two others), leaves arising from young shoots, produced on plants which had been cut down and kept in pots during thewinter in the green-house, slept like the leaves of a Trifolium, whilst theleaves on the fully-grown branches on these same plants afterwards sleptnormally like those of a Melilotus. If young shoots rising from the groundmay be considered as new individuals, partaking to a certain extent of thenature of seedlings, then the peculiar manner in which their leaves sleptmay be considered[page 416]as an embryological habit, probably the result of Melilotus being descendedfrom some form which slept like a Trifolium. This view is partiallysupported by the leaves on old and young branches of another species, M. Messanensis (not included in the above 15 species), always sleeping likethose of a Trifolium. 

The first true leaf of Mimosa albida consists of a simple petiole, oftenbearing three pairs of leaflets, all of which are of nearly equal size andof the same shape: the second leaf differs widely from the first, andresembles that on a mature plant (see Fig. 159, p. 379), for it consists oftwo pinnae, each of which bears two pairs of leaflets, of which the innerbasal one is very small. But at the base of each pinna there is a pair ofminute points, evidently rudiments of leaflets, for they are of unequalsizes, like the two succeeding leaflets. These rudiments are in one senseembryological, for they exist only during the youth of the leaf, fallingoff and disappearing as soon as it is fully grown. 

With Desmodium gyrans the two lateral leaflets are very much smaller thanthe corresponding leaflets in most of the species in this large genus; theyvary also in position and size; one or both are sometimes absent; and theydo not sleep like the fully-developed leaflets. They may therefore beconsidered as almost rudimentary; and in accordance with the generalprinciples of embryology, they ought to be more constantly and fullydeveloped on very young than on old plants. But this is not the case, forthey were quite absent on some young seedlings, and did not appear untilfrom 10 to 20 leaves had been formed. This fact leads to the suspicion thatD. Gyrans is descended through a unifoliate form (of which some exist) froma trifoliate species; and that the little lateral leaflets reappear throughreversion. However this may be, [page 417]the interesting fact of the pulvini or organs of movement of these littleleaflets, not having been reduced nearly so much as their blades--takingthe large terminal leaflet as the standard of comparison--gives us probablythe proximate cause of their extraordinary power of gyration. [page 418]

CHAPTER VIII. 

MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT. 

Distinction between heliotropism and the effects of light on theperiodicity of the movements of leaves--Heliotropic movements of Beta, Solanum, Zea, and Avena--Heliotropic movements towards an obscure light inApios, Brassica, Phalaris, Tropaeolum, and Cassia--Apheliotropic movementsof tendrils of Bignonia--Of flower-peduncles of Cyclamen--Burying of thepods--Heliotropism and apheliotropism modified forms of circumnutation--Steps by which one movement is converted into the other--Transversal-heliotropismus or diaheliotropism influenced by epinasty, theweight of the part and apogeotropism--Apogeotropism overcome during themiddle of the day by diaheliotropism--Effects of the weight of the bladesof cotyledons--So called diurnal sleep--Chlorophyll injured by intenselight--Movements to avoid intense light

SACHS first clearly pointed out the important difference between the actionof light in modifying the periodic movements of leaves, and in causing themto bend towards its source. * The latter, or heliotropic movements aredetermined by the direction of the light, whilst periodic movements areaffected by changes in its intensity and not by its direction. Theperiodicity of the circumnutating movement often continues for some time indarkness, as we have seen in the last chapter; whilst heliotropic bendingceases very quickly when the light fails. Nevertheless, plants which haveceased through long-continued darkness to move periodically, if re-exposedto the light are still, according to Sachs, heliotropic. 

Apheliotropism, or, as usually designated, negative

* 'Physiologie Veg. ' (French Translation), 1868, pp. 42, 517, etc. [page 419]

heliotropism, implies that a plant, when unequally illuminated on the twosides, bends from the light, instead of, as in the last sub-class of cases, towards it; but apheliotropism is comparatively rare, at least in awell-marked degree. There is a third and large sub-class of cases, namely, those of "transversal-Heliotropismus" of Frank, which we will here calldiaheliotropism. Parts of plants, under this influence, place themselvesmore or less transversely to the direction whence the light proceeds, andare thus fully illuminated. There is a fourth sub-class, as far as thefinal cause of the movement is concerned; for the leaves of some plantswhen exposed to an intense and injurious amount of light direct themselves, by rising or sinking or twisting, so as to be less intensely illuminated. Such movements have sometimes been called diurnal sleep. If thoughtadvisable, they might be called paraheliotropic, and this term wouldcorrespond with our other terms. 

It will be shown in the present chapter that all the movements included inthese four sub-classes, consist of modified circumnutation. We do notpretend to say that if a part of a plant, whilst still growing, did notcircumnutate--though such a supposition is most improbable--it could notbend towards the light; but, as a matter of fact, heliotropism seems alwaysto consist of modified circumnutation. Any kind of movement in relation tolight will obviously be much facilitated by each part circumnutating orbending successively in all directions, so that an already existingmovement has only to be increased in some one direction, and to be lessenedor stopped in the other directions, in order that it should becomeheliotropic, apheliotropic, etc. , as the case may be. In the next chaptersome observations on the sensitiveness of plants to light, their[page 420]rate of bending towards it, and the accuracy with which they point towardsits source, etc. , will be given. Afterwards it will be shown--and thisseems to us a point of much interest--that sensitiveness to light issometimes confined to a small part of the plant; and that this part whenstimulated by light, transmits an influence to distant parts, exciting themto bend. 

Heliotropism. --When a plant which is strongly heliotropic (and speciesdiffer much in this respect) is exposed to a bright lateral light, it bendsquickly towards it, and the course pursued by the stem is quite or nearlystraight. But if the light is much dimmed, or occasionally interrupted, oradmitted in only a slightly oblique direction, the course pursued is moreor less zigzag; and as we have seen and shall again see, such zigzagmovement results from the elongation or drawing out of the ellipses, loops, etc. , which the plant would have described, if it had been illuminated fromabove. On several occasions we were much struck with this fact, whilstobserving the circumnutation of highly sensitive seedlings, which wereunintentionally illuminated rather obliquely, or only at successiveintervals of time. 

Fig. 168. Beta vulgaris: circumnutation of hypocotyl, deflected by thelight being slightly lateral, traced on a horizontal glass from 8. 30 A. M. To 5. 30 P. M. Direction of the lighted taper by which it was illuminatedshown by a line joining the first and penultimate dots. Figure reduced toone-third of the original scale. 

[For instance two young seedlings of Beta vulgaris were placed in themiddle of a room with north-east windows, and were kept covered up, exceptduring each observation which lasted for only a minute or two; but theresult was that their hypocotyls bowed themselves to the side, whence somelight occasionally entered, in lines which were[page 421]only slightly zigzag. Although not a single ellipse was even approximatelyformed, we inferred from the zigzag lines - and, as it proved, correctly--that their hypocotyls were circumnutating, for on the following day thesesame seedlings were placed in a completely darkened room, and were observedeach time by the aid of a small wax taper held almost directly above them, and their movements were traced on a horizontal glass above; and now theirhypocotyls clearly circumnutated (Fig. 168, and Fig. 39, formerly given, p. 52); yet they moved a short distance towards the side where the taper washeld up. If we look at these diagrams, and suppose that the taper had beenheld more on one side, and that the hypocotyls, still circumnutating, hadbent themselves within the same time much more towards the light, longzigzag lines would obviously have been the result. 

Fig. 169. Avena sativa: heliotropic movement and circumnutation ofsheath-like cotyledon (1 ½ inch in height) traced on horizontal glass from8 A. M. To 10. 25 P. M. Oct. 16th. 

Again, two seedlings of Solanum lycopersicum were illuminated from above, but accidentally a little more light entered on one than on any other side, and their hypocotyls became slightly bowed towards the brighter side; theymoved in a zigzag line and described in their course two little triangles, as seen in Fig. 37 (p. 50), and in another tracing not given. Thesheath-like cotyledons of Zea mays behaved, under nearly similarcircumstances, in a nearly similar manner as described in our first chapter(p. 64), for they bowed themselves during the whole day towards one side, making, however, in their course some conspicuous flexures. Before we knewhow greatly ordinary circumnutation was modified by a lateral light, someseedling oats, with rather old and therefore not highly sensitivecotyledons, were placed in front of a north-east window, towards which theybent all day in a strongly zigzag course. On the following day theycontinued to bend in the same direction (Fig. 169), but zigzagged muchless. The sky, however, became between 12. 40 and 2. 35 P. M. [page 422]overcast with extraordinarily dark thunder-clouds, and it was interestingto note how plainly the cotyledons circumnutated during this interval. 

The foregoing observations are of some value, from having been made when wewere not attending to heliotropism; and they led us to experiment onseveral kinds of seedlings, by exposing them to a dim lateral light, so asto observe the gradations between ordinary circumnutation and heliotropism. Seedlings in pots were placed in front of, and about a yard from, anorth-east window; on each side and over the pots black boards were placed;in the rear the pots were open to the diffused light of the room, which hada second north-east and a north-west window. By hanging up one or moreblinds before the window where the seedlings stood, it was easy to dim thelight, so that very little more entered on this side than on the oppositeone, which received the diffused light of the room. Late in the evening theblinds were successively removed, and as the plants had been subjectedduring the day to a very obscure light, they continued to bend towards thewindow later in the evening than would otherwise have occurred. Most of theseedlings were selected because they were known to be highly sensitive tolight, and some because they were but little sensitive, or had become sofrom having grown old. The movements were traced in the usual manner on ahorizontal glass cover; a fine glass filament with little triangles ofpaper having been cemented in an upright position to the hypocotyls. Whenever the stem or hypocotyl became much bowed towards the light, thelatter part of its course had to be traced on a vertical glass, parallel tothe window, and at right angles to the horizontal glass cover. Fig. 170. Apios graveolens: heliotropic movement of hypocotyl (. 45 of inchin height) towards a moderately bright lateral light, traced on ahorizontal glass from 8. 30 A. M. To 11. 30 A. M. Sept. 18th. Figure reduced toone-third of original scale. 

Apios graveolens. --The hypocotyl bends in a few hours rectan-[page 423]gularly towards a bright lateral light. In order to ascertain how straighta course it would pursue when fairly well illuminated on one side, seedlings were first placed before a south-west window on a cloudy andrainy morning; and the movement of two hypocotyls were traced for 3 h. , during which time they became greatly bowed towards the light. One of thesetracings is given on p. 422 (Fig. 170), and the course may be seen to bealmost straight. But the amount of light on this occasion was superfluous, for two seedlings were placed before a north-east window, protected by anordinary linen and two muslin blinds, yet their hypocotyls moved towardsthis rather dim light in only slightly zigzag lines; but after 4 P. M. , asthe light waned, the lines became distinctly zigzag. One of theseseedlings, moreover, described in the afternoon an ellipse of considerablesize, with its longer axis directed towards the window. 

We now determined that the light should be made dim enough, so we began byexposing several seedlings before a north-east window, protected by onelinen blind, three muslin blinds, and a towel. But so little light enteredthat a pencil cast no perceptible shadow on a white card, and thehypocotyls did not bend at all towards the window. During this time, from8. 15 to 10. 50 A. M. , the hypocotyls zigzagged or circumnutated near the samespot, as may be seen at A, in Fig. 171. The towel, therefore, was removedat 10. 50 A. M. , and replaced by two muslin blinds, and now the light passedthrough one ordinary linen and four muslin blinds. When a pencil was heldupright on a card close to the seedlings, it cast a shadow (pointing fromthe window) which could only just be distinguished. Yet this very slightexcess of light on one side sufficed to cause the hypocotyls of all theseedlings immediately to begin bending in zigzag lines towards the window. The course of one is shown at A (Fig. 171): after moving towards the windowfrom 10. 50 A. M. To 12. 48 P. M. It bent from the window, and then returned ina nearly parallel line; that is, it almost completed between 12. 48 and 2P. M. A narrow ellipse. Late in the evening, as the light waned, thehypocotyl ceased to bend towards the window, and circumnutated on a smallscale round the same spot; during the night it moved considerablybackwards, that is, became more upright, through the action ofapogeotropism. At B, we have a tracing of the movements of another seedlingfrom the hour (10. 50 A. M. ) when the towel was removed; and it is in allessential respects[page 424]similar to the previous one. In these two cases there could be no doubtthat the ordinary circumnutating movement of the hypocotyl was modified andrendered heliotropic. 

Fig. 171. Apios graveolens: heliotropic movement and circumnutation of thehypocotyls of two seedlings towards a dim lateral light, traced on ahorizontal glass during the day. The broken lines show their returnnocturnal courses. Height of hypocotyl of A . 5, and of B . 55 inch. Figurereduced to one-half of original scale. 

Brassica oleracea. --The hypocotyl of the cabbage, when not disturbed by alateral light, circumnutates in a complicated[page 425]manner over nearly the same space, and a figure formerly given is herereproduced (Fig. 172). If the hypocotyl is exposed to a moderately stronglateral light it moves quickly towards this side, travelling in a straight, or nearly straight, line. But when the lateral light is very dim its courseis extremely tortuous, and evidently consists of modified circumnutation. Seedlings were placed before a north-east window, protected by a linen andmuslin blind and by a towel. The sky was cloudy, and whenever the cloudsgrew a little lighter an additional muslin blind was temporarily suspended. The light from the window was

Fig. 172. Brassica oleracea: ordinary circumnutating movement of thehypocotyl of a seedling plant. 

thus so much obscured that, judging by the unassisted eye, the seedlingsappeared to receive more light from the interior of the room than from thewindow; but this was not really the case, as was shown by a very faintshadow cast by a pencil on a card. Nevertheless, this extremely smallexcess of light on one side caused the hypocotyls, which in the morning hadstood upright, to bend at right angles towards the window, so that in theevening (after 4. 23 P. M. ) their course had to be traced on a vertical glassparallel to the window. It should be stated that at 3. 30 P. M. , by whichtime the sky had become darker, the towel was removed and replaced by anadditional muslin blind, which itself was removed at 4 P. M. , the other two[page 426]blinds being left suspended. In Fig. 173 the course pursued, between 8. 9A. M. And 7. 10 P. M. , by one of the hypocotyls thus

Fig. 173. Brassica oleracea: heliotropic movement and circumnutation of ahypocotyl towards a very dim lateral light, traced during 11 hours, on ahorizontal glass in the morning, and on a vertical glass in the evening. Figure reduced to one-third of the original scale. 

exposed is shown. It may be observed that during the first 16 m. Thehypocotyl moved obliquely from the light, and this, [page 427]no doubt, was due to its then circumnutating in this direction. Similarcases were repeatedly observed, and a dim light rarely or never producedany effect until from a quarter to three-quarters of an hour had elapsed. After 5. 15 P. M. , by which time the light had become obscure, the hypocotylbegan to circumnutate about the same spot. The contrast between the twofigures (172 and 173) would have been more striking, if they had beenoriginally drawn on the same scale, and had been equally reduced. But themovements shown in Fig. 172 were at first more magnified, and have beenreduced to only one-half of the original scale; whereas those in Fig. 173were at first less magnified, and have been reduced to a one-third scale. Atracing made at the same time with the last of the movements of a secondhypocotyl, presented a closely analogous appearance; but it did not bendquite so much towards the light, and it circumnutated rather more plainly. 

Fig. 174. Phalaris Canariensis: heliotropic movement and circumnutation ofa rather old cotyledon, towards a dull lateral light, traced on ahorizontal glass from 8. 15 A. M. Sept. 16th to 7. 45 A. M. 17th. Figurereduced to one-third of original scale. 

Phalaris Canariensis. --The sheath-like cotyledons of this monocotyledonousplant were selected for trial, because they are very sensitive to light andcircumnutate well, as formerly shown (see Fig. 49, p. 63). Although we feltno doubt about the result, some seedlings were first placed before asouth-west window on a moderately bright morning, and the movements of onewere traced. As is so common, it moved[page 428]for the first 45 m. In a zigzag line; it then felt the full influence ofthe light, and travelled towards it for the next 2 h. 30 m. In an almoststraight line. The tracing has not been given, as it was almost identicalwith that of Apios under similar circumstances (Fig. 170). By noon it hadbowed itself to its full extent; it then circumnutated about the same spotand described two ellipses; by 5 P. M. It had retreated considerably fromthe light, through the action of apogeotropism. After some preliminarytrials for ascertaining the right degree of obscurity, some seedlings wereplaced (Sept. 16th) before a north-east window, and light was admittedthrough an ordinary linen and three muslin blinds. A pencil held close bythe pot now cast a very faint shadow on a white card, pointing from thewindow. In the evening, at 4. 30 and again at 6 P. M. , some of the blindswere removed. In Fig. 174 we see the course pursued under thesecircumstances by a rather old and not very sensitive cotyledon, 1. 9 inch inheight, which became much bowed, but was never rectangularly bent towardsthe light. From 11 A. M. , when the sky became rather duller, until 6. 30P. M. , the zigzagging was conspicuous, and evidently consisted of drawn-outellipses. After 6. 30 P. M. And during the night, it retreated in a crookedline from the window. Another and younger seedling moved during the sametime much more quickly and to a much greater distance, in an only slightlyzigzag line towards the light; by 11 A. M. It was bent almost rectangularlyin this direction, and now circumnutated about the same place. 

Tropaeolum majus. --Some very young seedlings, bearing only two leaves, andtherefore not as yet arrived at the climbing stage of growth, were firsttried before a north-east window without any blind. The epicotyls bowedthemselves towards the light so rapidly that in little more than 3 h. Theirtips pointed rectangularly towards it. The lines traced were either nearlystraight or slightly zigzag; and in this latter case we see that a trace ofcircumnutation was retained even under the influence of a moderately brightlight. Twice whilst these epicotyls were bending towards the window, dotswere made every 5 or 6 minutes, in order to detect any trace of lateralmovement, but there was hardly any; and the lines formed by their junctionwere nearly straight, or only very slightly zigzag, as in the other partsof the figures. After the epicotyls had bowed themselves to the full extenttowards the light, ellipses of considerable size were described in theusual manner. [page 429]

After having seen how the epicotyls moved towards a moderately brightlight, seedlings were placed at 7. 48 A. M. (Sept. 7th) before a north-eastwindow, covered by a towel, and shortly afterwards by an ordinary linenblind, but the epicotyls still moved towards the window. At 9. 13 A. M. Twoadditional muslin blinds were suspended, so that the seedlings receivedvery little more light from the window than from the interior of the room. The sky varied in brightness, and the seedlings occasionally

Fig. 175. Tropaeolum majus: heliotropic movement and circumnutation of theepicotyl of a young seedling towards a dull lateral light, traced on ahorizontal glass from 7. 48 A. M. To 10. 40 P. M. Figure reduced to one-half ofthe original scale. 

received for a short time less light from the window than from the oppositeside (as ascertained by the shadow cast), and then one of the blinds wastemporarily removed. In the evening the blinds were taken away, one by one. The course pursued by an epicotyl under these circumstances is shown inFig. 175. During the whole day, until 6. 45 P. M. , it plainly bowed itselftowards the light; and the tip moved over a considerable space. After 6. 45P. M. It moved backwards, or from the window, till[page 430]10. 40 P. M. , when the last dot was made. Here, then, we have a distinctheliotropic movement, effected by means of six elongated figures (which ifdots had been made every few minutes would have been more or less elliptic)directed towards the light, with the apex of each successive ellipse nearerto the window than the previous one. Now, if the light had been only alittle brighter, the epicotyl would have bowed itself more to the light, aswe may safely conclude from the previous trials; there would also have beenless lateral movement, and the ellipses or other figures would have beendrawn out into a strongly marked zigzag line, with probably one or twosmall loops still formed. If the light had been much brighter, we shouldhave had a slightly zigzag line, or one quite straight, for there wouldhave been more movement in the direction of the light, and much less fromside to side. 

Fig. 176. Tropaeolum majus: heliotropic movement and circumnutation of anold internode towards a lateral light, traced on a horizontal glass from 8A. M. Nov. 2nd to 10. 20 A. M. Nov. 4th. Broken lines show the nocturnalcourse. 

Sachs states that the older internodes of this Tropaeolum areapheliotropic; we therefore placed a plant, 11 3/4 inches high, in a box, blackened within, but open on one side in front of a north-east windowwithout any blind. A filament was fixed to the third internode from thesummit on one plant, and to the fourth internode of another. Theseinternodes were either not old enough, or the light was not sufficientlybright, to induce apheliotropism, for both plants bent slowly towards, instead of from the window during four days. The course, during two days ofthe first-mentioned internode, is given in Fig. 176; and we see that iteither circumnutated on a small scale, or travelled in a zigzag linetowards the light. We have thought this case of feeble heliotropism in oneof the older internodes of a plant, [page 431]which, whilst young, is so extremely sensitive to light, worth giving. 

Fig. 177. Cassia tora: heliotropic movement and circumnutation of ahypocotyl (1 ½ inch in height) traced on a horizontal glass from 8 A. M. To10. 10 P. M. Oct. 7th. Also its circumnutation in darkness from 7 A. M. Oct. 8th to 7. 45 A. M. Oct. 9th. 

Cassia tora. --The cotyledons of this plant are extremely sensitive tolight, whilst the hypocotyls are much less sensitive than those of mostother seedlings, as we had often observed with surprise. It seemedtherefore worth while to trace their movements. They were exposed to alateral light before a north-east window, which was at first covered merelyby a muslin blind, but as the sky grew brighter about 11 A. M. , anadditional linen blind was suspended. After 4 P. M. One blind and then theother was removed. The seedlings were protected on each side and above, butwere open to the diffused light of the room in the rear. Upright filamentswere fixed to the hypocotyls of two seedlings, which stood vertically inthe morning. The accompanying figure (Fig. 177) shows the course pursued byone of them during two days; but it should be particularly noticed thatduring the second day the seedlings were kept in darkness, and they thencircumnutated round nearly the same small space. On the first day (Oct. 7th) the hypocotyl moved from 8 A. M. To 12. 23 P. M. , toward the light in azigzag line, then turned abruptly to the left and afterwards described asmall ellipse. Another irregular[page 432]ellipse was completed between 3 P. M. And about 5. 30 P. M. , the hypocotylstill bending towards the light. The hypocotyl was straight and upright inthe morning, but by 6 P. M. Its upper half was bowed towards the light, sothat the chord of the arc thus formed stood at an angle of 20o with theperpendicular. After 6 P. M. Its course was reversed through the action ofapogeotropism, and it continued to bend from the window during the night, as shown by the broken line. On the next day it was kept in the dark(excepting when each observation was made by the aid of a taper), and thecourse followed from 7 A. M. On the 8th to 7. 45 A. M. On the 9th is herelikewise shown. The difference between the two parts of the figure (177), namely that described during the daytime on the 7th, when exposed to arather dim lateral light, and that on the 8th in darkness, is striking. Thedifference consists in the lines during the first day having been drawn outin the direction of the light. The movements of the other seedling, tracedunder the same circumstances, were closely similar. 

Apheliotropism. --We succeeded in observing only two cases ofapheliotropism, for these are somewhat rare; and the movements aregenerally so slow that they would have been very troublesome to trace. 

Fig. 178. Bignonia capreolata: apheliotropic movement of a tendril, tracedon a horizontal glass from 6. 45 A. M. July 19th to 10 A. M. 20th. Movementsas originally traced, little magnified, here reduced to two-thirds of theoriginal scale. 

Bignonia capreolata. --No organ of any plant, as far as we have seen, bendsaway so quickly from the light as do the tendrils of this Bignonia. Theyare also remarkable from circumnutating much less regularly than most othertendrils, often remaining stationary; they depend on apheliotropism forcoming into[page 433]contact with the trunks of trees. * The stem of a young plant was tied to astick at the base of a pair of fine tendrils, which projected almostvertically upwards; and it was placed in front of a north-east window, being protected on all other sides from the light. The first dot was madeat 6. 45 A. M. , and by 7. 35 A. M. Both tendrils felt the full influence of thelight, for they moved straight away from it until 9. 20 A. M. , when theycircumnutated for a time, still moving, but only a little, from the light(see Fig. 178 of the left-hand tendril). After 3 P. M. They again movedrapidly away from the light in zigzag lines. By a late hour in the eveningboth had moved so far, that they pointed in a direct line from the light. During the night they returned a little in a nearly opposite direction. Onthe following morning they again moved from the light and converged, sothat by the evening they had become interlocked, still pointing from thelight. The right-hand tendril, whilst converging, zigzagged much more thanthe one figured. Both tracings showed that the apheliotropic movement was amodified form of circumnutation. 

Cyclamen Persicum. --Whilst this plant is in flower the peduncles standupright, but their uppermost part is hooked so that the flower itself hangsdownwards. As soon as the pods begin to swell, the peduncles increase muchin length and slowly curve downwards, but the short, upper, hooked partstraightens itself. Ultimately the pods reach the ground, and if this iscovered with moss or dead leaves, they bury themselves. We have often seensaucer-like depressions formed by the pods in damp sand or sawdust; and onepod (. 3 of inch in diameter) buried itself in sawdust for three-quarters ofits length. ** We shall have occasion hereafter to consider the objectgained by this burying process. The peduncles can change the direction oftheir curvature, for if a pot, with plants having their peduncles alreadybowed downwards, be placed horizontally, they slowly bend at right anglesto their former direction towards the centre of the earth. We therefore atfirst attributed the movement to geotropism; but a pot which had lainhorizontally with the pods

* 'The Movements and Habits of Climbing Plants, ' 1875, p. 97. 

** The peduncles of several other species of Cyclamen twist themselves intoa spire, and according to Erasmus Darwin ('Botanic Garden, ' Canto. , iii. P. 126), the pods forcibly penetrate the earth. See also Grenier and Godron, 'Flore de France, ' tom. Ii. P. 459. [page 434]

all pointing to the ground, was reversed, being still kept horizontal, sothat the pods now pointed directly upwards; it was then placed in a darkcupboard, but the pods still pointed upwards after four days and nights. The pot, in the same position, was next brought back into the light, andafter two days there was some bending downwards of the peduncles, and onthe fourth day two of them pointed to the centre of the earth, as did theothers after an additional day or two. Another plant, in a pot which hadalways stood upright, was left in the dark cupboard for six days; it bore 3peduncles, and only one became within this

Fig. 179. Cyclamen Persicum: downward apheliotropic movement of aflower-peduncle, greatly magnified (about 47 times?), traced on ahorizontal glass from 1 P. M. Feb. 18th to 8 A. M. 21st. 

time at all bowed downwards, and that doubtfully. The weight, therefore, ofthe pods is not the cause of the bending down. This pot was then broughtback into the light, and after three days the peduncles were considerablybowed downwards. We are thus led to infer that the downward curvature isdue to apheliotropism; though more trials ought to have been made. 

In order to observe the nature of this movement, a peduncle bearing a largepod which had reached and rested on the ground, was lifted a little up andsecured to a stick. A filament was fixed across the pod with a markbeneath, and its move-[page 435]ment, greatly magnified, was traced on a horizontal glass during 67 h. Theplant was illuminated during the day from above. A copy of the tracing isgiven on p. 434 (Fig. 179); and there can be no doubt that the descendingmovement is one of modified circumnutation, but on an extremely smallscale. The observation was repeated on another pod, which had partiallyburied itself in sawdust, and which was lifted up a quarter of an inchabove the surface; it described three very small circles in 24 h. Considering the great length and thinness of the peduncles and thelightness of the pods, we may conclude that they would not be able toexcavate saucer-like depressions in sand or sawdust, or bury themselves inmoss, etc. , unless they were aided by their continued rocking orcircumnutating movement. ]

Relation between Circumnutation and Heliotropism. --Any one who will look atthe foregoing diagrams, showing the movements of the stems of variousplants towards a lateral and more or less dimmed light, will be forced toadmit that ordinary circumnutation and heliotropism graduate into oneanother. When a plant is exposed to a dim lateral light and continuesduring the whole day bending towards it, receding late in the evening, themovement unquestionably is one of heliotropism. Now, in the case ofTropaeolum (Fig. 175) the stem or epicotyl obviously circumnutated duringthe whole day, and yet it continued at the same time to moveheliotropically; this latter movement being effected by the apex of eachsuccessive elongated figure or ellipse standing nearer to the light thanthe previous one. In the case of Cassia (Fig. 177) the comparison of themovement of the hypocotyl, when exposed to a dim lateral light and todarkness, is very instructive; as is that between the ordinarycircumnutating movement of a seedling Brassica (Figs. 172, 173), or that ofPhalaris (Figs. 49, 174), and their heliotropic movement towards a windowprotected by blinds. In both these cases, [page 436]and in many others, it was interesting to notice how gradually the stemsbegan to circumnutate as the light waned in the evening. We have thereforemany kinds of gradations from a movement towards the light, which must beconsidered as one of circumnutation very slightly modified and stillconsisting of ellipses or circles, --though a movement more or less stronglyzigzag, with loops or ellipses occasionally formed, --to a nearly straight, or even quite straight, heliotropic course. 

A plant, when exposed to a lateral light, though this may be bright, commonly moves at first in a zigzag line, or even directly from the light;and this no doubt is due to its circumnutating at the time in a directioneither opposite to the source of the light, or more or less transversely toit. As soon, however, as the direction of the circumnutating movementnearly coincides with that of the entering light, the plant bends in astraight course towards the light, if this is bright. The course appears tobe rendered more and more rapid and rectilinear, in accordance with thedegree of brightness of the light--firstly, by the longer axes of theelliptical figures, which the plant continues to describe as long as thelight remains very dim, being directed more or less accurately towards itssource, and by each successive ellipse being described nearer to the light. Secondly, if the light is only somewhat dimmed, by the acceleration andincrease of the movement towards it, and by the retardation or arrestmentof that from the light, some lateral movement being still retained, for thelight will interfere less with a movement at right angles to its direction, than with one in its own direction. *

* In his paper, 'Ueber orthotrope und plagiotrope Pflanzentheile'('Arbeiten des Bot. Inst. In Würzburg, ' Band ii. Heft ii. [[page 437]]1879), Sachs has discussed the manner in which geotropism and heliotropismare affected by differences in the angles at which the organs of plantsstand with respect to the direction of the incident force. [page 437]

The result is that the course is rendered more or less zigzag and unequalin rate. Lastly, when the light is very bright all lateral movement islost; and the whole energy of the plant is expended in rendering thecircumnutating movement rectilinear and rapid in one direction alone, namely, towards the light. 

The common view seems to be that heliotropism is a quite distinct kind ofmovement from circumnutation; and it may be urged that in the foregoingdiagrams we see heliotropism merely combined with, or superimposed on, circumnutation. But if so, it must be assumed that a bright lateral lightcompletely stops circumnutation, for a plant thus exposed moves in astraight line towards it, without describing any ellipses or circles. Ifthe light be somewhat obscured, though amply sufficient to cause the plantto bend towards it, we have more or less plain evidence of still-continuedcircumnutation. It must further be assumed that it is only a lateral lightwhich has this extraordinary power of stopping circumnutation, for we knowthat the several plants above experimented on, and all the others whichwere observed by us whilst growing, continue to circumnutate, howeverbright the light may be, if it comes from above. Nor should it be forgottenthat in the life of each plant, circumnutation precedes heliotropism, forhypocotyls, epicotyls, and petioles circumnutate before they have brokenthrough the ground and have ever felt the influence of light. 

We are therefore fully justified, as it seems to us, in believing thatwhenever light enters laterally, it is the[page 438]movement of circumnutation which gives rise to, or is converted into, heliotropism and apheliotropism. On this view we need not assume againstall analogy that a lateral light entirely stops circumnutation; it merelyexcites the plant to modify its movement for a time in a beneficial manner. The existence of every possible gradation, between a straight coursetowards a lateral light and a course consisting of a series of loops orellipses, becomes perfectly intelligible. Finally, the conversion ofcircumnutation into heliotropism or apheliotropism, is closely analogous towhat takes place with sleeping plants, which during the daytime describeone or more ellipses, often moving in zigzag lines and making little loops;for when they begin in the evening to go to sleep, they likewise expend alltheir energy in rendering their course rectilinear and rapid. In the caseof sleep-movements, the exciting or regulating cause is a difference in theintensity of the light, coming from above, at different periods of thetwenty-four hours; whilst with heliotropic and apheliotropic movements, itis a difference in the intensity of the light on the two sides of theplant. 

Transversal-heliotropismus (of Frank*) or Diaheliotropism. --The cause ofleaves placing themselves more or less transversely to the light, withtheir upper surfaces directed towards it, has been of late the subject ofmuch controversy. We do not here refer to the object of the movement, whichno doubt is that their upper surfaces may be fully illuminated, but themeans by which this position is gained. Hardly a better or more simpleinstance can be given

* 'Die natürliche Wagerechte Richtung von Pflanzentheilen, ' 1870. See alsosome interesting articles by the same author, "Zur Frage überTransversal-Geo-und Heliotropismus, " 'Bot. Zeitung, ' 1873, p. 17 et seq. [page 439]

of diaheliotropism than that offered by many seedlings, the cotyledons ofwhich are extended horizontally. When they first burst from theirseed-coats they are in contact and stand in various positions, oftenvertically upwards; they soon diverge, and this is effected by epinasty, which, as we have seen, is a modified form of circumnutation. After theyhave diverged to their full extent, they retain nearly the same position, though brightly illuminated all day long from above, with their lowersurfaces close to the ground and thus much shaded. There is therefore agreat contrast in the degree of illumination of their upper and lowersurfaces, and if they were heliotropic they would bend quickly upwards. Itmust not, however, be supposed that such cotyledons are immovably fixed ina horizontal position. When seedlings are exposed before a window, theirhypocotyls, which are highly heliotropic, bend quickly towards it, and theupper surfaces of their cotyledons still remain exposed at right angles tothe light; but if the hypocotyl is secured so that it cannot bend, thecotyledons themselves change their position. If the two are placed in theline of the entering light, the one furthest from it rises up and thatnearest to it often sinks down; if placed transversely to the light, theytwist a little laterally; so that in every case they endeavour to placetheir upper surfaces at right angles to the light. So it notoriously iswith the leaves on plants nailed against a wall, or grown in front of awindow. A moderate amount of light suffices to induce such movements; allthat is necessary is that the light should steadily strike the plants in anoblique direction. With respect to the above twisting movement ofcotyledons, Frank has given many and much more striking instances in thecase of the leaves on[page 440]branches which had been fastened in various positions or turned upsidedown. 

In our observations on the cotyledons of seedling plants, we often feltsurprise at their persistent horizontal position during the day, and wereconvinced before we had read Frank's essay, that some special explanationwas necessary. De Vries has shown* that the more or less horizontalposition of leaves is in most cases influenced by epinasty, by their ownweight, and by apogeotropism. A young cotyledon or leaf after bursting freeis brought down into its proper position, as already remarked, by epinasty, which, according to De Vries, long continues to act on the midribs andpetioles. Weight can hardly be influential in the case of cotyledons, except in a few cases presently to be mentioned, but must be so with largeand thick leaves. With respect to apogeotropism, De Vries maintains that itgenerally comes into play, and of this fact we shall presently advance someindirect evidence. But over these and other constant forces we believe thatthere is in many cases, but we do not say in all, a preponderant tendencyin leaves and cotyledons to place themselves more or less transversely withrespect to the light. 

In the cases above alluded to of seedlings exposed to a lateral light withtheir hypocotyls secured, it is impossible that epinasty, weight andapogeotropism, either in opposition or combined, can be the cause of therising of one cotyledon, and of the sinking of the other, since the forcesin question act equally on both; and since epinasty, weight andapogeotropism all act in a vertical plane, they cannot cause the twistingof the petioles, which occurs in seedlings under the

* 'Arbeiten des Bot. Instituts in Würzburg, ' Heft. Ii. 1872, pp. 223-277. [page 441]

above conditions of illumination. All these movements evidently depend insome manner on the obliquity of the light, but cannot be calledheliotropic, as this implies bending towards the light; whereas thecotyledon nearest to the light bends in an opposed direction or downwards, and both place themselves as nearly as possible at right angles to thelight. The movement, therefore, deserves a distinct name. As cotyledons andleaves are continually oscillating up and down, and yet retain all day longtheir proper position with their upper surfaces directed transversely tothe light, and if displaced reassume this position, diaheliotropism must beconsidered as a modified form of circumnutation. This was often evidentwhen the movements of cotyledons standing in front of a window were traced. We see something analogous in the case of sleeping leaves or cotyledons, which after oscillating up and down during the whole day, rise into avertical position late in the evening, and on the following morning sinkdown again into their horizontal or diaheliotropic position, in directopposition to heliotropism. This return into their diurnal position, whichoften requires an angular movement of 90o, is analogous to the movement ofleaves on displaced branches, which recover their former positions. Itdeserves notice that any force such as apogeotropism, will act withdifferent degrees of power* in the different positions of those leaves orcotyledons which oscillate largely up and down during the day; and yet theyrecover their horizontal or diaheliotropic position. 

We may therefore conclude that diaheliotropic movements cannot be fullyexplained by the direct action of light, gravitation, weight, etc. , anymore

* See former note, in reference to Sachs' remarks on this subject. [page 442]

than can the nyctitropic movements of cotyledons and leaves. In the lattercase they place themselves so that their upper surfaces may radiate atnight as little as possible into open space, with the upper surfaces of theopposite leaflets often in contact. These movements, which are sometimesextremely complex, are regulated, though not directly caused, by thealternations of light and darkness. In the case of diaheliotropism, cotyledons and leaves place themselves so that their upper surfaces may beexposed to the light, and this movement is regulated, though not directlycaused, by the direction whence the light proceeds. In both cases themovement consists of circumnutation modified by innate or constitutionalcauses, in the same manner as with climbing plants, the circumnutation ofwhich is increased in amplitude and rendered more circular, or again withvery young cotyledons and leaves which are thus brought down into ahorizontal position by epinasty. 

We have hitherto referred only to those leaves and cotyledons which occupya permanently horizontal position; but many stand more or less obliquely, and some few upright. The cause of these differences of position is notknown; but in accordance with Wiesner's views, hereafter to be given, it isprobable that some leaves and cotyledons would suffer, if they were fullyilluminated by standing at right angles to the light. 

We have seen in the second and fourth chapters that those cotyledons andleaves which do not alter their positions at night sufficiently to be saidto sleep, commonly rise a little in the evening and fall again on the nextmorning, so that they stand during the night at a rather higher inclinationthan during the middle of the day. It is incredible that a rising movementof 2o or 3o, or even of 10o or 20o, can be of[page 443]any service to the plant, so as to have been specially acquired. It must bethe result of some periodical change in the conditions to which they aresubjected, and there can hardly be a doubt that this is the dailyalternations of light and darkness. De Vries states in the paper beforereferred to, that most petioles and midribs are apogeotropic;* andapogeotropism would account for the above rising movement, which is commonto so many widely distinct species, if we suppose it to be conquered bydiaheliotropism during the middle of the day, as long as it is ofimportance to the plant that its cotyledons and leaves should be fullyexposed to the light. The exact hour in the afternoon at which they beginto bend slightly upwards, and the extent of the movement, will depend ontheir degree of sensitiveness to gravitation and on their power ofresisting its action during the middle of the day, as well as on theamplitude of their ordinary circumnutating movements; and as thesequalities differ much in different species, we might expect that the hourin the afternoon at which they begin to rise would differ much in differentspecies, as is the case. Some other agency, however, besides apogeotropism, must come into play, either directly or indirectly, in this upwardmovement. Thus a young bean (Vicia faba), growing in a small pot, wasplaced in front of a window in a klinostat; and at night the leaves rose alittle, although

* According to Frank ('Die nat. Wagerechte Richtung von Pflanzentheilen, '1870, p. 46) the root-leaves of many plants, kept in darkness, rise up andeven become vertical; and so it is in some cases with shoots. (SeeRauwenhoff, 'Archives Néerlandaises, ' tom. Xii. P. 32. ) These movementsindicate apogeotropism; but when organs have been long kept in the dark, the amount of water and of mineral matter which they contain is so muchaltered, and their regular growth is so much disturbed, that it is perhapsrash to infer from their movements what would occur under normalconditions. (See Godlewski, 'Bot. Zeitung, ' Feb. 14th, 1879. )[page 444]

the action of apogeotropism was quite eliminated. Nevertheless, they didnot rise nearly so much at night, as when subjected to apogeotropism. Is itnot possible, or even probable, that leaves and cotyledons, which havemoved upwards in the evening through the action of apogeotropism duringcountless generations, may inherit a tendency to this movement? We haveseen that the hypocotyls of several Leguminous plants have from a remoteperiod inherited a tendency to arch themselves; and we know that thesleep-movements of leaves are to a certain extent inherited, independentlyof the alternations of light and darkness. 

In our observations on the circumnutation of those cotyledons and leaveswhich do not sleep at night, we met with hardly any distinct cases of theirsinking a little in the evening, and rising again in the morning, --that is, of movements the reverse of those just discussed. We have no doubt thatsuch cases occur, inasmuch as the leaves of many plants sleep by sinkingvertically downwards. How to account for the few cases which were observedmust be left doubtful. The young leaves of Cannabis sativa sink at nightbetween 30o and 40o beneath the horizon; and Kraus attributes this toepinasty in conjunction with the absorption of water. Whenever epinasticgrowth is vigorous, it might conquer diaheliotropism in the evening, atwhich time it would be of no importance to the plant to keep its leaveshorizontal. The cotyledons of Anoda Wrightii, of one variety of Gossypium, and of several species of Ipomoea, remain horizontal in the evening whilstthey are very young; as they grow a little older they curve a littledownwards, and when large and heavy sink so much that they come under ourdefinition of sleep. In the case of[page 445]the Anoda and of some species of Ipomoea, it was proved that the downwardmovement did not depend on the weight of the cotyledons; but from the factof the movement being so much more strongly pronounced after the cotyledonshave grown large and heavy, we may suspect that their weight aboriginallyplayed some part in determining that the modification of the circumnutatingmovement should be in a downward direction. 

The so-called Diurnal Sleep of Leaves, Or Paraheliotropism. --This isanother class of movements, dependent on the action of light, whichsupports to some extent the belief that the movements above described areonly indirectly due to its action. We refer to the movements of leaves andcotyledons which when moderately illuminated are diaheliotropic; but whichchange their positions and present their edges to the light, when the sunshines brightly on them. These movements have sometimes been called diurnalsleep, but they differ wholly with respect to the object gained from thoseproperly called nyctitropic; and in some cases the position occupied duringthe day is the reverse of that during the night. 

[It has long been known* that when the sun shines brightly on the leafletsof Robinia, they rise up and present their edges to the light; whilst theirposition at night is vertically downwards. We have observed the samemovement, when the sun shone brightly on the leaflets of an AustralianAcacia. Those of Amphicarpaea monoica turned their edges to the sun; and ananalogous movement of the little almost rudimentary basal leaflets ofMimosa albida was on one occasion so rapid that it could be distinctly seenthrough a lens. The elongated, unifoliate, first leaves of PhaseolusRoxburghii stood at 7 A. M. At 20o above the horizon, and no doubt theyafterwards sank a little lower. At noon, after having been exposed forabout 2 h. To

* Pfeffer gives the names and dates of several ancient writers in his 'DiePeriodischen Bewegungen, ' 1875, p. 62. [page 446]

a bright sun, they stood at 56o above the horizon; they were then protectedfrom the rays of the sun, but were left well illuminated from above, andafter 30 m. They had fallen 40o, for they now stood at only 16o above thehorizon. Some young plants of Phaseolus Hernandesii had been exposed to thesame bright sunlight, and their broad, unifoliate, first leaves now stoodup almost or quite vertically, as did many of the leaflets on thetrifoliate secondary leaves; but some of the leaflets had twisted round ontheir own axes by as much as 90o without rising, so as to present theiredges to the sun. The leaflets on the same leaf sometimes behaved in thesetwo different manners, but always with the result of being less intenselyilluminated. These plants were then protected from the sun, and were lookedat after 1 ½ h. ; and now all the leaves and leaflets had reassumed theirordinary sub-horizontal positions. The copper-coloured cotyledons of someseedlings of Cassia mimosoides were horizontal in the morning, but afterthe sun had shone on them, each had risen 45 1/2o above the horizon. Themovement in these several cases must not be confounded with the suddenclosing of the leaflets of Mimosa pudica, which may sometimes be noticedwhen a plant which has been kept in an obscure place is suddenly exposed tothe sun; for in this case the light seems to act, as if it were a touch. 

From Prof. Wiesner's interesting observations, it is probable that theabove movements have been acquired for a special purpose. The chlorophyllin leaves is often injured by too intense a light, and Prof. Wiesner*believes that it is protected by the most diversified means, such as thepresence of hairs, colouring matter, etc. , and amongst other means by theleaves presenting their edges to the sun, so that the blades then receivemuch less light. He experimented on the young leaflets of Robinia, byfixing them in such a position that they could not escape being intenselyilluminated, whilst others were allowed to place themselves obliquely; andthe former began to suffer from the light in the course of two days. 

In the cases above given, the leaflets move either upwards

* 'Die Näturlichen Einrichtungen zum Schutze des Chlorophylls, ' etc. , 1876. Pringsheim has recently observed under the microscope the destruction ofchlorophyll in a few minutes by the action of concentrated light from thesun, in the presence of oxygen. See, also, Stahl on the protection ofchlorophyll from intense light, in 'Bot. Zeitung, ' 1880. [page 447]

or twist laterally, so as to place their edges in the direction of thesun's light; but Cohn long ago observed that the leaflets of Oxalis benddownwards when fully exposed to the sun. We witnessed a striking instanceof this movement in the very large leaflets of O. Ortegesii. A similarmovement may frequently be observed with the leaflets of Averrhoa bilimbi(a member of the Oxalidae); and a leaf is here represented (Fig. 180) onwhich the sun had shone. A diagram (Fig. 134) was given in the lastchapter, representing the oscillations by which a leaflet rapidly descendedunder these circumstances; and the movement may be seen closely to resemblethat (Fig. 133) by

Fig. 180. Averrhoa bilimbi: leaf with leaflets depressed after exposure tosunshine; but the leaflets are sometimes more depressed than is here shown. Figure much reduced. 

which it assumed its nocturnal position. It is an interesting fact inrelation to our present subject that, as Prof. Batalin informs us in aletter, dated February, 1879, the leaflets of Oxalis acetosella may bedaily exposed to the sun during many weeks, and they do not suffer if theyare allowed to depress themselves; but if this be prevented, they losetheir colour and wither in two or three days. Yet the duration of a leaf isabout two months, when subjected only to diffused light; and in this casethe leaflets never sink downwards during the day. ]

As the upward movements of the leaflets of Robinia, and the downwardmovements of those of Oxalis, have been proved to be highly beneficial tothese plants when subjected to bright sunshine, it seems probable that theyhave been acquired for the special purpose of avoiding too intense anillumination. As it would have been very troublesome in all the above casesto[page 448]have watched for a fitting opportunity and to have traced the movement ofthe leaves whilst they were fully exposed to the sunshine, we did notascertain whether paraheliotropism always consisted of modifiedcircumnutation; but this certainly was the case with the Averrhoa, andprobably with the other species, as their leaves were continuallycircumnutating. [page 449]

CHAPTER IX. 

SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS. 

Uses of heliotropism--Insectivorous and climbing plants not heliotropic--Same organ heliotropic at one age and not at another--Extraordinarysensitiveness of some plants to light--The effects of light do notcorrespond with its intensity--Effects of previous illumination--Timerequired for the action of light--After-effects of light--Apogeotropismacts as soon as light fails--Accuracy with which plants bend to the light--This dependent on the illumination of one whole side of the part--Localisedsensitiveness to light and its transmitted effects--Cotyledons of Phalaris, manner of bending--Results of the exclusion of light from their tips--Effects transmitted beneath the surface of the ground--Lateral illuminationof the tip determines the direction of the curvature of the base--Cotyledons of Avena, curvature of basal part due to the illumination ofupper part--Similar results with the hypocotyls of Brassica and Beta--Radicles of Sinapis apheliotropic, due to the sensitiveness of their tips--Concluding remarks and summary of chapter--Means by which circumnutationhas been converted into heliotropism or apheliotropism. 

NO one can look at the plants growing on a bank or on the borders of athick wood, and doubt that the young stems and leaves place themselves sothat the leaves may be well illuminated. They are thus enabled to decomposecarbonic acid. But the sheath-like cotyledons of some Gramineae, forinstance, those of Phalaris, are not green and contain very little starch;from which fact we may infer that they decompose little or no carbonicacid. Nevertheless, they are extremely heliotropic; and this probablyserves them in another way, namely, as a guide from the buried seedsthrough fissures in the ground or through overlying masses of vegetation, into the light and air. This view[page 450]is strengthened by the fact that with Phalaris and Avena the first trueleaf, which is bright green and no doubt decomposes carbonic acid, exhibitshardly a trace of heliotropism. The heliotropic movements of many otherseedlings probably aid them in like manner in emerging from the ground; forapogeotropism by itself would blindly guide them upwards, against anyoverlying obstacle. 

Heliotropism prevails so extensively among the higher plants, that thereare extremely few, of which some part, either the stem, flower-peduncle, petiole, or leaf, does not bend towards a lateral light. Droserarotundifolia is one of the few plants the leaves of which exhibit no traceof heliotropism. Nor could we see any in Dionaea, though the plants werenot so carefully observed. Sir J. Hooker exposed the pitchers of Sarraceniafor some time to a lateral light, but they did not bend towards it. * We canunderstand the reason why these insectivorous plants should not beheliotropic, as they do not live chiefly by decomposing carbonic acid; andit is much more important to them that their leaves should occupy the bestposition for capturing insects, than that they should be fully exposed tothe light. 

Tendrils, which consist of leaves or of other organs modified, and thestems of twining plants, are, as Mohl long ago remarked, rarelyheliotropic; and here again we can see the reason why, for if they hadmoved towards a lateral light they would have been drawn away from theirsupports. But some tendrils are apheliotropic, for instance those ofBignonia capreolata

* According to F. Kurtz ('Verhandl. Des Bot. Vereins der ProvinzBrandenburg, ' Bd. Xx. 1878) the leaves or pitchers of DarlingtoniaCalifornica are strongly apheliotropic. We failed to detect this movementin a plant which we possessed for a short time. [page 451]

and of Smilax aspera; and the stems of some plants which climb by rootlets, as those of the Ivy and Tecoma radicans, are likewise apheliotropic, andthey thus find a support. The leaves, on the other hand, of most climbingplants are heliotropic; but we could detect no signs of any such movementin those of Mutisia clematis. 

As heliotropism is so widely prevalent, and as twining plants aredistributed throughout the whole vascular series, the apparent absence ofany tendency in their stems to bend towards the light, seemed to us soremarkable a fact as to deserve further investigation, for it implies thatheliotropism can be readily eliminated. When twining plants are exposed toa lateral light, their stems go on revolving or circumnutating about thesame spot, without any evident deflection towards the light; but we thoughtthat we might detect some trace of heliotropism by comparing the averagerate at which the stems moved to and from the light during their successiverevolutions. * Three young plants (about a foot in height) of Ipomoeacaerulea and four of I. Purpurea, growing in separate pots, were placed ona bright day before a north-east window in a room otherwise darkened, withthe tips of their revolving stems fronting the window. When the tip of eachplant pointed directly from the window, and when again towards it, thetimes were recorded. This was continued from 6. 45 A. M. Till a little after2 P. M. On June 17th. After a few observations we concluded that we couldsafely estimate the time

* Some erroneous statements are unfortunately given on this subject, in'The Movements and Habits of Climbing Plants, ' 1875, pp. 28, 32, 40, and53. Conclusions were drawn from an insufficient number of observations, forwe did not then know at how unequal a rate the stems and tendrils ofclimbing plants sometimes travel in different parts of the same revolution. [page 452]

taken by each semicircle, within a limit of error of at most 5 minutes. Although the rate of movement in different parts of the same revolutionvaried greatly, yet 22 semicircles to the light were completed, each on anaverage in 73. 95 minutes; and 22 semicircles from the light each in 73. 5minutes. It may, therefore, be said that they travelled to and from thelight at exactly the same average rate; though probably the accuracy of theresult was in part accidental. In the evening the stems were not in theleast deflected towards the window. Nevertheless, there appears to exist avestige of heliotropism, for with 6 out of the 7 plants, the firstsemicircle from the light, described in the early morning after they hadbeen subjected to darkness during the night and thus probably rendered moresensitive, required rather more time, and the first semicircle to the lightconsiderably less time, than the average. Thus with all 7 plants, takentogether, the mean time of the first semicircle in the morning from thelight, was 76. 8 minutes, instead of 73. 5 minutes, which is the mean of allthe semicircles during the day from the light; and the mean of the firstsemicircle to the light was only 63. 1, instead of 73. 95 minutes, which wasthe mean of all the semicircles during the day to the light. 

Similar observations were made on Wistaria Sinensis, and the mean of 9semicircles from the light was 117 minutes, and of 7 semicircles to thelight 122 minutes, and this difference does not exceed the probable limitof error. During the three days of exposure, the shoot did not become atall bent towards the window before which it stood. In this case the firstsemicircle from the light in the early morning of each day, required ratherless time for its performance than did the first semicircle to the light;and this result, [page 453]if not accidental, appears to indicate that the shoots retain a trace of anoriginal apheliotropic tendency. With Lonicera brachypoda the semicirclesfrom and to the light differed considerably in time; for 5 semicircles fromthe light required on a mean 202. 4 minutes, and 4 to the light, 229. 5minutes; but the shoot moved very irregularly, and under thesecircumstances the observations were much too few. 

It is remarkable that the same part on the same plant may be affected bylight in a widely different manner at different ages, and as it appears atdifferent seasons. The hypocotyledonous stems of Ipomoea caerulea andpurpurea are extremely heliotropic, whilst the stems of older plants, onlyabout a foot in height, are, as we have just seen, almost wholly insensibleto light. Sachs states (and we have observed the same fact) that thehypocotyls of the Ivy (Hedera helix) are slightly heliotropic; whereas thestems of plants grown to a few inches in height become so stronglyapheliotropic, that they bend at right angles away from the light. Nevertheless, some young plants which had behaved in this manner early inthe summer again became distinctly heliotropic in the beginning ofSeptember; and the zigzag courses of their stems, as they slowly curvedtowards a north-east window, were traced during 10 days. The stems of veryyoung plants of Tropaeolum majus are highly heliotropic, whilst those ofolder plants, according to Sachs, are slightly apheliotropic. In all thesecases the heliotropism of the very young stems serves to expose thecotyledons, or when the cotyledons are hypogean the first true leaves, fully to the light; and the loss of this power by the older stems, or theirbecoming apheliotropic, is connected with their habit of climbing. 

Most seedling plants are strongly heliotropic, and[page 454]it is no doubt a great advantage to them in their struggle for life toexpose their cotyledons to the light as quickly and as fully as possible, for the sake of obtaining carbon. It has been shown in the first chapterthat the greater number of seedlings circumnutate largely and rapidly; andas heliotropism consists of modified circumnutation, we are tempted to lookat the high development of these two powers in seedlings as intimatelyconnected. Whether there are any plants which circumnutate slowly and to asmall extent, and yet are highly heliotropic, we do not know; but there areseveral, and there is nothing surprising in this fact, which circumnutatelargely and are not at all, or only slightly, heliotropic. Of such casesDrosera rotundifolia offers an excellent instance. The stolons of thestrawberry circumnutate almost like the stems of climbing plants, and theyare not at all affected by a moderate light; but when exposed late in thesummer to a somewhat brighter light they were slightly heliotropic; insunlight, according to De Vries, they are apheliotropic. Climbing plantscircumnutate much more widely than any other plants, yet they are not atall heliotropic. 

Although the stems of most seedling plants are strongly heliotropic, somefew are but slightly heliotropic, without our being able to assign anyreason. This is the case with the hypocotyl of Cassia tora, and we werestruck with the same fact with some other seedlings, for instance, those ofReseda odorata. With respect to the degree of sensitiveness of the moresensitive kinds, it was shown in the last chapter that seedlings of severalspecies, placed before a north-east window protected by several blinds, andexposed in the rear to the diffused light of the room, moved with unerringcertainty towards the window, although[page 455]it was impossible to judge, excepting by the shadow cast by an uprightpencil on a white card, on which side most light entered, so that theexcess on one side must have been extremely small. 

A pot with seedlings of Phalaris Canariensis, which had been raised indarkness, was placed in a completely darkened room, at 12 feet from a verysmall lamp. After 3 h. The cotyledons were doubtfully curved towards thelight, and after 7 h. 40 m. From the first exposure, they were all plainly, though slightly, curved towards the lamp. Now, at this distance of 12 feet, the light was so obscure that we could not see the seedlings themselves, nor read the large Roman figures on the white face of a watch, nor see apencil line on paper, but could just distinguish a line made with Indianink. It is a more surprising fact that no visible shadow was cast by apencil held upright on a white card; the seedlings, therefore, were actedon by a difference in the illumination of their two sides, which the humaneye could not distinguish. On another occasion even a less degree of lightacted, for some cotyledons of Phalaris became slightly curved towards thesame lamp at a distance of 20 feet; at this distance we could not see acircular dot 2. 29 mm. (. 09 inch) in diameter made with Indian ink on whitepaper, though we could just see a dot 3. 56 mm. (. 14 inch) in diameter; yeta dot of the former size appears large when seen in the light. *

We next tried how small a beam of light would act; as this bears on lightserving as a guide to seedlings whilst they emerge through fissured orencumbered ground. A pot with seedlings of Phalaris was covered

* Strasburger says ('Wirkung des Lichtes auf Schwärmsporen, ' 1878, p. 52), that the spores of Haematococcus moved to a light which only just sufficedto allow middle-sized type to be read. [page 456]

by a tin-vessel, having on one side a circular hole 1. 23 mm. In diameter(i. E. A little less than the 1/20th of an inch); and the box was placed infront of a paraffin lamp and on another occasion in front of a window; andboth times the seedlings were manifestly bent after a few hours towards thelittle hole. 

A more severe trial was now made; little tubes of very thin glass, closedat their upper ends and coated with black varnish, were slipped over thecotyledons of Phalaris (which had germinated in darkness) and just fittedthem. Narrow stripes of the varnish had been previously scraped off oneside, through which alone light could enter; and their dimensions wereafterwards measured under the microscope. As a control experiment, similarunvarnished and transparent tubes were tried, and they did not prevent thecotyledons bending towards the light. Two cotyledons were placed before asouth-west window, one of which was illuminated by a stripe in the varnish, only . 004 inch (0. 1 mm. ) in breadth and . 016 inch (0. 4 mm. ) in length; andthe other by a stripe . 008 inch in breadth and . 06 inch in length. Theseedlings were examined after an exposure of 7 h. 40 m. , and were found tobe manifestly bowed towards the light. Some other cotyledons were at thesame time treated similarly, excepting that the little stripes weredirected not to the sky, but in such a manner that they received only thediffused light from the room; and these cotyledons did not become at allbowed. Seven other cotyledons were illuminated through narrow, butcomparatively long, cleared stripes in the varnish--namely, in breadthbetween . 01 and . 026 inch, and in length between . 15 and . 3 inch; and theseall became bowed to the side, by which light entered through the stripes, whether these were directed towards the sky or to one side of[page 457]the room. That light passing through a hole only . 004 inch in breadth by. 016 in length, should induce curvature, seems to us a surprising fact. 

Before we knew how extremely sensitive the cotyledons of Phalaris were tolight, we endeavoured to trace their circumnutation in darkness by the aidof a small wax taper, held for a minute or two at each observation innearly the same position, a little on the left side in front of thevertical glass on which the tracing was made. The seedlings were thusobserved seventeen times in the course of the day, at intervals of fromhalf to three-quarters of an hour; and late in the evening we weresurprised to find that all the 29 cotyledons were greatly curved andpointed towards the vertical glass, a little to the left where the taperhad been held. The tracings showed that they had travelled in zigzag lines. Thus, an exposure to a feeble light for a very short time at the abovespecified intervals, sufficed to induce well-marked heliotropism. Ananalogous case was observed with the hypocotyls of Solanum lycopersicum. Weat first attributed this result to the after-effects of the light on eachoccasion; but since reading Wiesner's observations, * which will be referredto in the last chapter, we cannot doubt that an intermittent light is moreefficacious than a continuous one, as plants are especially sensitive toany contrast in its amount. 

The cotyledons of Phalaris bend much more slowly towards a very obscurelight than towards a bright one. Thus, in the experiments with seedlingsplaced in a dark room at 12 feet from a very small lamp, they were justperceptibly and doubtfully curved towards it after 3 h. , and only slightly, yet certainly, after 4 h. 

* 'Sitz. Der k. Akad. Der Wissensch. ' (Vienna), Jan. 1880, p. 12. [page 458]

After 8 h. 40 m. The chords of their arcs were deflected from theperpendicular by an average angle of only 16o. Had the light been bright, they would have become much more curved in between 1 and 2 h. Severaltrials were made with seedlings placed at various distances from a smalllamp in a dark room; but we will give only one trial. Six pots were placedat distances of 2, 4, 8, 12, 16, and 20 feet from the lamp, before whichthey were left for 4 h. As light decreases in a geometrical ratio, theseedlings in the 2nd pot received 1/4th, those in the 3rd pot 1/16th, thosein the 4th 1/36th, those in the 5th 1/64th, and those in the 6th 1/100th ofthe light received by the seedlings in the first or nearest pot. Thereforeit might have been expected that there would have been an immensedifference in the degree of their heliotropic curvature in the severalpots; and there was a well-marked difference between those which stoodnearest and furthest from the lamp, but the difference in each successivepair of pots was extremely small. In order to avoid prejudice, we askedthree persons, who knew nothing about the experiment, to arrange the potsin order according to the degree of curvature of the cotyledons. The firstperson arranged them in proper order, but doubted long between the 12 feetand 16 feet pots; yet these two received light in the proportion of 36 to64. The second person also arranged them properly, but doubted between the8 feet and 12 feet pots, which received light in the proportion of 16 to36. The third person arranged them in wrong order, and doubted about fourof the pots. This evidence shows conclusively how little the curvature ofthe seedlings differed in the successive pots, in comparison with the greatdifference in the amount of light which they received; and it should benoted that there was no[page 459]excess of superfluous light, for the cotyledons became but little andslowly curved even in the nearest pot. Close to the 6th pot, at thedistance of 20 feet from the lamp, the light allowed us just to distinguisha dot 3. 56 mm. (. 14 inch) in diameter, made with Indian ink on white paper, but not a dot 2. 29 mm. (. 09 inch) in diameter. 

The degree of curvature of the cotyledons of Phalaris within a given time, depends not merely on the amount of lateral light which they may thenreceive, but on that which they have previously received from above and onall sides. Analogous facts have been given with respect to the nyctitropicand periodic movements of plants. Of two pots containing seedlings ofPhalaris which had germinated in darkness, one was still kept in the dark, and the other was exposed (Sept. 26th) to the light in a greenhouse duringa cloudy day and on the following bright morning. On this morning (27th), at 10. 30 A. M. , both pots were placed in a box, blackened within and open infront, before a north-east window, protected by a linen and muslin blindand by a towel, so that but little light was admitted, though the sky wasbright. Whenever the pots were looked at, this was done as quickly aspossible, and the cotyledons were then held transversely with respect tothe light, so that their curvature could not have been thus increased ordiminished. After 50 m. The seedlings which had previously been kept indarkness, were perhaps, and after 70 m. Were certainly, curved, though veryslightly, towards the window. After 85 m. Some of the seedlings, which hadpreviously been illuminated, were perhaps a little affected, and after 100m. Some of the younger ones were certainly a little curved towards thelight. At this time (i. E. After 100 m. ) there was a plain difference[page 460]in the curvature of the seedlings in the two pots. After 2 h. 12 m. Thechords of the arcs of four of the most strongly curved seedlings in eachpot were measured, and the mean angle from the perpendicular of those whichhad previously been kept in darkness was 19o, and of those which hadpreviously been illuminated only 7o. Nor did this difference diminishduring two additional hours. As a check, the seedlings in both pots werethen placed in complete darkness for two hours, in order that apogeotropismshould act on them; and those in the one pot which were little curvedbecame in this time almost completely upright, whilst the more curved onesin the other pot still remained plainly curved. 

Two days afterwards the experiment was repeated, with the sole differencethat even less light was admitted through the window, as it was protectedby a linen and muslin blind and by two towels; the sky, moreover, wassomewhat less bright. The result was the same as before, excepting thateverything occurred rather slower. The seedlings which had been previouslykept in darkness were not in the least curved after 54 m. , but were soafter 70 m. Those which had previously been illuminated were not at allaffected until 130 m. Had elapsed, and then only slightly. After 145 m. Some of the seedlings in this latter pot were certainly curved towards thelight; and there was now a plain difference between the two pots. After 3h. 45 m. The chords of the arcs of 3 seedlings in each pot were measured, and the mean angle from the perpendicular was 16o for those in the potwhich had previously been kept in darkness, and only 5o for those which hadpreviously been illuminated. 

The curvature of the cotyledons of Phalaris towards a lateral light istherefore certainly influenced by the[page 461]degree to which they have been previously illuminated. We shall presentlysee that the influence of light on their bending continues for a short timeafter the light has been extinguished. These facts, as well as that of thecurvature not increasing or decreasing in nearly the same ratio with thatof the amount of light which they receive, as shown in the trials with theplants before the lamp, all indicate that light acts on them as a stimulus, in somewhat the same manner as on the nervous system of animals, and not ina direct manner on the cells or cell-walls which by their contraction orexpansion cause the curvature. 

It has already been incidentally shown how slowly the cotyledons ofPhalaris bend towards a very dim light; but when they were placed before abright paraffin lamp their tips were all curved rectangularly towards it in2 h. 20 m. The hypocotyls of Solanum lycopersicum had bent in the morningat right angles towards a north-east window. At 1 P. M. (Oct. 21st) the potwas turned round, so that the seedlings now pointed from the light, but by5 P. M. They had reversed their curvature and again pointed to the light. They had thus passed through 180o in 4 h. , having in the morning previouslypassed through about 90o. But the reversal of the first half of thecurvature will have been aided by apogeotropism. Similar cases wereobserved with other seedlings, for instance, with those of Sinapis alba. 

We attempted to ascertain in how short a time light acted on the cotyledonsof Phalaris, but this was difficult on account of their rapidcircumnutating movement; moreover, they differ much in sensibility, according to age; nevertheless, some of our observations are worth giving. Pots with seedlings were[page 462]placed under a microscope provided with an eye-piece micrometer, of whicheach division equalled 1/500th of an inch (0. 051 mm. ); and they were atfirst illuminated by light from a paraffin lamp passing through a solutionof bichromate of potassium, which does not induce heliotropism. Thus thedirection in which the cotyledons were circumnutating could be observedindependently of any action from the light; and they could be made, byturning round the pots, to circumnutate transversely to the line in whichthe light would strike them, as soon as the solution was removed. The factthat the direction of the circumnutating movement might change at anymoment, and thus the plant might bend either towards or from the lampindependently of the action of the light, gave an element of uncertainty tothe results. After the solution had been removed, five seedlings which werecircumnutating transversely to the line of light, began to move towards it, in 6, 4, 7 1/2, 6, and 9 minutes. In one of these cases, the apex of thecotyledon crossed five of the divisions of the micrometer (i. E. 1/100th ofan inch, or 0. 254 mm. ) towards the light in 3 m. Of two seedlings whichwere moving directly from the light at the time when the solution wasremoved, one began to move towards it in 13 m. , and the other in 15 m. Thislatter seedling was observed for more than an hour and continued to movetowards the light; it crossed at one time 5 divisions of the micrometer(0. 254 mm. ) in 2 m. 30 s. In all these cases, the movement towards thelight was extremely unequal in rate, and the cotyledons often remainedalmost stationary for some minutes, and two of them retrograded a little. Another seedling which was circumnutating transversely to the line oflight, moved towards it in 4 m. After the solution was removed; it thenremained[page 463]almost stationary for 10 m. ; then crossed 5 divisions of the micrometer in6 m. ; and then 8 divisions in 11m. This unequal rate of movement, interrupted by pauses, and at first with occasional retrogressions, accordswell with our conclusion that heliotropism consists of modifiedcircumnutation. 

In order to observe how long the after-effects of light lasted, a pot withseedlings of Phalaris, which had germinated in darkness, was placed at10. 40 A. M. Before a north-east window, being protected on all other sidesfrom the light; and the movement of a cotyledon was traced on a horizontalglass. It circumnutated about the same space for the first 24 m. , andduring the next 1 h. 33 m. Moved rapidly towards the light. The light wasnow (i. E. After 1 h. 57 m. ) completely excluded, but the cotyledoncontinued bending in the same direction as before, certainly for more than15 m. , probably for about 27 m. The doubt arose from the necessity of notlooking at the seedlings often, and thus exposing them, though momentarily, to the light. This same seedling was now kept in the dark, until 2. 18 P. M. , by which time it had reacquired through apogeotropism its original uprightposition, when it was again exposed to the light from a clouded sky. By 3P. M. It had moved a very short distance towards the light, but during thenext 45 m. Travelled quickly towards it. After this exposure of 1 h. 27 m. To a rather dull sky, the light was again completely excluded, but thecotyledon continued to bend in the same direction as before for 14 m. Within a very small limit of error. It was then placed in the dark, and itnow moved backwards, so that after 1 h. 7 m. It stood close to where it hadstarted from at 2. 18 P. M. These observations show that the cotyledons ofPhalaris, after being exposed to a lateral[page 464]light, continue to bend in the same direction for between a quarter andhalf an hour. 

In the two experiments just given, the cotyledons moved backwards or fromthe window shortly after being subjected to darkness; and whilst tracingthe circumnutation of various kinds of seedlings exposed to a laterallight, we repeatedly observed that late in the evening, as the light waned, they moved from it. This fact is shown in some of the diagrams given in thelast chapter. We wished therefore to learn whether this was wholly due toapogeotropism, or whether an organ after bending towards the light tendedfrom any other cause to bend from it, as soon as the light failed. Accordingly, two pots of seedling Phalaris and one pot of seedling Brassicawere exposed for 8 h. Before a paraffin lamp, by which time the cotyledonsof the former and the hypocotyls of the latter were bent rectangularlytowards the light. The pots were now quickly laid horizontally, so that theupper parts of the cotyledons and of the hypocotyls of 9 seedlingsprojected vertically upwards, as proved by a plumb-line. In this positionthey could not be acted on by apogeotropism, and if they possessed anytendency to straighten themselves or to bend in opposition to their formerheliotropic curvature, this would be exhibited, for it would be opposed atfirst very slightly by apogeotropism. They were kept in the dark for 4 h. , during which time they were twice looked at; but no uniform bending inopposition to their former heliotropic curvature could be detected. We havesaid uniform bending, because they circumnutated in their new position, andafter 2 h. Were inclined in different directions (between 4o and 11o) fromthe perpendicular. Their directions were also changed after two additionalhours, and again on the following morning. We may[page 465]therefore conclude that the bending back of plants from a light, when thisbecomes obscure or is extinguished, is wholly due to apogeotropism. *

In our various experiments we were often struck with the accuracy withwhich seedlings pointed to a light although of small size. To test this, many seedlings of Phalaris, which had germinated in darkness in a verynarrow box several feet in length, were placed in a darkened room near toand in front of a lamp having a small cylindrical wick. The cotyledons atthe two ends and in the central part of the box, would therefore have tobend in widely different directions in order to point to the light. Afterthey had become rectangularly bent, a long white thread was stretched bytwo persons, close over and parallel, first to one and then to anothercotyledon; and the thread was found in almost every case actually tointersect the small circular wick of the now extinguished lamp. Thedeviation from accuracy never exceeded, as far as we could judge, a degreeor two. This extreme accuracy seems at first surprising, but is not reallyso, for an upright cylindrical stem, whatever its position may be withrespect to the light, would have exactly half its circumference illuminatedand half in shadow; and as the difference in illumination of the two sidesis the exciting cause of heliotropism, a cylinder would naturally bend withmuch accuracy towards the light. The cotyledons, however, of Phalaris arenot cylindrical, but oval in section; and the longer axis was to theshorter axis (in the one which was measured) as 100 to 70. Nevertheless, nodifference could be

* It appears from a reference in Wiesner ('Die Undulirende Nutation derInternodien, ' p. 7), that H. Müller of Thurgau found that a stem which isbending heliotropically is at the same time striving, throughapogeotropism, to raise itself into a vertical position. [page 466]

detected in the accuracy of their bending, whether they stood with theirbroad or narrow sides facing the light, or in any intermediate position;and so it was with the cotyledons of Avena sativa, which are likewise ovalin section. Now, a little reflection will show that in whatever positionthe cotyledons may stand, there will be a line of greatest illumination, exactly fronting the light, and on each side of this line an equal amountof light will be received; but if the oval stands obliquely with respect tothe light, this will be diffused over a wider surface on one side of thecentral line than on the other. We may therefore infer that the same amountof light, whether diffused over a wider surface or concentrated on asmaller surface, produces exactly the same effect; for the cotyledons inthe long narrow box stood in all sorts of positions with reference to thelight, yet all pointed truly towards it. 

That the bending of the cotyledons to the light depends on the illuminationof one whole side or on the obscuration of the whole opposite side, and noton a narrow longitudinal zone in the line of the light being affected, wasshown by the effects of painting longitudinally with Indian ink one side offive cotyledons of Phalaris. These were then placed on a table near to asouth-west window, and the painted half was directed either to the right orleft. The result was that instead of bending in a direct line towards thewindow, they were deflected from the window and towards the unpainted side, by the following angles, 35o, 83o, 31o, 43o, and 39o. It should be remarkedthat it was hardly possible to paint one-half accurately, or to place allthe seedlings which are oval in section in quite the same positionrelatively to the light; and this will account for the differences in theangles. Five coty-[page 467]ledons of Avena were also painted in the same manner, but with greatercare; and they were laterally deflected from the line of the window, towards the unpainted side, by the following angles, 44o, 44o, 55o, 51o, and57o. This deflection of the cotyledons from the window is intelligible, forthe whole unpainted side must have received some light, whereas theopposite and painted side received none; but a narrow zone on the unpaintedside directly in front of the window will have received most light, and allthe hinder parts (half an oval in section) less and less light in varyingdegrees; and we may conclude that the angle of deflection is the resultantof the action of the light over the whole of the unpainted side. 

It should have been premised that painting with Indian ink does not injureplants, at least within several hours; and it could injure them only bystopping respiration. To ascertain whether injury was thus soon caused, theupper halves of 8 cotyledons of Avena were thickly coated with transparentmatter, --4 with gum, and 4 with gelatine; they were placed in the morningbefore a window, and by the evening they were normally bowed towards thelight, although the coatings now consisted of dry crusts of gum andgelatine. Moreover, if the seedlings which were painted longitudinally withIndian ink had been injured on the painted side, the opposite side wouldhave gone on growing, and they would consequently have become bowed towardsthe painted side; whereas the curvature was always, as we have seen, in theopposite direction, or towards the unpainted side which was exposed to thelight. We witnessed the effects of injuring longitudinally one side of thecotyledons of Avena and Phalaris; for before we knew that grease was highlyinjurious to them, several were painted down one side[page 468]with a mixture of oil and lamp-black, and were then exposed before awindow; others similarly treated were afterwards tried in darkness. Thesecotyledons soon became plainly bowed towards the blackened side, evidentlyowing to the grease on this side having checked their growth, whilst growthcontinued on the opposite side. But it deserves notice that the curvaturediffered from that caused by light, which ultimately becomes abrupt nearthe ground. These seedlings did not afterwards die, but were much injuredand grew badly. 

LOCALISED SENSITIVENESS TO LIGHT, AND ITS TRANSMITTED EFFECTS. 

Phalaris Canariensis. --Whilst observing the accuracy with which thecotyledons of this plant became bent towards the light of a small lamp, wewere impressed with the idea that the uppermost part determined thedirection of the curvature of the lower part. When the cotyledons areexposed to a lateral light, the upper part bends first, and afterwards thebending gradually extends down to the base, and, as we shall presently see, even a little beneath the ground. This holds good with cotyledons from lessthan . 1 inch (one was observed to act in this manner which was only . 03 inheight) to about . 5 of an inch in height; but when they have grown tonearly an inch in height, the basal part, for a length of . 15 to . 2 of aninch above the ground, ceases to bend. As with young cotyledons the lowerpart goes on bending, after the upper part has become well arched towards alateral light, the apex would ultimately point to the ground instead of tothe light, did not the upper part reverse its curvature and straightenitself, as[page 469]soon as the upper convex surface of the bowed-down portion received morelight than the lower concave surface. The position ultimately assumed byyoung and upright cotyledons, exposed to light entering obliquely fromabove through a window, is shown in the accompanying figure (Fig. 181); andhere it may be seen that the whole upper part has become very nearlystraight. When the cotyledons were exposed before a bright lamp, standingon the same level with them, the upper part, which was at first

Fig. 181. Phalaris Canariensis: cotyledons after exposure in a box open onone side in front of a south-west window during 8 h. Curvature towards thelight accurately traced. The short horizontal lines show the level of theground. 

greatly arched towards the light, became straight and strictly parallelwith the surface of the soil in the pots; the basal part being nowrectangularly bent. All this great amount of curvature, together with thesubsequent straightening of the upper part, was often effected in a fewhours. 

[After the uppermost part has become bowed a little to the light, itsoverhanging weight must tend to increase the curvature of the lower part;but any such effect was shown in several ways to be quite insignificant. When little caps of tin-foil (hereafter to be described) were placed on thesummits of the cotyledons, though this must have added considerably totheir weight, the rate or amount of bending was not thus increased. But thebest evidence was afforded by placing pots with seedlings of Phalarisbefore a lamp in such a position, that the cotyledons were horizontallyextended and projected at right angles to the line of light. In the courseof 5 ½ h. They were directed towards the light with their bases bent atright angles; and this abrupt[page 470]curvature could not have been aided in the least by the weight of the upperpart, which acted at right angles to the plane of curvature. 

It will be shown that when the upper halves of the cotyledons of Phalarisand Avena were enclosed in little pipes of tin-foil or of blackened glass, in which case the upper part was mechanically prevented from bending, thelower and unenclosed part did not bend when exposed to a lateral light; andit occurred to us that this fact might be due, not to the exclusion of thelight from the upper part, but to some necessity of the bending graduallytravelling down the cotyledons, so that unless the upper part first becamebent, the lower could not bend, however much it might be stimulated. It wasnecessary for our purpose to ascertain whether this notion was true, and itwas proved false; for the lower halves of several cotyledons became bowedto the light, although their upper halves were enclosed in little glasstubes (not blackened), which prevented, as far as we could judge, theirbending. Nevertheless, as the part within the tube might possibly bend avery little, fine rigid rods or flat splinters of thin glass were cementedwith shellac to one side of the upper part of 15 cotyledons; and in sixcases they were in addition tied on with threads. They were thus forced toremain quite straight. The result was that the lower halves of all becamebowed to the light, but generally not in so great a degree as thecorresponding part of the free seedlings in the same pots; and this mayperhaps be accounted for by some slight degree of injury having been causedby a considerable surface having been smeared with shellac. It may beadded, that when the cotyledons of Phalaris and Avena are acted on byapogeotropism, it is the upper part which begins first to bend; and whenthis part was rendered rigid in the manner just described, the upwardcurvature of the basal part was not thus prevented. 

To test our belief that the upper part of the cotyledons of Phalaris, whenexposed to a lateral light, regulates the bending of the lower part, manyexperiments were tried; but most of our first attempts proved useless fromvarious causes not worth specifying. Seven cotyledons had their tips cutoff for lengths varying between . 1 and . 16 of an inch, and these, when leftexposed all day to a lateral light, remained upright. In another set of 7cotyledons, the tips were cut off for a length of only about . 05 of an inch(1. 27 mm. ) and these became bowed towards[page 471]a lateral light, but not nearly so much as the many other seedlings in thesame pots. This latter case shows that cutting off the tips does not byitself injure the plants so seriously as to prevent heliotropism; but wethought at the time, that such injury might follow when a greater lengthwas cut off, as in the first set of experiments. Therefore, no more trialsof this kind were made, which we now regret; as we afterwards found thatwhen the tips of three cotyledons were cut off for a length of . 2 inch, andof four others for lengths of . 14, . 12, . 1, and . 07 inch, and they wereextended horizontally, the amputation did not interfere in the least withtheir bending vertically upwards, through the action of apogeotropism, likeunmutilated specimens. It is therefore extremely improbable that theamputation of the tips for lengths of from . 1 to . 14 inch, could from theinjury thus caused have prevented the lower part from bending towards thelight. 

We next tried the effects of covering the upper part of the cotyledons ofPhalaris with little caps which were impermeable to light; the whole lowerpart being left fully exposed before a south-west window or a brightparaffin lamp. Some of the caps were made of extremely thin tin-foilblackened within; these had the disadvantage of occasionally, thoughrarely, being too heavy, especially when twice folded. The basal edgescould be pressed into close contact with the cotyledons; though this againrequired care to prevent injuring them. Nevertheless, any injury thuscaused could be detected by removing the caps, and trying whether thecotyledons were then sensitive to light. Other caps were made of tubes ofthe thinnest glass, which when painted black served well, with the onegreat disadvantage that the lower ends could not be closed. But tubes wereused which fitted the cotyledons almost closely, and black paper was placedon the soil round each, to check the upward reflection of light from thesoil. Such tubes were in one respect far better than caps of tin-foil, asit was possible to cover at the same time some cotyledons with transparentand others with opaque tubes; and thus our experiments could be controlled. It should be kept in mind that young cotyledons were selected for trial, and that these when not interfered with become bowed down to the groundtowards the light. 

We will begin with the glass-tubes. The summits of nine cotyledons, differing somewhat in height, were enclosed for rather less than half theirlengths in uncoloured or transparent[page 472]tubes; and these were then exposed before a south-west window on a brightday for 8 h. All of them became strongly curved towards the light, in thesame degree as the many other free seedlings in the same pots; so that theglass-tubes certainly did not prevent the cotyledons from bending towardsthe light. Nineteen other cotyledons were, at the same time, similarlyenclosed in tubes thickly painted with Indian ink. On five of them, thepaint, to our surprise, contracted after exposure to the sunlight, and verynarrow cracks were formed, through which a little light entered; and thesefive cases were rejected. Of the remaining 14 cotyledons, the lower halvesof which had been fully exposed to the light for the whole time, 7continued quite straight and upright; 1 was considerably bowed to thelight, and 6 were slightly bowed, but with the exposed bases of most ofthem almost or quite straight. It is possible that some light may have beenreflected upwards from the soil and entered the bases of these 7 tubes, asthe sun shone brightly, though bits of blackened paper had been placed onthe soil round them. Nevertheless, the 7 cotyledons which were slightlybowed, together with the 7 upright ones, presented a most remarkablecontrast in appearance with the many other seedlings in the same pots towhich nothing had been done. The blackened tubes were then removed from 10of these seedlings, and they were now exposed before a lamp for 8 h. ; 9 ofthem became greatly, and 1 moderately, curved towards the light, provingthat the previous absence of any curvature in the basal part, or thepresence of only a slight degree of curvature there, was due to theexclusion of light from the upper part. 

Similar observations were made on 12 younger cotyledons with their upperhalves enclosed within glass-tubes coated with black varnish, and withtheir lower halves fully exposed to bright sunshine. In these youngerseedlings the sensitive zone seems to extend rather lower down, as wasobserved on some other occasions, for two became almost as much curvedtowards the light as the free seedlings; and the remaining ten wereslightly curved, although the basal part of several of them, which normallybecomes more curved than any other part, exhibited hardly a trace ofcurvature. These 12 seedlings taken together differed greatly in theirdegree of curvature from all the many other seedlings in the same pots. 

Better evidence of the efficiency of the blackened tubes was incidentallyafforded by some experiments hereafter to be given, [page 473]in which the upper halves of 14 cotyledons were enclosed in tubes fromwhich an extremely narrow stripe of the black varnish had been scraped off. These cleared stripes were not directed towards the window, but obliquelyto one side of the room, so that only a very little light could act on theupper halves of the cotyledons. These 14 seedlings remained during eighthours of exposure before a south-west window on a hazy day quite upright;whereas all the other many free seedlings in the same pots became greatlybowed towards the light. 

We will now turn to the trials with caps made of very thin tin-foil. Thesewere placed at different times on the summits of 24 cotyledons, and theyextended down for a length of between . 15 and . 2 of an inch. The seedlingswere exposed to a lateral light for periods varying between 6 h. 30 m. And7 h. 45 m. , which sufficed to cause all the other seedlings in the samepots to become almost rectangularly bent towards the light. They varied inheight from only . 04 to 1. 15 inch, but the greater number were about . 75inch. Of the 24 cotyledons with their summits thus protected, 3 became muchbent, but not in the direction of the light, and as they did not straightenthemselves through apogeotropism during the following night, either thecaps were too heavy or the plants themselves were in a weak condition; andthese three cases may be excluded. There are left for consideration 21cotyledons; of these 17 remained all the time quite upright; the other 4became slightly inclined to the light, but not in a degree comparable withthat of the many free seedlings in the same pots. As the glass-tubes, whenunpainted, did not prevent the cotyledons from becoming greatly bowed, itcannot be supposed that the caps of very thin tin-foil did so, exceptthrough the exclusion of the light. To prove that the plants had not beeninjured, the caps were removed from 6 of the upright seedlings, and thesewere exposed before a paraffin lamp for the same length of time as before, and they now all became greatly curved towards the light. 

As caps between . 15 and . 2 of an inch in depth were thus proved to behighly efficient in preventing the cotyledons from bending towards thelight, 8 other cotyledons were protected with caps between only . 06 and . 12in depth. Of these, two remained vertical, one was considerably and fiveslightly curved towards the light, but far less so than the free seedlingsin the same pots. [page 474]

Another trial was made in a different manner, namely, by bandaging withstrips of tin-foil, about . 2 in breadth, the upper part, but not the actualsummit, of eight moderately young seedlings a little over half an inch inheight. The summits and the basal parts were thus left fully exposed to alateral light during 8 h. ; an upper intermediate zone being protected. Withfour of these seedlings the summits were exposed for a length of . 05 inch, and in two of them this part became curved towards the light, but the wholelower part remained quite upright; whereas the entire length of the othertwo seedlings became slightly curved towards the light. The summits of thefour other seedlings were exposed for a length of . 04 inch, and of theseone remained almost upright, whilst the other three became considerablycurved towards the light. The many free seedlings in the same pots were allgreatly curved towards the light. 

From these several sets of experiments, including those with theglass-tubes, and those when the tips were cut off, we may infer that theexclusion of light from the upper part of the cotyledons of Phalarisprevents the lower part, though fully exposed to a lateral light, frombecoming curved. The summit for a length of . 04 or . 05 of an inch, thoughit is itself sensitive and curves towards the light, has only a slightpower of causing the lower part to bend. Nor has the exclusion of lightfrom the summit for a length of . 1 of an inch a strong influence on thecurvature of the lower part. On the other hand, an exclusion for a lengthof between . 15 and . 2 of an inch, or of the whole upper half, plainlyprevents the lower and fully illuminated part from becoming curved in themanner (see Fig. 181) which invariably occurs when a free cotyledon isexposed to a lateral light. With very young seedlings the sensitive zoneseems to extend rather lower down relatively to their height than in olderseedlings. We must therefore conclude that when seedlings are freelyexposed to a lateral light some influence is transmitted from the upper tothe lower part, causing the latter to bend. 

This conclusion is supported by what may be seen to occur on a small scale, especially with young cotyledons, without any artificial exclusion of thelight; for they bend beneath the earth where no light can enter. Seeds ofPhalaris were covered with a layer one-fourth of an inch in thickness ofvery fine sand, consisting of extremely minute grains of silex coated with[page 475]oxide of iron. A layer of this sand, moistened to the same degree as thatover the seeds, was spread over a glass-plate; and when the layer was . 05of an inch in thickness (carefully measured) no light from a bright skycould be seen to pass through it, unless it was viewed through a longblackened tube, and then a trace of light could be detected, but probablymuch too little to affect any plant. A layer . 1 of an inch in thickness wasquite impermeable to light, as judged by the eye aided by the tube. It maybe worth adding that the layer, when dried, remained equally impermeable tolight. This sand yielded to very slight pressure whilst kept moist, and inthis state did not contract or crack in the least. In a first trial, cotyledons which had grown to a moderate height were exposed for 8 h. Before a paraffin lamp, and they became greatly bowed. At their bases onthe shaded side opposite to the light, well-defined, crescentic, openfurrows were formed, which (measured under a microscope with a micrometer)were from . 02 to . 03 of an inch in breadth, and these had evidently beenleft by the bending of the buried bases of the cotyledons towards thelight. On the side of the light the cotyledons were in close contact withthe sand, which was a very little heaped up. By removing with a sharp knifethe sand on one side of the cotyledons in the line of the light, the bentportion and the open furrows were found to extend down to a depth of about. 1 of an inch, where no light could enter. The chords of the short buriedarcs formed in four cases angles of 11o, 13o, 15o, and 18o, with theperpendicular. By the following morning these short bowed portions hadstraightened themselves through apogeotropism. 

In the next trial much younger cotyledons were similarly treated, but wereexposed to a rather obscure lateral light. After some hours, a bowedcotyledon, . 3 inch in height, had an open furrow on the shaded side . 04inch in breadth; another cotyledon, only . 13 inch in height, had left afurrow . 02 inch in breadth. But the most curious case was that of acotyledon which had just protruded above the ground and was only . 03 inchin height, and this was found to be bowed in the direction of the light toa depth of . 2 of an inch beneath the surface. From what we know of theimpermeability of this sand to light, the upper illuminated part in theseseveral cases must have determined the curvature of the lower buriedportions. But an apparent cause of doubt may be suggested: as thecotyledons are continually circumnutating, they tend to form a minute[page 476]crack or furrow all round their bases, which would admit a little light onall sides; but this would not happen when they were illuminated laterally, for we know that they quickly bend towards a lateral light, and they thenpress so firmly against the sand on the illuminated side as to furrow it, and this would effectually exclude light on this side. Any light admittedon the opposite and shaded side, where an open furrow is formed, would tendto counteract the curvature towards the lamp or other source of the light. It may be added, that the use of fine moist sand, which yields easily topressure, was indispensable in the above experiments; for seedlings raisedin common soil, not kept especially damp, and exposed for 9 h. 30 m. To astrong lateral light, did not form an open furrow at their bases on theshaded side, and were not bowed beneath the surface. Perhaps the most striking proof of the action of the upper on the lowerpart of the cotyledons of Phalaris, when laterally illuminated, wasafforded by the blackened glass-tubes (before alluded to) with very narrowstripes of the varnish scraped off on one side, through which a littlelight was admitted. The breadth of these stripes or slits varied between. 01 and . 02 inch (. 25 and . 51 mm. ). Cotyledons with their upper halvesenclosed in such tubes were placed before a south-west window, in such aposition, that the scraped stripes did not directly face the window, butobliquely to one side. The seedlings were left exposed for 8 h. , before theclose of which time the many free seedlings in the same pots had becomegreatly bowed towards the window. Under these circumstances, the wholelower halves of the cotyledons, which had their summits enclosed in thetubes, were fully exposed to the light of the sky, whilst their upperhalves received exclusively or chiefly diffused light from the room, andthis only through a very narrow slit on one side. Now, if the curvature ofthe lower part had been determined by the illumination of this part, allthe cotyledons assuredly would have become curved towards the window; butthis was far from being the case. Tubes of the kind just described wereplaced on several occasions over the upper halves of 27 cotyledons; 14 ofthem remained all the time quite vertical; so that sufficient diffusedlight did not enter through the narrow slits to produce any effectwhatever; and they behaved in the same manner as if their upper halves hadbeen enclosed in completely blackened tubes. The lower halves of the 13other cotyledons became bowed[page 477]not directly in the line of the window, but obliquely towards it; onepointed at an angle of only 18o, but the remaining 12 at angles varyingbetween 45o and 62o from the line of the window. At the commencement of theexperiment, pins had been laid on the earth in the direction towards whichthe slits in the varnish faced; and in this direction alone a small amountof diffused light entered. At the close of the experiment, 7 of the bowedcotyledons pointed exactly in the line of the pins, and 6 of them in a linebetween that of the pins and that of the window. This intermediate positionis intelligible, for any light from the sky which entered obliquely throughthe slits would be much more efficient than the diffused light whichentered directly through them. After the 8 h. Exposure, the contrast inappearance between these 13 cotyledons and the many other seedlings in thesame pots, which were all (excepting the above 14 vertical ones) greatlybowed in straight and parallel lines towards the window, was extremelyremarkable. It is therefore certain that a little weak light striking theupper halves of the cotyledons of Phalaris, is far more potent indetermining the direction of the curvature of the lower halves, than thefull illumination of the latter during the whole time of exposure. 

In confirmation of the above results, the effect of thickly painting withIndian ink one side of the upper part of three cotyledons of Phalaris, fora length of . 2 inch from their tips, may be worth giving. These were placedso that the unpainted surface was directed not towards the window, but alittle to one side; and they all became bent towards the unpainted side, and from the line of the window by angles amounting to 31o, 35o, and 83o. The curvature in this direction extended down to their bases, although thewhole lower part was fully exposed to the light from the window. 

Finally, although there can be no doubt that the illumination of the upperpart of the cotyledons of Phalaris greatly affects the power and manner ofbending of the lower part, yet some observations seemed to render itprobable that the simultaneous stimulation of the lower part by lightgreatly favours, or is almost necessary, for its well-marked curvature; butour experiments were not conclusive, owing to the difficulty of excludinglight from the lower halves without mechanically preventing theircurvature. 

Avena sativa. --The cotyledons of this plant become quickly bowed towards alateral light, exactly like those of Phalaris. [page 478]Experiments similar to the foregoing ones were tried, and we will give theresults as briefly as possible. They are somewhat less conclusive than inthe case of Phalaris, and this may possibly be accounted for by thesensitive zone varying in extension, in a species so long cultivated andvariable as the common Oat. Cotyledons a little under three-quarters of aninch in height were selected for trial: six had their summits protectedfrom light by tin-foil caps, . 25 inch in depth, and two others by caps . 3inch in depth. Of these 8 cotyledons, five remained upright during 8 hoursof exposure, although their lower parts were fully exposed to the light allthe time; two were very slightly, and one considerably, bowed towards it. Caps only . 2 or . 22 inch in depth were placed over 4 other cotyledons, andnow only one remained upright, one was slightly, and two considerably bowedto the light. In this and the following cases all the free seedlings in thesame pots became greatly bowed to the light. 

Our next trial was made with short lengths of thin and fairly transparentquills; for glass-tubes of sufficient diameter to go over the cotyledonswould have been too heavy. Firstly, the summits of 13 cotyledons wereenclosed in unpainted quills, and of these 11 became greatly and 2 slightlybowed to the light; so that the mere act of enclosure did not prevent thelower part from becoming bowed. Secondly, the summits of 11 cotyledons wereenclosed in quills . 3 inch in length, painted so as to be impermeable tolight; of these, 7 did not become at all inclined towards the light, but 3of them were slightly bent more or less transversely with respect to theline of light, and these might perhaps have been altogether excluded; onealone was slightly bowed towards the light. Painted quills, . 25 inch inlength, were placed over the summits of 4 other cotyledons; of these, onealone remained upright, a second was slightly bowed, and the two others asmuch bowed to the light as the free seedlings in the same pots. These twolatter cases, considering that the caps were . 25 in length, areinexplicable. 

Lastly, the summits of 8 cotyledons were coated with flexible and highlytransparent gold-beaters' skin, and all became as much bowed to the lightas the free seedlings. The summits of 9 other cotyledons were similarlycoated with gold-beaters' skin, which was then painted to a depth ofbetween . 25 and . 3 inch, so as to be impermeable to light; of these 5remained upright, and 4 were well bowed to the light, almost or quite aswell as[page 479]the free seedlings. These latter four cases, as well as the two in the lastparagraph, offer a strong exception to the rule that the illumination ofthe upper part determines the curvature of the lower part. Nevertheless, 5of these 8 cotyledons remained quite upright, although their lower halveswere fully illuminated all the time; and it would almost be a prodigy tofind five free seedlings standing vertically after an exposure for severalhours to a lateral light. 

The cotyledons of Avena, like those of Phalaris, when growing in soft, damp, fine sand, leave an open crescentric furrow on the shaded side, afterbending to a lateral light; and they become bowed beneath the surface at adepth to which, as we know, light cannot penetrate. The arcs of the chordsof the buried bowed portions formed in two cases angles of 20o and 21o withthe perpendicular. The open furrows on the shaded side were, in four cases, . 008, . 016, . 024, and . 024 of an inch in breadth. Brassica oleracea (Common Red). --It will here be shown that the upper halfof the hypocotyl of the cabbage, when illuminated by a lateral light, determines the curvature of the lower half. It is necessary toexperimentise on young seedlings about half an inch or rather less inheight, for when grown to an inch and upwards the basal part ceases tobend. We first tried painting the hypocotyls with Indian ink, or cuttingoff their summits for various lengths; but these experiments are not worthgiving, though they confirm, as far as they can be trusted, the results ofthe following ones. These were made by folding gold-beaters' skin onceround the upper halves of young hypocotyls, and painting it thickly withIndian ink or with black grease. As a control experiment, the sametransparent skin, left unpainted, was folded round the upper halves of 12hypocotyls; and these all became greatly curved to the light, exceptingone, which was only moderately curved. Twenty other young hypocotyls hadthe skin round their upper halves painted, whilst their lower halves wereleft quite uncovered. These seedlings were then exposed, generally forbetween 7 and 8 h. , in a box blackened within and open in front, eitherbefore a south-west window or a paraffin lamp. This exposure was amplysufficient, as was shown by the strongly-marked heliotropism of all thefree seedlings in the same pots; nevertheless, some were left exposed tothe light for a much longer time. Of the 20 hypocotyls thus treated, 14remained quite upright, and 6 became slightly bowed to the light; but 2 ofthese latter cases were not really[page 480]exceptions, for on removing the skin the paint was found imperfect and waspenetrated by many small transparent spaces on the side which faced thelight. Moreover, in two other cases the painted skin did not extend quitehalfway down the hypocotyl. Although there was a wonderful contrast in theseveral pots between these 20 hypocotyls and the other many free seedlings, which were all greatly bowed down to their bases in the direction of thelight, some being almost prostrate on the ground. 

The most successful trial on any one day (included in the above results) isworth describing in detail. Six young seedlings were selected, thehypocotyls of which were nearly . 45 inch, excepting one, which was . 6 inchin height, measured from the bases of their petioles to the ground. Theirupper halves, judged as accurately as could be done by the eye, were foldedonce round with gold-beaters' skin, and this was painted thickly withIndian ink. They were exposed in an otherwise darkened room before a brightparaffin lamp, which stood on a level with the two pots containing theseedlings. They were first looked at after an interval of 5 h. 10 m. , andfive of the protected hypocotyls were found quite erect, the sixth beingvery slightly inclined to the light; whereas all the many free seedlings inthe same two pots were greatly bowed to the light. They were again examinedafter a continuous exposure to the light of 20 h. 35m. ; and now thecontrast between the two sets was wonderfully great; for the free seedlingshad their hypocotyls extended almost horizontally in the direction of thelight, and were curved down to the ground; whilst those with the upperhalves protected by the painted skin, but with their lower halves fullyexposed to the light, still remained quite upright, with the exception ofthe one which retained the same slight inclination to the light which ithad before. This latter seedling was found to have been rather badlypainted, for on the side facing the light the red colour of the hypocotylcould be distinguished through the paint. 

We next tried nine older seedlings, the hypocotyls of which varied between1 and 1. 6 inch in height. The gold-beaters' skin round their upper partswas painted with black grease to a depth of only . 3 inch, that is, fromless than a third to a fourth or fifth of their total heights. They wereexposed to the light for 7 h. 15 m. ; and the result showed that the wholeof the sensitive zone, which determines the curvature of the lower[page 481]part, was not protected from the action of the light; for all 9 becamecurved towards it, 4 of them very slightly, 3 moderately, and 2 almost asmuch as the unprotected seedlings. Nevertheless, the whole 9 taken togetherdiffered plainly in their degree of curvature from the many free seedlings, and from some which were wrapped in unpainted skin, growing in the same twopots. 

Seeds were covered with about a quarter of an inch of the fine sanddescribed under Phalaris; and when the hypocotyls had grown to a height ofbetween . 4 and . 55 inch, they were exposed during 9 h. Before a paraffinlamp, their bases being at first closely surrounded by the damp sand. Theyall became bowed down to the ground, so that their upper parts lay near toand almost parallel to the surface of the soil. On the side of the lighttheir bases were in close contact with the sand, which was here a verylittle heaped up; on the opposite or shaded side there were open, crescentic cracks or furrows, rather above . 01 of an inch in width; butthey were not so sharp and regular as those made by Phalaris and Avena, andtherefore could not be so easily measured under the microscope. Thehypocotyls were found, when the sand was removed on one side, to be curvedto a depth beneath the surface in three cases of at least . 1 inch, in afourth case of . 11, and in a fifth of . 15 inch. The chords of the arcs ofthe short, buried, bowed portions formed angles of between 11o and 15o withthe perpendicular. From what we have seen of the impermeability of thissand to light, the curvature of the hypocotyls certainly extended down to adepth where no light could enter; and the curvature must have been causedby an influence transmitted from the upper illuminated part. 

The lower halves of five young hypocotyls were surrounded by unpaintedgold-beaters' skin, and these, after an exposure of 8 h. Before a paraffinlamp, all became as much bowed to the light as the free seedlings. Thelower halves of 10 other young hypocotyls, similarly surrounded with theskin, were thickly painted with Indian ink; their upper and unprotectedhalves became well curved to the light, but their lower and protectedhalves remained vertical in all the cases excepting one, and on this thelayer of paint was imperfect. This result seems to prove that the influencetransmitted from the upper part is not sufficient to cause the lower partto bend, unless it be at the same time illuminated; but there remains thedoubt, as in[page 482]the case of Phalaris, whether the skin covered with a rather thick crust ofdry Indian ink did not mechanically prevent their curvature. 

Beta vulgaris. --A few analogous experiments were tried on this plant, whichis not very well adapted for the purpose, as the basal part of thehypocotyl, after it has grown to above half an inch in height, does notbend much on exposure to a lateral light. Four hypocotyls were surroundedclose beneath their petioles with strips of thin tin-foil, . 2 inch inbreadth, and they remained upright all day before a paraffin lamp; twoothers were surrounded with strips . 15 inch in breadth, and one of theseremained upright, the other becoming bowed; the bandages in two other caseswere only . 1 inch in breadth, and both of these hypocotyls became bowed, though one only slightly, towards the light. The free seedlings in the samepots were all fairly well curved towards the light; and during thefollowing night became nearly upright. The pots were now turned round andplaced before a window, so that the opposite sides of the seedlings wereexposed to the light, towards which all the unprotected hypocotyls becamebent in the course of 7 h. Seven out of the 8 seedlings with bandages oftin-foil remained upright, but one which had a bandage only . 1 inch inbreadth, became curved to the light. On another occasion, the upper halvesof 7 hypocotyls were surrounded with painted gold-beaters' skin; of these 4remained upright, and 3 became a little curved to the light: at the sametime 4 other seedlings surrounded with unpainted skin, as well as the freeones in the same pots, all became bowed towards the lamp, before which theyhad been exposed during 22 hours. 

Radicles of Sinapis alba. --The radicles of some plants are indifferent, asfar as curvature is concerned, to the action of light; whilst others bendtowards and others from it. * Whether these movements are of any service tothe plant is very doubtful, at least in the case of subterranean roots;they probably result from the radicles being sensitive to contact, moisture, and gravitation, and as a consequence to other irritants whichare never naturally encountered. The radicles of Sinapis alba, whenimmersed in water and exposed to a lateral light, bend from it, or areapheliotropic. They become bent for a length of about 4 mm. From theirtips. To ascertain whether this movement

* Sachs, 'Physiologie Végétale, ' 1868, p. 44. [page 483]

generally occurred, 41 radicles, which had germinated in damp sawdust, wereimmersed in water and exposed to a lateral light; and they all, with twodoubtful exceptions, became curved from the light. At the same time thetips of 54 other radicles, similarly exposed, were just touched withnitrate of silver. They were blackened for a length of from . 05 to . 07 mm. , and probably killed; but it should be observed that this did not checkmaterially, if at all, the growth of the upper part; for several, whichwere measured, increased in the course of only 8 -9 h. By 5 to 7 mm. Inlength. Of the 54 cauterised radicles one case was doubtful, 25 curvedthemselves from the light in the normal manner, and 28, or more than half, were not in the least apheliotropic. There was a considerable difference, which we cannot account for, in the results of the experiments triedtowards the end of April and in the middle of September. Fifteen radicles(part of the above 54) were cauterised at the former period and wereexposed to sunshine, of which 12 failed to be apheliotropic, 2 were stillapheliotropic, and 1 was doubtful. In September, 39 cauterised radicleswere exposed to a northern light, being kept at a proper temperature; andnow 23 continued to be apheliotropic in the normal manner, and only 16failed to bend from the light. Looking at the aggregate results at bothperiods, there can be no doubt that the destruction of the tip for lessthan a millimeter in length destroyed in more than half the cases theirpower of moving from the light. It is probable that if the tips had beencauterised for the length of a whole millimeter, all signs ofapheliotropism would have disappeared. It may be suggested that althoughthe application of caustic does not stop growth, yet enough may be absorbedto destroy the power of movement in the upper part; but this suggestionmust be rejected, for we have seen and shall again see, that cauterisingone side of the tip of various kinds of radicles actually excites movement. The conclusion seems inevitable that sensitiveness to light resides in thetip of the radicle of Sinapis alba; and that the tip when thus stimulatedtransmits some influence to the upper part, causing it to bend. The case inthis respect is parallel with that of the radicles of several plants, thetips of which are sensitive to contact and to other irritants, and, as willbe shown in the eleventh chapter, to gravitation. [page 484]

CONCLUDING REMARKS AND SUMMARY OF CHAPTER. 

We do not know whether it is a general rule with seedling plants that theillumination of the upper part determines the curvature of the lower part. But as this occurred in the four species examined by us, belonging to suchdistinct families as the Gramineae, Cruciferae, and Chenopodeae, it isprobably of common occurrence. It can hardly fail to be of service toseedlings, by aiding them to find the shortest path from the buried seed tothe light, on nearly the same principle that the eyes of most of the lowercrawling animals are seated at the anterior ends of their bodies. It isextremely doubtful whether with fully developed plants the illumination ofone part ever affects the curvature of another part. The summits of 5 youngplants of Asparagus officinalis (varying in height between 1. 1 and 2. 7inches, and consisting of several short internodes) were covered with capsof tin-foil from 0. 3 to 0. 35 inch in depth; and the lower uncovered partsbecame as much curved towards a lateral light, as were the free seedlingsin the same pots. Other seedlings of the same plant had their summitspainted with Indian ink with the same negative result. Pieces of blackenedpaper were gummed to the edges and over the blades of some leaves on youngplants of Tropaeolum majus and Ranunculus ficaria; these were then placedin a box before a window, and the petioles of the protected leaves becamecurved towards the light, as much as those of the unprotected leaves. 

The foregoing cases with respect to seedling plants have been fullydescribed, not only because the transmission of any effect from light is anew physiological fact, but because we think it tends to modify somewhatthe current views on heliotropic movements. Until[page 485]lately such movements were believed to result simply from increased growthon the shaded side. At present it is commonly admitted* that diminishedlight increases the turgescence of the cells, or the extensibility of thecell-walls, or of both together, on the shaded side, and that this isfollowed by increased growth. But Pfeffer has shown that a difference inthe turgescence on the two sides of a pulvinus, --that is, an aggregate ofsmall cells which have ceased to grow at an early age, --is excited by adifference in the amount of light received by the two sides; and thatmovement is thus caused without being followed by increased growth on themore turgescent side. ** All observers apparently believe that light actsdirectly on the part which bends, but we have seen with the above describedseedlings that this is not the case. Their lower halves were brightlyilluminated for hours, and yet did not bend in the least towards the light, though this is the part which under ordinary circumstances bends the most. It is a still more striking fact, that the faint illumination of a narrowstripe on one side of the upper part of the cotyledons of Phalarisdetermined the direction of the curvature of the lower part; so that thislatter part did not bend towards the bright light by which it had beenfully illuminated, * Emil Godlewski has given ('Bot. Zeitung, ' 1879, Nos. 6-9) an excellentaccount (p. 120) of the present state of the question. See also Vines in'Arbeiten des Bot. Inst. In Würzburg, ' 1878, B. Ii. Pp. 114-147. Hugo deVries has recently published a still more important article on thissubject: 'Bot Zeitung, ' Dec. 19th and 26th, 1879. 

** 'Die Periodischen Bewegungen der Blattorgane, ' 1875, pp. 7, 63, 123, etc. Frank has also insisted ('Die Naturliche wägerechte Richtung vonPflanzentheilen, ' 1870, p. 53) on the important part which the pulvini ofthe leaflets of compound leaves play in placing the leaflets in a properposition with respect to the light. This holds good, especially with theleaves of climbing plants, which are carried into all sorts of positions, ill-adapted for the action of the light. [page 486]

but obliquely towards one side where only a little light entered. Theseresults seem to imply the presence of some matter in the upper part whichis acted on by light, and which transmits its effects to the lower part. Ithas been shown that this transmission is independent of the bending of theupper sensitive part. We have an analogous case of transmission in Drosera, for when a gland is irritated, the basal and not the upper or intermediatepart of the tentacle bends. The flexible and sensitive filament of Dionaealikewise transmits a stimulus, without itself bending; as does the stem ofMimosa. 

Light exerts a powerful influence on most vegetable tissues, and there canbe no doubt that it generally tends to check their growth. But when the twosides of a plant are illuminated in a slightly different degree, it doesnot necessarily follow that the bending towards the illuminated side iscaused by changes in the tissues of the same nature as those which lead toincreased growth in darkness. We know at least that a part may bend fromthe light, and yet its growth may not be favoured by light. This is thecase with the radicles of Sinapis alba, which are plainly apheliotropic;nevertheless, they grow quicker in darkness than in light. * So it is withmany aërial roots, according to Wiesner;** but there are other opposedcases. It appears, therefore, that light does not determine the growth ofapheliotropic parts in any uniform manner. 

We should bear in mind that the power of bending to the light is highlybeneficial to most plants. There

* Francis Darwin, 'Über das Wachsthum negativ heliotropischer Wurzeln':'Arbeiten des Bot. Inst. In Würzburg, ' B. Ii. , Heft iii. , 1880, p. 521. 

** 'Sitzb. Der k. Akad. Der Wissensch' (Vienna), 1880, p. 12. [page 487]

is therefore no improbability in this power having been specially acquired. In several respects light seems to act on plants in nearly the same manneras it does on animals by means of the nervous system. * With seedlings theeffect, as we have just seen, is transmitted from one part to another. Ananimal may be excited to move by a very small amount of light; and it hasbeen shown that a difference in the illumination of the two sides of thecotyledons of Phalaris, which could not be distinguished by the human eye, sufficed to cause them to bend. It has also been shown that there is noclose parallelism between the amount of light which acts on a plant and itsdegree of curvature; it was indeed hardly possible to perceive anydifference in the curvature of some seedlings of Phalaris exposed to alight, which, though dim, was very much brighter than that to which othershad been exposed. The retina, after being stimulated by a bright light, feels the effect for some time; and Phalaris continued to bend for nearlyhalf an hour towards the side which had been illuminated. The retina cannotperceive a dim light after it has been exposed to a bright one; and plantswhich had been kept in the daylight during the previous day and morning, did not move so soon towards an obscure lateral light as did others whichhad been kept in complete darkness. 

Even if light does act in such a manner on the growing parts of plants asalways to excite in them a tendency to bend towards the more illuminatedside--a supposition contradicted by the foregoing experiments on seedlingsand by all apheliotropic* Sachs has made some striking remarks to the same effect with respect tothe various stimuli which excite movement in plants. See his paper 'Ueberorthotrope und plagiotrope Pflanzentheile, ' 'Arb. Des Bot. Inst. InWürzburg, ' 1879, B. Ii. P. 282. [page 488]

organs--yet the tendency differs greatly in different species, and isvariable in degree in the individuals of the same species, as may be seenin almost any pot of seedlings of a long cultivated plant. * There istherefore a basis for the modification of this tendency to almost anybeneficial extent. That it has been modified, we see in many cases: thus, it is of more importance for insectivorous plants to place their leaves inthe best position for catching insects than to turn their leaves to thelight, and they have no such power. If the stems of twining plants were tobend towards the light, they would often be drawn away from their supports;and as we have seen they do not thus bend. As the stems of most otherplants are heliotropic, we may feel almost sure that twining plants, whichare distributed throughout the whole vascular series, have lost a powerthat their non-climbing progenitors possessed. Moreover, with Ipomoea, andprobably all other twiners, the stem of the young plant, before it beginsto twine, is highly heliotropic, evidently in order to expose thecotyledons or the first true leaves fully to the light. With the Ivy thestems of seedlings are moderately heliotropic, whilst those of the sameplants when grown a little older

* Strasburger has shown in his interesting work ('Wirkung des Lichtes... AufSchwärmsporen, ' 1878), that the movement of the swarm-spores of variouslowly organised plants to a lateral light is influenced by their stage ofdevelopment, by the temperature to which they are subjected, by the degreeof illumination under which they have been raised, and by other unknowncauses; so that the swarm-spores of the same species may move across thefield of the microscope either to or from the light. Some individuals, moreover, appear to be indifferent to the light; and those of differentspecies behave very differently. The brighter the light, the straighter istheir course. They exhibit also for a short time the after-effects oflight. In all these respects they resemble the higher plants. See, also, Stahl, 'Ueber den einfluss der Lichts auf die Bewegungs-erscheinungen derSchwärmsporen' Verh. D. Phys. -med. Geselsshalft in Würzburg, B. Xii. 1878. [page 489]

are apheliotropic. Some tendrils which consist of modified leaves--organsin all ordinary cases strongly diaheliotropic--have been renderedapheliotropic, and their tips crawl into any dark crevice. 

Even in the case of ordinary heliotropic movements, it is hardly crediblethat they result directly from the action of the light, without any specialadaptation. We may illustrate what we mean by the hygroscopic movements ofplants: if the tissues on one side of an organ permit of rapid evaporation, they will dry quickly and contract, causing the part to bend to this side. Now the wonderfully complex movements of the pollinia of Orchispyramidalis, by which they clasp the proboscis of a moth and afterwardschange their position for the sake of depositing the pollen-masses on thedouble stigma--or again the twisting movements, by which certain seeds burythemselves in the ground*--follow from the manner of drying of the parts inquestion; yet no one will suppose that these results have been gainedwithout special adaptation. Similarly, we are led to believe in adaptationwhen we see the hypocotyl of a seedling, which contains chlorophyll, bending to the light; for although it thus receives less light, being nowshaded by its own cotyledons, it places them--the more important organs--inthe best position to be fully illuminated. The hypocotyl may therefore besaid to sacrifice itself for the good of the cotyledons, or rather of thewhole plant. But if it be prevented from bending, as must sometimes occurwith seedlings springing up in an entangled mass of vegetation, thecotyledons themselves bend so as to face the light; the one farthest offrising

* Francis Darwin, 'On the Hygroscopic Mechanism, ' etc. , 'Transactions Linn. Soc. , ' series ii. Vol. I. P. 149, 1876. [page 490]

up, and that nearest to the light sinking down, or both twistinglaterally. * We may, also, suspect that the extreme sensitiveness to lightof the upper part of the sheath-like cotyledons of the Gramineae, and theirpower of transmitting its effects to the lower part, are specialisedarrangements for finding the shortest path to the light. With plantsgrowing on a bank, or thrown prostrate by the wind, the manner in which theleaves move, even rotating on their own axes, so that their upper surfacesmay be again directed to the light, is a striking phenomenon. Such factsare rendered more striking when we remember that too intense a lightinjures the chlorophyll, and that the leaflets of several Leguminosae whenthus exposed bend upwards and present their edges to the sun, thus escapinginjury. On the other hand, the leaflets of Averrhoa and Oxalis, whensimilarly exposed, bend downwards. 

It was shown in the last chapter that heliotropism is a modified form ofcircumnutation; and as every growing part of every plant circumnutates moreor less, we can understand how it is that the power of bending to the lighthas been acquired by such a multitude of plants throughout the vegetablekingdom. The manner in which a circumnutating movement--that is, oneconsisting of a succession of irregular ellipses or loops--is graduallyconverted into a rectilinear course towards the light, has been alreadyexplained. First, we have a succession of ellipses with their longer axesdirected towards the light, each of which

* Wiesner has made remarks to nearly the same effect with respect toleaves: 'Die undulirende Nutation der Internodien, ' p. 6, extracted from B. Lxxvii. (1878). Sitb. Der k. Akad. Der Wissensch. Wien. [page 491]

is described nearer and nearer to its source; then the loops are drawn outinto a strongly pronounced zigzag line, with here and there a small loopstill formed. At the same time that the movement towards the light isincreased in extent and accelerated, that in the opposite direction islessened and retarded, and at last stopped. The zigzag movement to eitherside is likewise gradually lessened, so that finally the course becomesrectilinear. Thus under the stimulus of a fairly bright light there is nouseless expenditure of force. 

As with plants every character is more or less variable, there seems to beno great difficulty in believing that their circumnutating movements mayhave been increased or modified in any beneficial manner by thepreservation of varying individuals. The inheritance of habitual movementsis a necessary contingent for this process of selection, or the survival ofthe fittest; and we have seen good reason to believe that habitualmovements are inherited by plants. In the case of twining species thecircumnutating movements have been increased in amplitude and rendered morecircular; the stimulus being here an internal or innate one. With sleepingplants the movements have been increased in amplitude and often changed indirection; and here the stimulus is the alternation of light and darkness, aided, however, by inheritance. In the case of heliotropism, the stimulusis the unequal illumination of the two sides of the plant, and thisdetermines, as in the foregoing cases, the modification of thecircumnutating movement in such a manner that the organ bends to the light. A plant which has been rendered heliotropic by the above means, mightreadily lose this tendency, judging from the cases already given, as soonas it became useless or[page 492]injurious. A species which has ceased to be heliotropic might also berendered apheliotropic by the preservation of the individuals which tendedto circumnutate (though the cause of this and most other variations isunknown) in a direction more or less opposed to that whence the lightproceeded. In like manner a plant might be rendered diaheliotropic. [page 493]

CHAPTER X. 

MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION. 

Means of observation - Apogeotropism--Cytisus--Verbena--Beta--Gradualconversion of the movement of circumnutation into apogeotropism in Rubus, Lilium, Phalaris, Avena, and Brassica--Apogeotropism retarded byheliotropism--Effected by the aid of joints or pulvini--Movements offlower-peduncles of Oxalis--General remarks on apogeotropism--Geotropism--Movements of radicles--Burying of seed-capsules--Use of process--Trifoliumsubterraneum--Arachis--Amphicarpaea--Diageotropism--Conclusion

OUR object in the present chapter is to show that geotropism, apogeotropism, and diageotropism are modified forms of circumnutation. Extremely fine filaments of glass, bearing two minute triangles of paper, were fixed to the summits of young stems, frequently to the hypocotyls ofseedlings, to flower-peduncles, radicles, etc. , and the movements of theparts were then traced in the manner already described on vertical andhorizontal glass-plates. It should be remembered that as the stems or otherparts become more and more oblique with respect to the glasses, the figurestraced on them necessarily become more and more magnified. The plants wereprotected from light, excepting whilst each observation was being made, andthen the light, which was always a dim one, was allowed to enter so as tointerfere as little as possible with the movement in progress; and we didnot detect any evidence of such interference. 

When observing the gradations between circumnu-[page 494]tation and heliotropism, we had the great advantage of being able to lessenthe light; but with geotropism analogous experiments were of courseimpossible. We could, however, observe the movements of stems placed atfirst only a little from the perpendicular, in which case geotropism didnot act with nearly so much power, as when the stems were horizontal and atright angles to the force. Plants, also, were selected which were butfeebly geotropic or apogeotropic, or had become so from having grown ratherold. Another plan was to place the stems at first so that they pointed 30or 40o beneath the horizon, and then apogeotropism had a great amount ofwork to do before the stem was rendered upright; and in this case ordinarycircumnutation was often not wholly obliterated. Another plan was toobserve in the evening plants which during the day had become greatlycurved heliotropically; for their stems under the gradually waning lightvery slowly became upright through the action of apogeotropism; and in thiscase modified circumnutation was sometimes well displayed. 

[Apogeotropism. --Plants were selected for observation almost by chance, excepting that they were taken from widely different families. If the stemof a plant which is even moderately sensitive to apogeotropism be placedhorizontally, the upper growing part bends quickly upwards, so as to becomeperpendicular; and the line traced by joining the dots successively made ona glass-plate, is generally almost straight. For instance, a young Cytisusfragrans, 12 inches in height, was placed so that the stem projected 10obeneath the horizon, and its course was traced during 72 h. At first itbent a very little downwards (Fig. 182), owing no doubt to the weight ofthe stem, as this occurred with most of the other plants observed, though, as they were of course circumnutating, the short downward lines were oftenoblique. After three-quarters of an hour the stem began to curve upwards, quickly during the first two hours, but much more slowly during theafternoon and night, [page 495]and on the following day. During the second night it fell a little, andcircumnutated during the following day; but it also moved a short distanceto the right, which was caused by a little light having been accidentallyadmitted on this side. The stem was now inclined 60o above the horizon, andhad therefore risen 70o. With time allowed it would probably have becomeupright, and no doubt would have continued circumnutating. The soleremarkable feature in the figure here given is the straightness of thecourse pursued. The stem, however, did not move upwards at an equable rate, and it sometimes stood almost or quite still. Such periods probablyrepresent attempts to circumnutate in a direction opposite toapogeotropism. 

Fig. 182. Cytisus fragrans: apogeotropic movement of stem from 10o beneathto 60o above horizon, traced on vertical glass, from 8. 30 A. M. March 12thto 10. 30 P. M. 13th. The subsequent circumnutating movement is likewiseshown up to 6. 45 A. M. On the 15th. Nocturnal course represented, as usual, by a broken line. Movement not greatly magnified, and tracing reduced totwo-thirds of original scale. 

 The herbaceous stem of a Verbena melindres (?) laid horizontally, rose in7 h. So much that it could no longer be observed on the vertical glasswhich stood in front of the plant. The long line which was traced wasalmost absolutely straight. After the 7 h. It still continued to rise, butnow circumnutated slightly. On the following day it stood upright, andcircumnutated regularly, as shown in Fig. 82, given in the fourth chapter. The stems of several other plants which were highly sensitive toapogeotropism rose up in almost straight lines, and[page 496]then suddenly began to circumnutate. A partially etiolated and somewhat oldhypocotyl of a seedling cabbage (2 3/4 inches in height) was so sensitivethat when placed at an angle of only 23o from the perpendicular, it becamevertical in 33 minutes. As it could not have been strongly acted upon byapogeotropism in the above slightly inclined position, we expected that itwould have circumnutated, or at least have moved in a zigzag course. Accordingly, dots were made every 3 minutes; but, when these were joined, the line was nearly straight. After this hypocotyl had become upright itstill moved onwards for half an hour in the same general direction, but ina zigzag manner. During the succeeding 9 h. It circumnutated regularly, anddescribed 3 large ellipses. In this case apogeotropism, although acting ata very unfavourable angle, quite overcame the ordinary circumnutatingmovement. 

Fig. 183. Beta vulgaris: apogeotropic movement of hypocotyl from 19obeneath horizon to a vertical position, with subsequent circumnutation, traced on a vertical and on a horizontal glass-plate, from 8. 28 A. M. Sept. 28th to 8. 40 A. M. 29th. Figure reduced to one-third of original scale. 

The hypocotyls of Beta vulgaris are highly sensitive to apogeotropism. Onewas placed so as to project 19o beneath the horizon; it fell at first avery little (see Fig. 183), no doubt owing to its weight; but as it wascircumnutating the line was[page 497]oblique. During the next 3 h. 8 m. It rose in a nearly straight line, passing through an angle of 109o, and then (at 12. 3 P. M. ) stood upright. Itcontinued for 55 m. To move in the same general direction beyond theperpendicular, but in a zigzag course. It returned also in a zigzag line, and then circumnutated regularly, describing three large ellipses duringthe remainder of the day. It should be observed that the ellipses in thisfigure are exaggerated in size, relatively to the length of the upwardstraight line, owing to the position of the vertical and horizontalglass-plates. Another and somewhat old hypocotyl was placed so as to standat only 31o from the perpendicular, in which position apogeotropism actedon it with little force, and its course accordingly was slightly zigzag. 

The sheath-like cotyledons of Phalaris Canariensis are extremely sensitiveto apogeotropism. One was placed so as to project 40o beneath the horizon. Although it was rather old and 1. 3 inch in height, it became vertical in 4h. 30 m. , having passed through an angle of 130o in a nearly straight line. It then suddenly began to circumnutate in the ordinary manner. Thecotyledons of this plant, after the first leaf has begun to protrude, arebut slightly apogeotropic, though they still continue to circumnutate. Oneat this stage of development was placed horizontally, and did not becomeupright even after 13 h. , and its course was slightly zigzag. So, again, arather old hypocotyl of Cassia tora (1 1/4 inch in height) required 28 h. To become upright, and its course was distinctly zigzag; whilst youngerhypocotyls moved much more quickly and in a nearly straight line. 

When a horizontally placed stem or other organ rises in a zigzag line, wemay infer from the many cases given in our previous chapters, that we havea modified form of circumnutation; but when the course is straight, thereis no evidence of circumnutation, and any one might maintain that thislatter movement had been replaced by one of a wholly distinct kind. Thisview seems the more probable when (as sometimes occurred with thehypocotyls of Brassica and Beta, the stems of Cucurbita, and the cotyledonsof Phalaris) the part in question, after bending up in a straight course, suddenly begins to circumnutate to the full extent and in the usual manner. A fairly good instance of a sudden change of this kind--that is, from anearly straight upward movement to one of circumnutation--is shown in Fig. 183; but more striking instances were occasionally observed with Beta, Brassica, and Phalaris. 

We will now describe a few cases in which it may be[page 498]seen how gradually circumnutation becomes changed into apogeotropism, undercircumstances to be specified in each instance. 

Rubus idaeus (hybrid). --A young plant, 11 inches in height, growing in apot, was placed horizontally; and the upward movement was traced duringnearly 70 h. ; but the plant, though growing vigorously, was not highlysensitive to apogeotropism, or it was not capable of quick movement, forduring the above time it rose only 67o. We may see in the diagram (Fig. 184) that during the first day of 12 h. It rose in a nearly straight line. When placed horizontally, it was evidently circumnutating, for it rose atfirst a little, notwithstanding the weight of the stem, and then sank down;so that it did not start on its permanently upward course until 1 h. 25 m. Had elapsed. On the second day, by which time it had risen considerably, and when apogeotropism acted on it with somewhat less power, its courseduring 15 ½ h. Was clearly zigzag, and the rate of the upward movement wasnot equable. During the third day, also of 15 ½ h. , when apogeotropismacted on it with still less power, the stem plainly circumnutated, for itmoved during this day 3 times up and 3 times down, 4 times to the left and4 to the right. But the course was so complex that it could hardly betraced on the glass. We can, however, see that the successively formedirregular ellipses rose higher and higher. Apogeotropism continued to acton the fourth morning, as the stem was still rising, though it now stoodonly 23o from the perpendicular. In this diagram the several stages may befollowed by which an almost rectilinear, upward, apogeotropic course firstbecomes zigzag, and then changes into a circumnutating movement, with mostof the successively formed, irregular ellipses directed upwards. 

Fig 184: Rubus idaeus (hybrid): apogeotropic movement of stem, traced on avertical glass during 3 days and 3 nights, from 10. 40 A. M. March 18th to 8A. M. 21st. Figure reduced to one-half of the original scale. 

Lilium auratum. --A plant 23 inches in height was placed[page 499]horizontally, and the upper part of the stem rose 58o in 46 h. , in themanner shown in the accompanying diagram (Fig. 185). We here see thatduring the whole of the second day of 15 ½ h. , the stem plainlycircumnutated whilst bending upwards through apogeotropism. It had still torise considerably, for when the last dot in the figure was made, it stood32o from an upright position. 

Fig. 185. Lilium auratum: apogeotropic movement of stem, traced on avertical glass during 2 days and 2 nights, from 10. 40 A. M. March 18th to 8A. M. 20th. Figure reduced to one-half of the original scale. 

Phalaris Canariensis. --A cotyledon of this plant (1. 3 inch in height) hasalready been described as rising in 4 h. 30 m. From 40o beneath the horizoninto a vertical position, passing through an angle of 130o in a nearlystraight line, and then abruptly beginning to circumnutate. Anothersomewhat old cotyledon of the same height (but from which a true leaf hadnot yet protruded), was similarly placed at 40o beneath the horizon. Forthe first 4 h. It rose in a nearly straight course (Fig. 186), so that by1. 10 P. M. It was highly inclined, and now apogeotropism acted on it withmuch less power than before, and it began to zigzag. At 4. 15 P. M. (i. E. In7 h. From the commencement) it stood vertically, and afterwards continuedto circumnutate in the usual manner about the same spot. Here then we havea graduated change from a straight upward apogeotropic course intocircumnutation, instead of an abrupt change, as in the former case. 

Avena sativa. --The sheath-like cotyledons, whilst young, are stronglyapogeotropic; and some which were placed at 45o beneath the horizon rose90o in 7 or 8 h. In lines almost absolutely straight. An oldish cotyledon, from which the first leaf began to[page 500]protrude whilst the following observations were being made, was placed at10o beneath the horizon, and it rose only 59o in 24h. It behaved ratherdifferently from any other plant, observed by us, for during the first 4 ½h. It rose in a line not far from straight; during the next 6 ½ h. Itcircumnutated, that is, it descended and again ascended in a stronglymarked zigzag course; it then resumed its upward movement in a moderatelystraight line, and, with time allowed, no doubt would have become upright. In this case, after the first 4 ½ h. , ordinary circumnutation almostcompletely conquered for a time apogeotropism. 

Fig 186. Phalaris Canariensis: apogeotropic movement of cotyledon, tracedon a vertical and horizontal glass, from 9. 10 A. M. Sept. 19th to 9 A. M. 20th. Figure here reduced to one-fifth of original scale. 

Brassica oleracea. --The hypocotyls of several young seedlings placedhorizontally, rose up vertically in the course of 6 or 7 h. In nearlystraight lines. A seedling which had grown in darkness to a height of 2 1/4inches, and was therefore rather old and not highly sensitive, was placedso that the hypocotyl projected at between 30o and 40o beneath the horizon. The upper part alone became curved[page 501]upwards, and rose during the first 3 h. 10 m. In a nearly straight line(Fig. 187); but it was not possible to trace the upward movement on thevertical glass for the first 1 h. 10 m. , so that the nearly straight linein the diagram ought to have been much longer. During the next 11 h. Thehypocotyl circumnutated, describing irregular figures, each of which rose alittle above the one previously formed. During the night and followingearly morning it continued to rise in a zigzag course, so thatapogeotropism was still acting. At the close of our observations, after 23h. (represented by the highest dot in the diagram) the hypocotyl was still32o from the perpendicular. There can be little doubt that it wouldultimately have become upright by describing an additional number ofirregular ellipses, one above the other. 

Fig 187. Brassica oleracea: apogeotropic movement of hypocotyl, traced onvertical glass, from 9. 20 A. M. , Sept. 12th to 8. 30 A. M. 13th. The upperpart of the figure is more magnified than the lower part. If the wholecourse had been traced, the straight upright line would have been muchlonger. Figure here reduced to one-third of the original scale. 

Apogeotropism retarded by Heliotropism. --When the stem of any plant bendsduring the day towards a lateral light, the movement is opposed byapogeotropism; but as the light gradually wanes in the evening the latterpower slowly gains the upper hand, and draws the stem back into a verticalposition. Here then we have a good opportunity for observing howapogeotropism acts when very nearly balanced by an opposing force. Forinstance, the plumule of Tropaeolum majus (see former Fig. 175) movedtowards the dim evening light in a slightly zigzag line until 6. 45 P. M. , itthen returned on its course until[page 502]10. 40 P. M. , during which time it zigzagged and described an ellipse ofconsiderable size. The hypocotyl of Brassica oleracea (see former Fig. 173)moved in a straight line to the light until 5. 15 P. M. , and then from thelight, making in its backward course a great rectangular bend, and thenreturned for a short distance towards the former source of the light; noobservations were made after 7. 10 P. M. , but during the night it recoveredits vertical position. A hypocotyl of Cassia tora moved in the evening in asomewhat zigzag line towards the failing light until 6 P. M. , and was nowbowed 20o from the perpendicular; it then returned on its course, makingbefore 10. 30 P. M. Four great, nearly rectangular bends and almostcompleting an ellipse. Several other analogous cases were casuallyobserved, and in all of them the apogeotropic movement could be seen toconsist of modified circumnutation. 

Apogeotropic Movements effected by the aid of joints or pulvini. --Movementsof this kind are well known to occur in the Gramineae, and are effected bymeans of the thickened bases of their sheathing leaves; the stem withinbeing in this part thinner than elsewhere. * According to the analogy of allother pulvini, such joints ought to continue circumnutating for a longperiod, after the adjoining parts have ceased to grow. We therefore wishedto ascertain whether this was the case with the Gramineae; for if so, theupward curvature of their stems, when extended horizontally or laidprostrate, would be explained in accordance with our view--namely, thatapogeotropism results from modified circumnutation. After these joints havecurved upwards, they are fixed in their new position by increased growthalong their lower sides. 

Lolium perenne. --A young stem, 7 inches in height, consisting of 3internodes, with the flower-head not yet protruded, was selected forobservation. A long and very thin glass filament was cemented horizontallyto the stem close above the second joint, 3 inches above the ground. Thisjoint was subsequently proved to be in an active condition, as its lowerside swelled much through the action of apogeotropism (in the mannerdescribed by De Vries) after the haulm had been fastened down for 24 h. Ina horizontal position. The pot was

* This structure has been recently described by De Vries in an interestingarticle, 'Ueber die Aufrichtung des gelagerten Getreides, ' in'Landwirthschaftliche Jahrbücher, ' 1880, p. 473. [page 503]

so placed that the end of the filament stood beneath the 2-inch objectglass of a microscope with an eye-piece micrometer, each division of whichequalled 1/500 of an inch. The end of the filament was repeatedly observedduring 6 h. , and was seen to be in constant movement; and it crossed 5divisions of the micrometer (1/100 inch) in 2 h. Occasionally it movedforwards by jerks, some of which were 1/1000 inch in length, and thenslowly retreated a little, afterwards again jerking forwards. Theseoscillations were exactly like those described under Brassica and Dionaea, but they occurred only occasionally. We may therefore conclude that thismoderately old joint was continually circumnutating on a small scale. 

Alopecurus pratensis. --A young plant, 11 inches in height, with theflower-head protruded, but with the florets not yet expanded, had a glassfilament fixed close above the second joint, at a height of only 2 inchesabove the ground. The basal internode, 2 inches in length, was cemented toa stick to prevent any possibility of its circumnutating. The extremity ofthe filament, which projected about 50o above the horizon, was oftenobserved during 24 h. In the same manner as in the last case. Wheneverlooked at, it was always in movement, and it crossed 30 divisions of themicrometer (3/50 inch) in 3 ½ h. ; but it sometimes moved at a quicker rate, for at one time it crossed 5 divisions in 1 ½ h. The pot had to be movedoccasionally, as the end of the filament travelled beyond the field ofvision; but as far as we could judge it followed during the daytime asemicircular course; and it certainly travelled in two different directionsat right angles to one another. It sometimes oscillated in the same manneras in the last species, some of the jerks forwards being as much as 1/1000of an inch. We may therefore conclude that the joints in this and the lastspecies of grass long continue to circumnutate; so that this movement wouldbe ready to be converted into an apogeotropic movement, whenever the stemwas placed in an inclined or horizontal position. 

Movements of the Flower-peduncles of Oxalis carnosa, due to apogeotropismand other forces. --The movements of the main peduncle, and of the three orfour sub-peduncles which each main peduncle of this plant bears, areextremely complex, and are determined by several distinct causes. Whilstthe flowers are expanded, both kinds of peduncles circumnutate about thesame spot, as we have seen (Fig. 91) in the fourth chapter. But soon afterthe flowers have begun to wither the sub-[page 504]peduncles bend downwards, and this is due to epinasty; for on two occasionswhen pots were laid horizontally, the sub-peduncles assumed the sameposition relatively to the main peduncle, as would have been the case ifthey had remained upright; that is, each of them formed with it an angle ofabout 40o. If they had been acted on by geotropism or apheliotropism (forthe plant was illuminated from above), they would have directed themselvesto the centre of the earth. A main peduncle was secured to a stick in anupright position, and one of the upright sub-peduncles which had beenobserved circumnutating whilst the flower was expanded, continued to do sofor at least 24 h. After it had withered. It then began to bend downwards, and after 36 h. Pointed a little beneath the horizon. A new figure was nowbegun (A, Fig. 188), and the sub-peduncle was traced descending in a zigzagline from 7. 20 P. M. On the 19th to 9 A. M. On the 22nd. It now pointedalmost perpendicularly downwards, and the glass filament had to be removedand fastened transversely across the base of the young capsule. We expectedthat the sub-peduncle would have been motionless in its new position; butit continued slowly to swing, like a pendulum, from side to side, that is, in a plane at right angles to that in which it had descended. Thiscircumnutating movement was observed from 9 A. M. On 22nd to 9 A. M. 24th, asshown at B in the diagram. We were not able to observe this particularsub-peduncle any longer; but it would certainly have gone on circumnutatinguntil the capsule was nearly ripe (which requires only a short time), andit would then have moved upwards. 

The upward movement (C, Fig. 188) is effected in part by the wholesub-peduncle rising in the same manner as it had previously descendedthrough epinasty--namely, at the joint where united to the main peduncle. As this upward movement occurred with plants kept in the dark and inwhatever position the main peduncle was fastened, it could not have beencaused by heliotropism or apogeotropism, but by hyponasty. Besides thismovement at the joint, there is another of a very different kind, for thesub-peduncle becomes upwardly bent in the middle part. If the sub-pedunclehappens at the time to be inclined much downwards, the upward curvature isso great that the whole forms a hook. The upper end bearing the capsule, thus always places itself upright, and as this occurs in darkness, and inwhatever position the main peduncle may have been secured, [page 505]the upward curvature cannot be due to heliotropism or hyponasty, but toapogeotropism. 

Fig. 188. Oxalis carnosa: movements of flower-peduncle, traced on avertical glass: A, epinastic downward movement; B, circumnutation whilstdepending vertically; C, subsequent upward movement, due to apogeotropismand hyponasty combined. [page 506]

In order to trace this upward movement, a filament was fixed to asub-peduncle bearing a capsule nearly ripe, which was beginning to bendupwards by the two means just described. Its course was traced (see C, Fig188) during 53 h. , by which time it had become nearly upright. The courseis seen to be strongly zigzag, together with some little loops. We maytherefore conclude that the movement consists of modified circumnutation. 

The several species of Oxalis probably profit in the following manner bytheir sub-peduncles first bending downwards and then upwards. They areknown to scatter their seeds by the bursting of the capsule; the walls ofwhich are so extremely thin, like silver paper, that they would easily bepermeated by rain. But as soon as the petals wither, the sepals rise up andenclose the young capsule, forming a perfect roof over it as soon as thesub-peduncle has bent itself downwards. By its subsequent upward movement, the capsule stands when ripe at a greater height above the ground by twicethe length of the sub-peduncle, than it did when dependent, and is thusable to scatter its seeds to a greater distance. The sepals, which enclosethe ovarium whilst it is young, present an additional adaptation byexpanding widely when the seeds are ripe, so as not to interfere with theirdispersal. In the case of Oxalis acetosella, the capsules are saidsometimes to bury themselves under loose leaves or moss on the ground, butthis cannot occur with those of O. Carnosa, as the woody stem is too high. 

Oxalis acetosella. --The peduncles are furnished with a joint in

Fig. 189. Oxalis acetosella: course pursued by the upper part of apeduncle, whilst rising, traced from 11 A. M. June 1st to 9 A. M. 3rd. Figurehere reduced to one-half of the original scale. 

the middle, so that the lower part answers to the main peduncle, [page 507]and the upper part to one of the sub-peduncles of O. Carnosa. The upperpart bends downwards, after the flower has begun to wither, and the wholepeduncle then forms a hook; that this bending is due to epinasty we mayinfer from the case of O. Carnosa. When the pod is nearly ripe, the upperpart straightens itself and becomes erect; and this is due to hyponasty orapogeotropism, or both combined, and not to heliotropism, for it occurredin darkness. The short, hooked part of the peduncle of a cleistogamicflower, bearing a pod nearly ripe, was observed in the dark during threedays. The apex of the pod at first pointed perpendicularly down, but in thecourse of three days rose 90o, so that it now projected horizontally. Thecourse during the two latter days is shown in Fig. 189; and it may be seenhow greatly the peduncle, whilst rising, circumnutated. The lines of chiefmovement were at right angles to the plane of the originally hooked part. The tracing was not continued any longer; but after two additional days, the peduncle with its capsule had become straight and stood upright. ]

Concluding Remarks on Apogeotropism. --When apogeotropism is rendered by anymeans feeble, it acts, as shown in the several foregoing cases, byincreasing the always present circumnutating movement in a directionopposed to gravity, and by diminishing that in the direction of gravity, aswell as that to either side. The upward movement thus becomes unequal inrate, and is sometimes interrupted by stationary periods. Wheneverirregular ellipses or loops are still formed, their longer axes are almostalways directed in the line of gravity, in an analogous manner as occurredwith heliotropic movements in reference to the light. As apogeotropism actsmore and more energetically, ellipses or loops cease to be formed, and thecourse becomes at first strongly, and then less and less zigzag, andfinally rectilinear. From this gradation in the nature of the movement, andmore especially from all growing parts, which alone (except when pulviniare present) are acted on by apogeotropism, con-[page 508]tinually circumnutating, we may conclude that even a rectilinear course ismerely an extremely modified form of circumnutation. It is remarkable thata stem or other organ which is highly sensitive to apogeotropism, and whichhas bowed itself rapidly upwards in a straight line, is often carriedbeyond the vertical, as if by momentum. It then bends a little backwards toa point round which it finally circumnutates. Two instances of this wereobserved with the hypocotyls of Beta vulgaris, one of which is shown inFig. 183, and two other instances with the hypocotyls of Brassica. Thismomentum-like movement probably results from the accumulated effects ofapogeotropism. For the sake of observing how long such after-effectslasted, a pot with seedlings of Beta was laid on its side in the dark, andthe hypocotyls in 3 h. 15 m. Became highly inclined. The pot, still in thedark, was then placed upright, and the movements of the two hypocotyls weretraced; one continued to bend in its former direction, now in opposition toapogeotropism, for about 37 m. , perhaps for 48 m. ; but after 61 m. It movedin an opposite direction. The other hypocotyl continued to move in itsformer course, after being placed upright, for at least 37 m. 

Different species and different parts of the same species are acted on byapogeotropism in very different degrees. Young seedlings, most of whichcircumnutate quickly and largely, bend upwards and become vertical in muchless time than do any older plants observed by us; but whether this is dueto their greater sensitiveness to apogeotropism, or merely to their greaterflexibility we do not know. A hypocotyl of Beta traversed an angle of 109oin 3 h. 8 m. , and a cotyledon of Phalaris an angle of 130o in 4 h. 30 m. Onthe other hand, the stem of a herbaceous[page 509]Verbena rose 90o in about 24 h. ; that of Rubus 67o, in 70 h. ; that ofCytisus 70o, in 72 h. ; that of a young American Oak only 37o, in 72 h. Thestem of a young Cyperus alternifolius rose only 11o in 96 h. ; the bendingbeing confined to near its base. Though the sheath-like cotyledons ofPhalaris are so extremely sensitive to apogeotropism, the first true leaveswhich protrude from them exhibited only a trace of this action. Two frondsof a fern, Nephrodium molle, both of them young and one with the tip stillinwardly curled, were kept in a horizontal position for 46 h. , and duringthis time they rose so little that it was doubtful whether there was anytrue apogeotropic movement. 

The most curious case known to us of a difference in sensitiveness togravitation, and consequently of movement, in different parts of the sameorgan, is that offered by the petioles of the cotyledons of Ipomoealeptophylla. The basal part for a short length where united to theundeveloped hypocotyl and radicle is strongly geotropic, whilst the wholeupper part is strongly apogeotropic. But a portion near the blades of thecotyledons is after a time acted on by epinasty and curves downwards, forthe sake of emerging in the form of an arch from the ground; itsubsequently straightens itself, and is then again acted on byapogeotropism. 

A branch of Cucurbita ovifera, placed horizontally, moved upwards during 7h. In a straight line, until it stood at 40o above the horizon; it thenbegan to circumnutate, as if owing to its trailing nature it had notendency to rise any higher. Another upright branch was secured to a stick, close to the base of a tendril, and the pot was then laid horizontally inthe dark. In this position the tendril circumnutated and made[page 510]several large ellipses during 14 h. , as it likewise did on the followingday; but during this whole time it was not in the least affected byapogeotropism. On the other hand, when branches of another Cucurbitaceousplant, Echinocytis lobata, were fixed in the dark so that the tendrilsdepended beneath the horizon, these began immediately to bend upwards, andwhilst thus moving they ceased to circumnutate in any plain manner; but assoon as they had become horizontal they recommenced to revolveconspicuously. * The tendrils of Passiflora gracilis are likewiseapogeotropic. Two branches were tied down so that their tendrils pointedmany degrees beneath the horizon. One was observed for 8 h. , during whichtime it rose, describing two circles, one above the other. The othertendril rose in a moderately straight line during the first 4 h. , makinghowever one small loop in its course; it then stood at about 45o above thehorizon, where it circumnutated during the remaining 8 h. Of observation. 

A part or organ which whilst young is extremely sensitive to apogeotropismceases to be so as it grows old; and it is remarkable, as showing theindependence of this sensitiveness and of the circumnutating movement, thatthe latter sometimes continues for a time after all power of bending fromthe centre of the earth has been lost. Thus a seedling Orange bearing only3 young leaves, with a rather stiff stem, did not curve in the leastupwards during 24 h. Whilst extended horizontally; yet it circumnutated allthe time over a small space. The hypocotyl of a young seedling of Cassiatora, similarly placed, became vertical in 12 h. ; that of an olderseedling, 1 1/4 inch in height, * For details see 'The Movements and Habits of Climbing Plants, ' 1875, p. 131. [page 511]

became so in 28 h. ; and that of another still older one, 1 ½ inch inheight, remained horizontal during two days, but distinctly circumnutatedduring this whole time. 

When the cotyledons of Phalaris or Avena are laid horizontally, theuppermost part first bends upwards, and then the lower part; consequently, after the lower part has become much curved upwards, the upper part iscompelled to curve backwards in an opposite direction, in order tostraighten itself and to stand vertically; and this subsequentstraightening process is likewise due to apogeotropism. The upper part of 8young cotyledons of Phalaris were made rigid by being cemented to thinglass rods, so that this part could not bend in the least; nevertheless, the basal part was not prevented from curving upward. A stem or other organwhich bends upwards through apogeotropism exerts considerable force; itsown weight, which has of course to be lifted, was sufficient in almostevery instance to cause the part at first to bend a little downwards; butthe downward course was often rendered oblique by the simultaneouscircumnutating movement. The cotyledons of Avena placed horizontally, besides lifting their own weight, were able to furrow the soft sand abovethem, so as to leave little crescentic open spaces on the lower sides oftheir bases; and this is a remarkable proof of the force exerted. 

As the tips of the cotyledons of Phalaris and Avena bend upwards throughthe action of apogeotropism before the basal part, and as these same tipswhen excited by a lateral light transmit some influence to the lower part, causing it to bend, we thought that the same rule might hold good withapogeotropism. Consequently, the tips of 7 cotyledons of Phalaris were[page 512]cut off for a length in three cases of . 2 inch and in the four other casesof . 14, . 12, . 1, and . 07 inch. But these cotyledons, after being extendedhorizontally, bowed themselves upwards as effectually as the unmutilatedspecimens in the same pots, showing that sensitiveness to gravitation isnot confined to their tips. 

GEOTROPISM. 

This movement is directly the reverse of apogeotropism. Many organs benddownwards through epinasty or apheliotropism or from their own weight; butwe have met with very few cases of a downward movement in sub-aërial organsdue to geotropism. We shall however, give one good instance in thefollowing section, in the case of Trifolium subterraneum, and probably inthat of Arachis hypogaea. 

On the other hand, all roots which penetrate the ground (including themodified root-like petioles of Megarrhiza and Ipomoea leptophylla) areguided in their downward course by geotropism; and so are many aërialroots, whilst others, as those of the Ivy, appear to be indifferent to itsaction. In our first chapter the movements of the radicles of severalseedlings were described. We may there see (Fig. 1) how a radicle of thecabbage, when pointing vertically upwards so as to be very little acted onby geotropism, circumnutated; and how another (Fig. 2) which was at firstplaced in an inclined position bowed itself downwards in a zigzag line, sometimes remaining stationary for a time. Two other radicles of thecabbage travelled downwards in almost rectilinear courses. A radicle of thebean placed upright (Fig. 20) made a great sweep and zigzagged; but as itsank downwards and was more strongly acted on by geotropism, it moved in an[page 513]almost straight course. A radicle of Cucurbita, directed upwards (Fig. 26), also zigzagged at first, and described small loops; it then moved in astraight line. Nearly the same result was observed with the radicles of Zeamays. But the best evidence of the intimate connection betweencircumnutation and geotropism was afforded by the radicles of Phaseolus, Vicia, and Quercus, and in a less degree by those of Zea and Aesculus (seeFigs. 18, 19, 21, 41, and 52); for when these were compelled to grow andslide down highly inclined surfaces of smoked glass, they left distinctlyserpentine tracks. 

[The Burying of Seed-capsules: Trifolium subterraneum. --The flower-heads ofthis plant are remarkable from producing only 3 or 4 perfect flowers, whichare situated exteriorly. All the other many flowers abort, and are modifiedinto rigid points, with a bundle of vessels running up their centres. Aftera time 5 long, elastic, claw-like projections, which represent thedivisions of the calyx, are developed on their summits. As soon as theperfect flowers wither they bend downwards, supposing the peduncle to standupright, and they then closely surround its upper part. This movement isdue to epinasty, as is likewise the case with the flowers of T. Repens. Theimperfect central flowers ultimately follow, one after the other, the samecourse. Whilst the perfect flowers are thus bending down, the wholepeduncle curves downwards and increases much in length, until theflower-head reaches the ground. Vaucher* says that when the plant is soplaced that the heads cannot soon reach the ground, the peduncles grow tothe extraordinary length of from 6 to 9 inches. In whatever position thebranches may be placed, the upper part of the peduncle at first bendsvertically upwards through heliotropism; but as soon as the flowers beginto wither the downward curvature of the whole peduncle commences. As thislatter movement occurred in complete darkness, and with peduncles arisingfrom upright and from dependent branches, it cannot be due toapheliotropism or to epinasty, but must be attributed to geotropism. Nineteen

* 'Hist. Phys. Des Plantes d'Europe, ' tom. Ii. 1841, p. 106. [page 514]

upright flower-heads, arising from branches in all sorts of positions, onplants growing in a warm greenhouse, were marked with thread, and after 24h. Six of them were vertically dependent; these therefore had travelledthrough 180o in this time. Ten were extended sub-horizontally, and thesehad moved through about 90o. Three very young peduncles had as yet movedonly a little downwards, but after an additional 24 h. Were greatlyinclined. 

At the time when the flower-heads reach the ground, the younger imperfectflowers in the centre are still pressed closely together, and form aconical projection; whereas the perfect and imperfect flowers on theoutside are upturned and closely surround the peduncle. They are thusadapted to offer as little resistance, as the case admits of, inpenetrating the ground, though the diameter of the flower-head is stillconsiderable. The means by which this penetration is effected willpresently be described. The flower-heads are able to bury themselves incommon garden mould, and easily in sand or in fine sifted cinders packedrather closely. The depth to which they penetrated, measured from thesurface to the base of the head, was between 1/4 and ½ inch, but in onecase rather above 0. 6 inch. With a plant kept in the house, a head partlyburied itself in sand in 6 h. : after 3 days only the tips of the reflexedcalyces were visible, and after 6 days the whole had disappeared. But withplants growing out of doors we believe, from casual observations, that theybury themselves in a much shorter time. 

After the heads have buried themselves, the central aborted flowersincrease considerably in length and rigidity, and become bleached. Theygradually curve, one after the other, upwards or towards the peduncle, inthe same manner as did the perfect flowers at first. In thus moving, thelong claws on their summits carry with them some earth. Hence a flower-headwhich has been buried for a sufficient time, forms a rather large ball, consisting of the aborted flowers, separated from one another by earth, andsurrounding the little pods (the product of the perfect flowers) which lieclose round the upper part of the peduncle. The calyces of the perfect andimperfect flowers are clothed with simple and multicellular hairs, whichhave the power of absorption; for when placed in a weak solution ofcarbonate of ammonia (2 gr. To 1 oz. Of water) their protoplasmic contentsimmediately became aggregated and afterwards displayed the usual slowmovements. This clover generally[page 515]grows in dry soil, but whether the power of absorption by the hairs on theburied flower-heads is of any importance to them we do not know. Only a fewof the flower-heads, which from their position are not able to reach theground and bury themselves, yield seeds; whereas the buried ones neverfailed, as far as we observed, to produce as many seeds as there had beenperfect flowers. 

We will now consider the movements of the peduncle whilst

Fig. 190. Trifolium subterraneum: downward movement of peduncle from 19obeneath the horizon to a nearly vertically dependent position, traced from11 A. M. July 22nd to the morning of 25th. Glass filament fixed transverselyacross peduncle, at base of flower-head. 

curving down to the ground. We have seen in Chap. IV. , Fig. 92, p. 225, that an upright young flower-head circumnutated conspicuously; and thatthis movement continued after the peduncle had begun to bend downwards. Thesame peduncle was observed when inclined at an angle of 19o above thehorizon, and it circumnutated during two days. Another[page 516]which was already curved 36o beneath the horizon, was observed from 11 A. M. July 22nd to the 27th, by which latter date it had become verticallydependent. Its course during the first 12 h. Is shown in Fig. 190, and itsposition on the three succeeding mornings until the 25th, when it wasnearly vertical. During the first day the peduncle clearly circumnutated, for it moved 4 times down and 3 times up; and on each succeeding day, as itsank downwards, the same movement continued, but was only occasionallyobserved and was less strongly marked. It should be stated that thesepeduncles were observed under a double skylight in the house, and that theygenerally moved downwards very much more slowly than those on plantsgrowing out of doors or in the greenhouse. 

Fig. 191. Trifolium subterraneum: circumnutating movement of peduncle, whilst the flower-head was burying itself in sand, with the reflexed tipsof the calyx still visible; traced from 8 A. M. July 26th to 9 A. M. On 27th. Glass filament fixed transversely across peduncle, near flower-head. 

Fig. 192. Trifolium subterraneum: movement of same peduncle, withflower-head completely buried beneath the sand; traced from 8 A. M. To 7. 15P. M. On July 29th. 

The movement of another vertically dependent peduncle with the flower-headstanding half an inch above the ground, was traced, and again when it firsttouched the ground; in both cases irregular ellipses were described every 4or 5 h. A peduncle on a plant which had been brought into the house, movedfrom an upright into a vertically dependent position in a single day; andhere the course during the first 12 h. Was nearly straight, but with a fewwell-marked zigzags which betrayed the essential nature of the movement. Lastly the circumnutation of a peduncle was traced during 51 h. Whilst inthe act of burying itself obliquely in a little heap of sand. After it hadburied itself to such a depth that the tips of the sepals were alonevisible, the above figure (Fig 191) was traced during 25 h. When theflower-head had completely disappeared beneath the sand, another tracingwas made during 11 h. 45 m. (Fig. 192); and here again we see that thepeduncle was circumnutating. [page 517]

Any one who will observe a flower-head burying itself, will be convincedthat the rocking movement, due to the continued circumnutation of thepeduncle, plays an important part in the act. Considering that theflower-heads are very light, that the peduncles are long, thin, andflexible, and that they arise from flexible branches, it is incredible thatan object as blunt as one of these flower-heads could penetrate the groundby means of the growing force of the peduncle, unless it were aided by therocking movement. After a flower-head has penetrated the ground to a smalldepth, another and efficient agency comes into play; the central rigidaborted flowers, each terminating in five long claws, curve up towards thepeduncle; and in doing so can hardly fail to drag the head down to agreater depth, aided as this action is by the circumnutating movement, which continues after the flower-head has completely buried itself. Theaborted flowers thus act something like the hands of the mole, which forcethe earth backwards and the body forwards. 

It is well known that the seed-capsules of various widely distinct plantseither bury themselves in the ground, or are produced from imperfectflowers developed beneath the surface. Besides the present case, two otherwell-marked instances will be immediately given. It is probable that onechief good thus gained is the protection of the seeds from animals whichprey on them. In the case of T. Subterraneum, the seeds are not onlyconcealed by being buried, but are likewise protected by being closelysurrounded by the rigid, aborted flowers. We may the more confidently inferthat protection is here aimed at, because the seeds of several species inthis same genus are protected in other ways;* namely, by the swelling andclosure of the calyx, or by the persistence and bending down of thestandard-petal, etc. But the most curious instance is that of T. Globosum, in which the upper flowers are sterile, as in T. Subterraneum, but are heredeveloped into large brushes of hairs which envelop and protect theseed-bearing flowers. Nevertheless, in all these cases the capsules, withtheir seeds, may profit, as Mr. T. Thiselton Dyer has remarked, ** by theirbeing kept somewhat damp; and the advantage of such dampness perhaps throwslight on the presence of the absorbent hairs on the buried flower-heads ofT. Subterraneum. According to Mr. Bentham, as quoted by Mr. Dyer, * Vaucher, 'Hist. Phys. Des Plantes d'Europe, ' tom. Ii. P. 110. 

** See his interesting article in 'Nature, ' April 4th, 1878, p. 446. [page 518]

the prostrate habit of Helianthemum prostratum "brings the capsules incontact with the surface of the ground, postpones their maturity, and sofavours the seeds attaining a larger size. " The capsules of Cyclamen and ofOxalis acetosella are only occasionally buried, and this only beneath deadleaves or moss. If it be an advantage to a plant that its capsules shouldbe kept damp and cool by being laid on the ground, we have in these lattercases the first step, from which the power of penetrating the ground, withthe aid of the always present movement of circumnutation, might afterwardshave been gained. 

Arachis hypogoea. --The flowers which bury themselves, rise from stiffbranches a few inches above the ground, and stand upright. After they havefallen off, the gynophore, that is the part which supports the ovarium, grows to a great length, even to 3 or 4 inches, and bends perpendicularlydownwards. It resembles closely a peduncle, but has a smooth and pointedapex, which contains the ovules, and is at first not in the least enlarged. The apex after reaching the ground penetrates it, in one case observed byus to a depth of 1 inch, and in another to 0. 7 inch. It there becomesdeveloped into a large pod. Flowers which are seated too high on the plantfor the gynophore to reach the ground are said* never to produce pods. 

The movement of a young gynophore, rather under an inch in length andvertically dependent, was traced during 46 H. By means of a glass filament(with sights) fixed transversely a little above the apex. It plainlycircumnutated (Fig. 193) whilst increasing in length and growing downwards. It was then raised up, so as to be extended almost horizontally, and theterminal part curved itself downwards, following a nearly straight courseduring 12 h. , but with one attempt to circumnutate, as shown in Fig. 194. After 24 h. It had become nearly vertical. Whether the exciting cause ofthe downward movement is geotropism or apheliotropism was not ascertained;but probably it is not apheliotropism, as all the gynophores grew straightdown towards the ground, whilst the light in the hot-house entered from oneside as well as from above. Another and older gynophore, the apex of whichhad nearly reached the ground, was observed during 3 days in the samemanner as the first-mentioned short one; and it was found to be alwayscircumnutating. During the first 34 h. It described a figure which

* 'Gard. Chronicle, ' 1857, p. 566. [page 519]

represented four ellipses. Lastly, a long gynophore, the apex of which hadburied itself to the depth of about half an inch, was

Fig. 193 Arachis hypogoea: circumnutation of vertically dependent younggynophore, traced on a vertical glass from 10 A. M. July 31st to 8 A. M. Aug. 2nd. 

Fig. 194. Arachis hypogoea: downward movement of same young gynophore, after being extended horizontally; traced on a vertical glass from 8. 30A. M. To 8. 30 P. M. Aug. 2nd. 

pulled up and extended horizontally: it quickly began to curve downwards ina zigzag line; but on the following day the ter-[page 520]minal bleached portion was a little shrivelled. As the gynophores are rigidand arise from stiff branches, and as they terminate in sharp smoothpoints, it is probable that they could penetrate the ground by the mereforce of growth. But this action must be aided by the circumnutatingmovement, for fine sand, kept moist, was pressed close round the apex of agynophore which had reached the ground, and after a few hours it wassurrounded by a narrow open crack. After three weeks this gynophore wasuncovered, and the apex was found at a depth of rather above half an inchdeveloped into a small, white, oval pod. 

Amphicarpoea monoica. --This plant produces long thin shoots, which twineround a support and of course circumnutate. Early in the summer shortershoots are produced from the lower parts of the plant, which growperpendicularly downwards and penetrate the ground. One of these, terminating in a minute bud, was observed to bury itself in sand to a depthof 0. 2 inch in 24 h. It was lifted up and fixed in an inclined positionabout 25o beneath the horizon, being feebly illuminated from above. In thisposition it described two vertical ellipses in 24 h. ; but on the followingday, when brought into the house, it circumnutated only a very little roundthe same spot. Other branches were seen to penetrate the ground, and wereafterwards found running like roots beneath the surface for a length ofnearly two inches, and they had grown thick. One of these, after thusrunning, had emerged into the air. How far circumnutation aids thesedelicate branches in entering the ground we do not know; but the reflexedhairs with which they are clothed will assist in the work. This plantproduces pods in the air, and others beneath the ground; which differgreatly in appearance. Asa Gray says* that it is the imperfect flowers onthe creeping branches near the base of the plant which produce thesubterranean pods; these flowers, therefore, must bury themselves likethose of Arachis. But it may be suspected that the branches which were seenby us to penetrate the ground also produce subterranean flowers and pods. ]

DIAGEOTROPISM. 

Besides geotropism and apogeotropism, there is, according to Frank, anallied form of movement, * 'Manual of the Botany of the Northern United States, ' 1856, p. 106. [page 521]

namely, "transverse-geotropism, " or diageotropism, as we may call it forthe sake of matching our other terms. Under the influence of gravitationcertain parts are excited to place themselves more or less transversely tothe line of its action. * We made no observations on this subject, and willhere only remark that the position of the secondary radicles of variousplants, which extend horizontally or are a little inclined downwards, wouldprobably be considered by Frank as due to transverse-geotropism. As it hasbeen shown in Chap. I. That the secondary radicles of Cucurbita madeserpentine tracks on a smoked glass-plate, they clearly circumnutated, andthere can hardly be a doubt that this holds good with other secondaryradicles. It seems therefore highly probable that they place themselves intheir diageotropic position by means of modified circumnutation. 

Finally, we may conclude that the three kinds of movement which have nowbeen described and which are excited by gravitation, consist of modifiedcircumnutation. Different parts or organs on the same plant, and the samepart in different species, are thus excited to act in a widely differentmanner. We can see no reason why the attraction of gravity should directlymodify the state of turgescence and subsequent growth of one part on theupper side and of another part on the lower side. We are therefore led toinfer that both geotropic, apogeotropic, and diageotropic movements, thepurpose of which we can generally understand, * Elfving has lately described ('Arbeiten des Bot. Instituts in Würzburg, 'B. Ii. 1880, p. 489) an excellent instance of such movements in therhizomes of certain plants. [page 522]

have been acquired for the advantage of the plant by the modification ofthe ever-present movement of circumnutation. This, however, implies thatgravitation produces some effect on the young tissues sufficient to serveas a guide to the plant. [page 523]

CHAPTER XI. 

LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS. 

General considerations--Vicia faba, effects of amputating the tips of theradicles--Regeneration of the tips--Effects of a short exposure of the tipsto geotropic action and their subsequent amputation--Effects of amputatingthe tips obliquely--Effects of cauterising the tips--Effects of grease onthe tips--Pisum sativum, tips of radicles cauterised transversely, and ontheir upper and lower sides--Phaseolus, cauterisation and grease on thetips--Gossypium--Cucurbita, tips cauterised transversely, and on theirupper and lower sides--Zea, tips cauterised--Concluding remarks and summaryof chapter--Advantages of the sensibility to geotropism being localised inthe tips of the radicles. 

CIESIELSKI states* that when the roots of Pisum, Lens and Vicia wereextended horizontally with their tips cut off, they were not acted on bygeotropism; but some days afterwards, when a new root-cap and vegetativepoint had been formed, they bent themselves perpendicularly downwards. Hefurther states that if the tips are cut off, after the roots have been leftextended horizontally for some little time, but before they have begun tobend downwards, they may be placed in any position, and yet will bend as ifstill acted on by geotropism; and this shows that some influence had beenalready transmitted to the bending part from the tip before it wasamputated. Sachs repeated these experiments; he cut off a length of between. 05 and 1 mm. (measured from the apex of the

* 'Abwartskrümmung der Wurzel, ' Inaug. Dissert. Breslau, 1871, p. 29. [page 524]

vegetative point) of the tips of the radicles of the bean (Vicia faba), andplaced them horizontally or vertically in damp air, earth, and water, withthe result that they became bowed in all sorts of directions. * He thereforedisbelieved in Ciesielski's conclusions. But as we have seen with severalplants that the tip of the radicle is sensitive to contact and to otherirritants, and that it transmits some influence to the upper growing partcausing it to bend, there seemed to us to be no a priori improbability inCiesielski's statements. We therefore determined to repeat his experiments, and to try others on several species by different methods. 

Vicia faba. --Radicles of this plant were extended horizontally either overwater or with their lower surfaces just touching it. Their tips hadpreviously been cut off, in a direction as accurately transverse as couldbe done, to different lengths, measured from the apex of the root-cap, andwhich will be specified in each case. Light was always excluded. We hadpreviously tried hundreds of unmutilated radicles under similarcircumstances, and found that every one that was healthy became plainlygeotropic in under 12 h. In the case of four radicles which had their tipscut off for a length of 1. 5 mm. , new root caps and new vegetative pointswere re-formed after an interval of 3 days 20 h. ; and these when placedhorizontally were acted on by geotropism. On some other occasions thisregeneration of the tips and reacquired sensitiveness occurred within asomewhat shorter time. Therefore, radicles having their tips amputatedshould be observed in from 12 to 48 h. After the operation. 

Four radicles were extended horizontally with their lower surfaces touchingthe water, and with their tips cut off for a length of only 0. 5 mm. : after23 h. Three of them were still horizontal; after 47 h. One of the threebecame fairly geotropic; and after 70 h. The other two showed a trace ofthis action. The fourth radicle was vertically geotropic after 23 h. ; butby an

* 'Arbeiten des Bot. Instituts in Würzburg, ' Heft. Iii. 1873, p. 432. [page 525]

accident the root-cap alone and not the vegetative point was found to havebeen amputated; so that this case formed no real exception and might havebeen excluded. 

Five radicles were extended horizontally like the last, and had their tipscut off for a length of 1 mm. ; after 22-23 h. , four of them were stillhorizontal, and one was slightly geotropic; after 48 h. The latter hadbecome vertical; a second was also somewhat geotropic; two remainedapproximately horizontal; and the last or fifth had grown in a disorderedmanner, for it was inclined upwards at an angle of 65o above the horizon. 

Fourteen radicles were extended horizontally at a little height over thewater with their tips cut off for a length of 1. 5 mm. ; after 12 h. All werehorizontal, whilst five control or standard specimens in the same jar wereall bent greatly downwards. After 24 h. Several of the amputated radiclesremained horizontal, but some showed a trace of geotropism, and one wasplainly geotropic, for it was inclined at 40o beneath the horizon. 

Seven horizontally extended radicles from which the tips had been cut offfor the unusual length of 2 mm. Unfortunately were not looked at until 35h. Had elapsed; three were still horizontal, but to our surprise, four weremore or less plainly geotropic. 

The radicles in the foregoing cases were measured before their tips wereamputated, and in the course of 24 h. They had all increased greatly inlength; but the measurements are not worth giving. It is of more importancethat Sachs found that the rate of growth of the different parts of radicleswith amputated tips was the same as with unmutilated ones. Altogethertwenty-nine radicles were operated on in the manner above described, and ofthese only a few showed any geotropic curvature within 24 h. ; whereasradicles with unmutilated tips always became, as already stated, much bentdown in less than half of this time. The part of the radicle which bendsmost lies at the distance of from 3 to 6 mm. From the tip, and as thebending part continues to grow after the operation, there does not seem anyreason why it should not have been acted on by geotropism, unless itscurvature depended on some influence transmitted from the tip. And we haveclear evidence of such transmission in Ciesielski's experiments, which werepeated and extended in the following manner. 

Beans were embedded in friable peat with the hilum downwards, and aftertheir radicles had grown perpendicularly down for a length of from ½ to 1inch, sixteen were selected which[page 526]were perfectly straight, and these were placed horizontally on the peat, being covered by a thin layer of it. They were thus left for an averageperiod of 1 h. 37 m. The tips were then cut off transversely for a lengthof 1. 5 mm. , and immediately afterwards they were embedded vertically in thepeat. In this position geotropism would not tend to induce any curvature, but if some influence had already been transmitted from the tip to the partwhich bends most, we might expect that this part would become curved in thedirection in which geotropism had previously acted; for it should be notedthat these radicles being now destitute of their sensitive tips, would notbe prevented by geotropism from curving in any direction. The result wasthat of the sixteen vertically embedded radicles, four continued forseveral days to grow straight downwards, whilst twelve became more or lessbowed laterally. In two of the twelve, a trace of curvature was perceptiblein 3 h. 30 m. , counting from the time when they had first been laidhorizontally; and all twelve were plainly bowed in 6 h. , and still moreplainly in 9 h. In every one of them the curvature was directed towards theside which had been downwards whilst the radicles remained horizontal. Thecurvature extended for a length of from 5 to, in one instance, 8 mm. , measured from the cut-off end. Of the twelve bowed radicles five becamepermanently bent into a right angle; the other seven were at first muchless bent, and their curvature generally decreased after 24 h. , but did notwholly disappear. This decrease of curvature would naturally follow, if anexposure of only 1 h. 37 m. To geotropism, served to modify the turgescenceof the cells, but not their subsequent growth to the full extent. The fiveradicles which were rectangularly bent became fixed in this position, andthey continued to grow out horizontally in the peat for a length of about 1inch during from 4 to 6 days. By this time new tips had been formed; and itshould be remarked that this regeneration occurred slower in the peat thanin water, owing perhaps to the radicles being often looked at and thusdisturbed. After the tips had been regenerated, geotropism was able to acton them, so that they now became bowed vertically downwards. An accuratedrawing (Fig. 195) is given on the opposite page of one of these fiveradicles, reduced to half the natural size. 

We next tried whether a shorter exposure to geotropism would suffice toproduce an after-effect. Seven radicles were extended horizontally for anhour, instead of 1 h. 37 m. As in the[page 527]former trial; and after their tips (1. 5 mm. In length) had been amputated, they were placed vertically in damp peat. Of these, three were not in theleast affected and continued for days to grow straight downwards. Fourshowed after 8 h. 30 m. A mere trace of curvature in the direction in whichthey had been acted on by geotropism; and in this respect they differedmuch from those which had been exposed for 1 h. 37 m. , for many of thelatter were plainly curved in 6 h. The curvature of one of these fourradicles almost disappeared after 24 h. In the second, the curvatureincreased during two days and then decreased. The third radicle becamepermanently bent, so that its terminal part made an angle of about 45o withits original vertical direction. The fourth radicle became horizontal. These two, latter radicles continued during two more days to grow in thepeat in the same directions, that is, at an angle of 45o beneath thehorizon and horizontally. By the fourth morning new tips had beenre-formed, and now geotropism was able to act on them again, and theybecame bent perpendicularly downwards, exactly as in the case of the fiveradicles described in the last paragraph and as is shown in (Fig. 195) heregiven. 

Fig. 195. Vicia faba: radicle, rectangularly bent at A, after theamputation of the tip, due to the previous influence of geotropism. L, sideof bean which lay on the peat, whilst geotropism acted on the radicle. A, point of chief curvature of the radicle, whilst standing verticallydownwards. B, point of chief curvature after the regeneration of the tip, when geotropism again acted. C, regenerated tip. 

Lastly, five other radicles were similarly treated, but were exposed togeotropism during only 45 m. After 8 h. 30 m. Only one was doubtfullyaffected; after 24 h. Two were just perceptibly curved towards the sidewhich had been acted on by geotropism; after 48 h. The one first mentionedhad a radius of curvature of 60 mm. That this curvature was due to theaction of geotropism during the horizontal position of the radicle, wasshown after 4 days, when a new tip had been re-formed, for it then grewperpendicularly downwards. We learn from this[page 528]case that when the tips are amputated after an exposure to geotropism ofonly 45 m. , though a slight influence is sometimes transmitted to theadjoining part of the radicle, yet this seldom suffices, and then onlyslowly, to induce even moderately well-pronounced curvature. 

In the previously given experiments on 29 horizontally extended radicleswith their tips amputated, only one grew irregularly in any marked manner, and this became bowed upwards at an angle of 65o. In Ciesielski'sexperiments the radicles could not have grown very irregularly, for if theyhad done so, he could not have spoken confidently of the obliteration ofall geotropic action. It is therefore remarkable that Sachs, whoexperimented on many radicles with their tips amputated, found extremelydisordered growth to be the usual result. As horizontally extended radicleswith amputated tips are sometimes acted on slightly by geotropism within ashort time, and are often acted on plainly after one or two days, wethought that this influence might possibly prevent disordered growth, though it was not able to induce immediate curvature. Therefore 13radicles, of which 6 had their tips amputated transversely for a length of1. 5 mm. , and the other 7 for a length of only 0. 5 mm. , were suspendedvertically in damp air, in which position they would not be affected bygeotropism; but they exhibited no great irregularity of growth, whilstobserved during 4 to 6 days. We next thought that if care were not taken incutting off the tips transversely, one side of the stump might be irritatedmore than the other, either at first or subsequently during theregeneration of the tip, and that this might cause the radicle to bend toone side. It has also been shown in Chapter III. That if a thin slice becut off one side of the tip of the radicle, this causes the radicle to bendfrom the sliced side. Accordingly, 30 radicles, with tips amputated for alength of 1. 5 mm. , were allowed to grow perpendicularly downwards intowater. Twenty of them were amputated at an angle of 20o with a linetransverse to their longitudinal axes; and such stumps appeared onlymoderately oblique. The remaining ten radicles were amputated at an angleof about 45o. Under these circumstances no less than 19 out of the 30became much distorted in the course of 2 or 3 days. Eleven other radicleswere similarly treated, excepting that only 1 mm. (including in this andall other cases the root-cap) was amputated; and of these only one grewmuch, and two others slightly[page 529]distorted; so that this amount of oblique amputation was not sufficient. Out of the above 30 radicles, only one or two showed in the first 24 h. Anydistortion, but this became plain in the 19 cases on the second day, andstill more conspicuous at the close of the third day, by which time newtips had been partially or completely regenerated. When therefore a new tipis reformed on an oblique stump, it probably is developed sooner on oneside than on the other: and this in some manner excites the adjoining partto bend to one side. Hence it seems probable that Sachs unintentionallyamputated the radicles on which he experimented, not strictly in atransverse direction. 

This explanation of the occasional irregular growth of radicles withamputated tips, is supported by the results of cauterising their tips; foroften a greater length on one side than on the other was unavoidablyinjured or killed. It should be remarked that in the following trials thetips were first dried with blotting-paper, and then slightly rubbed with adry stick of nitrate of silver or lunar caustic. A few touches with thecaustic suffice to kill the root-cap and some of the upper layers of cellsof the vegetative point. Twenty-seven radicles, some young and very short, others of moderate length, were suspended vertically over water, afterbeing thus cauterised. Of these some entered the water immediately, andothers on the second day. The same number of uncauterised radicles of thesame age were observed as controls. After an interval of three or four daysthe contrast in appearance between the cauterised and control specimens waswonderfully great. The controls had grown straight downwards, with theexception of the normal curvature, which we have called Sachs' curvature. Of the 27 cauterised radicles, 15 had become extremely distorted; 6 of themgrew upwards and formed hoops, so that their tips sometimes came intocontact with the bean above; 5 grew out rectangularly to one side; only afew of the remaining 12 were quite straight, and some of these towards theclose of our observations became hooked at their extreme lower ends. Radicles, extended horizontally instead of vertically, with their tipscauterised, also sometimes grew distorted, but not so commonly, as far aswe could judge, as those suspended vertically; for this occurred with only5 out of 19 radicles thus treated. 

Instead of cutting off the tips, as in the first set of experiments, wenext tried the effects of touching horizontally extended radicles withcaustic in the manner just described. But[page 530]some preliminary remarks must first be made. It may be objected that thecaustic would injure the radicles and prevent them from bending; but ampleevidence was given in Chapter III. That touching the tips of verticallysuspended radicles with caustic on one side, does not stop their bending;on the contrary, it causes them to bend from the touched side. We alsotried touching both the upper and the lower sides of the tips of someradicles of the bean, extended horizontally in damp friable earth. The tipsof three were touched with caustic on their upper sides, and this would aidtheir geotropic bending; the tips of three were touched on their lowersides, which would tend to counteract the bending downwards; and three wereleft as controls. After 24 h. An independent observer was asked to pick outof the nine radicles, the two which were most and the two which were leastbent; he selected as the latter, two of those which had been touched ontheir lower sides, and as the most bent, two of those which had beentouched on the upper side. Hereafter analogous and more strikingexperiments with Pisum sativum and Cucurbita ovifera will be given. We maytherefore safely conclude that the mere application of caustic to the tipdoes not prevent the radicles from bending. 

In the following experiments, the tips of young horizontally extendedradicles were just touched with a stick of dry caustic; and this was heldtransversely, so that the tip might be cauterised all round assymmetrically as possible. The radicles were then suspended in a closedvessel over water, kept rather cool, viz. , 55o - 59o F. This was donebecause we had found that the tips were more sensitive to contact under alow than under a high temperature; and we thought that the same rule mightapply to geotropism. In one exceptional trial, nine radicles (which wererather too old, for they had grown to a length of from 3 to 5 cm. ), wereextended horizontally in damp friable earth, after their tips had beencauterised and were kept at too high a temperature, viz. , of 68o F. , or 20oC. The result in consequence was not so striking as in the subsequent casesfor although when after 9 h. 40 m. Six of them were examined, these did notexhibit any geotropic bending, yet after 24 h. , when all nine wereexamined, only two remained horizontal, two exhibited a trace ofgeotropism, and five were slightly or moderately geotropic, yet notcomparable in degree with the control specimens. Marks had been made onseven of these cauterised radicles at 10 mm. From the tips, which includes[page 531]the whole growing portion; and after the 24 h. This part had a mean lengthof 37 mm. , so that it had increased to more than 3 ½ times its originallength; but it should be remembered that these beans had been exposed to arather high temperature. 

Nineteen young radicles with cauterised tips were extended at differenttimes horizontally over water. In every trial an equal number of controlspecimens were observed. In the first trial, the tips of three radicleswere lightly touched with the caustic for 6 or 7 seconds, which was alonger application than usual. After 23 h. 30 m. (temp. 55o - 56o F. ) thesethree radicles, Fig. 196. Vicia faba: state of radicles which had been extendedhorizontally for 23 h. 30 m. ; A, B, C, tips touched with caustic; D, E, F, tips uncauterised. Lengths of radicles reduced to one-half scale, but by anaccident the beans themselves not reduced in the same degree. 

A, B, C (Fig. 196), were still horizontal, whilst the three controlspecimens had become within 8 h. Slightly geotropic, and strongly so (D, E, F) in 23 h. 30 m. A dot had been made on all six radicles at 10 mm. Fromtheir tips, when first placed horizontally. After the 23 h. 30 m. Thisterminal part, originally 10 mm. In length, had increased in the cauterisedspecimens to a mean length of 17. 3 mm. , and to 15. 7 mm. In the controlradicles, as shown in the figures by the unbroken transverse line; thedotted line being at 10 mm. From the apex. The control or uncauterisedradicles, therefore, had actually grown less[page 532]than the cauterised; but this no doubt was accidental, for radicles ofdifferent ages grow at different rates, and the growth of differentindividuals is likewise affected by unknown causes. The state of the tipsof these three radicles, which had been cauterised for a rather longer timethan usual, was as follows: the blackened apex, or the part which had beenactually touched by the caustic, was succeeded by a yellowish zone, dueprobably to the absorption of some of the caustic; in A, both zonestogether were 1. 1 mm. In length, and 1. 4 mm. In diameter at the base of theyellowish zone; in B, the length of both was only 0. 7 mm. , and the diameter0. 7 mm. ; in C, the length was 0. 8 mm. , and the diameter 1. 2 mm. 

Three other radicles, the tips of which had been touched with causticcuring 2 or 3 seconds, remained (temp. 58o - 59o F. ) horizontal for 23 h. ;the control radicles having, of course, become geotropic within this time. The terminal growing part, 10 mm. In length, of the cauterised radicles hadincreased in this interval to a mean length of 24. 5 mm. , and of thecontrols to a mean of 26 mm. A section of one of the cauterised tips showedthat the blackened part was 0. 5 mm. In length, of which 0. 2 mm. Extendedinto the vegetative point; and a faint discoloration could be detected evento 1. 6 mm. From the apex of the root-cap. 

In another lot of six radicles (temp. 55o - 57o F. ) the three controlspecimens were plainly geotropic in 8 ½ h. ; and after 24 h. The mean lengthof their terminal part had increased from 10 mm. To 21 mm. When the causticwas applied to the three cauterised specimens, it was held quite motionlessduring 5 seconds, and the result was that the black marks were extremelyminute. Therefore, caustic was again applied, after 8 ½ h. , during whichtime no geotropic action had occurred. When the specimens were re-examinedafter an additional interval of 15 ½ h. , one was horizontal and the othertwo showed, to our surprise, a trace of geotropism which in one of themsoon afterwards became strongly marked; but in this latter specimen thediscoloured tip was only 2/3 mm. In length. The growing part of these threeradicles increased in 24 h. From 10 mm. To an average of 16. 5 mm. 

It would be superfluous to describe in detail the behaviour of the 10remaining cauterised radicles. The corresponding control specimens allbecame geotropic in 8 h. Of the cauterised, 6 were first looked at after 8h. , and one alone showed a trace[page 533]of geotropism; 4 were first looked at after 14 h. , and one alone of thesewas slightly geotropic. After 23 - 24h. , 5 of the 10 were still horizontal, 4 slightly, and 1 decidedly, geotropic. After 48 h. Some of them becamestrongly geotropic. The cauterised radicles increased greatly in length, but the measurements are not worth giving. 

As five of the last-mentioned cauterised radicles had become in 24 h. Somewhat geotropic, these (together with three which were still horizontal)had their positions reversed, so that their tips were now a littleupturned, and they were again touched with caustic. After 24 h. They showedno trace of geotropism; whereas the eight corresponding control specimens, which had likewise been reversed, in which position the tips of severalpointed to the zenith, all became geotropic; some having passed in the 24h. Through an angle of 180o, others through about 135o, and others throughonly 90o. The eight radicles, which had been twice cauterised, wereobserved for an additional day (i. E. For 48 h. After being reversed), andthey still showed no signs of geotropism. Nevertheless, they continued togrow rapidly; four were measured 24 h. After being reversed, and they hadin this time increased in length between 8 and 11 mm. ; the other four weremeasured 48 h. After being reversed, and these had increased by 20, 18, 23, and 28 mm. 

In coming to a conclusion with respect to the effects of cauterising thetips of these radicles, we should bear in mind, firstly, that horizontallyextended control radicles were always acted on by geotropism, and becamesomewhat bowed downwards in 8 or 9 h. ; secondly, that the chief seat of thecurvature lies at a distance of from 3 to 6 mm. From the tip; thirdly, thatthe tip was discoloured by the caustic rarely for more than 1 mm. Inlength; fourthly, that the greater number of the cauterised radicles, although subjected to the full influence of geotropism during the wholetime, remained horizontal for 24 h. , and some for twice as long; and thatthose which did become bowed were so only in a slight degree; fifthly, thatthe cauterised radicles continued to grow almost, and sometimes quite, aswell as the uninjured ones along the part which bends most. And lastly, that a touch on the tip with caustic, if on one side, far from preventingcurvature, actually induces it. Bearing all these facts in mind, we mustinfer that under normal conditions the geotropic curvature of the root isdue to an influence transmitted from the apex to the adjoining part wherethe bending[page 534]takes place; and that when the tip of the root is cauterised it is unableto originate the stimulus necessary to produce geotropic curvature. 

As we had observed that grease was highly injurious to some plants, wedetermined to try its effects on radicles. When the cotyledons of Phalarisand Avena were covered with grease along one side, the growth of this sidewas quite stopped or greatly checked, and as the opposite side continued togrow, the cotyledons thus treated became bowed towards the greased side. This same matter quickly killed the delicate hypocotyls and young leaves ofcertain plants. The grease which we employed was made by mixing lamp-blackand olive oil to such a consistence that it could be laid on in a thicklayer. The tips of five radicles of the bean were coated with it for alength of 3 mm. , and to our surprise this part increased in length in 23 h. To 7. 1 mm. ; the thick layer of grease being curiously drawn out. It thuscould not have checked much, if at all, the growth of the terminal part ofthe radicle. With respect to geotropism, the tips of seven horizontallyextended radicles were coated for a length of 2 mm. , and after 24 h. Noclear difference could be perceived between their downward curvature andthat of an equal number of control specimens. The tips of 33 other radicleswere coated on different occasions for a length of 3 mm. ; and they werecompared with the controls after 8 h. , 24 h. , and 48 h. On one occasion, after 24 h. , there was very little difference in curvature between thegreased and control specimens; but generally the difference wasunmistakable, those with greased tips being considerably less curveddownwards. The whole growing part (the greased tips included) of six ofthese radicles was measured and was found to have increased in 23 h. From10 mm. To a mean length of 17. 7 mm. ; whilst the corresponding part of thecontrols had increased to 20. 8 mm. It appears therefore, that although thetip itself, when greased, continues to grow, yet the growth of the wholeradicle is somewhat checked, and that the geotropic curvature of the upperpart, which was free from grease, was in most cases considerably lessened. 

Pisum sativum. --Five radicles, extended horizontally over water, had theirtips lightly touched two or three times with dry caustic. These tips weremeasured in two cases, and found to be blackened for a length of only halfa millimeter. Five other radicles were left as controls. The part which ismost bowed through geotropism lies at a distance of several millimetersfrom[page 535]the apex. After 24 h. , and again after 32 h. From the commencement, four ofthe cauterised radicles were still horizontal, but one was plainlygeotropic, being inclined at 45o beneath the horizon. The five controlswere somewhat geotropic after 7 h. 20 m. , and after 24 h. Were all stronglygeotropic; being inclined at the following angles beneath the horizon, viz. , 59o, 60o, 65o, 57o, and 43o. The length of the radicles was notmeasured in either set, but it was manifest that the cauterised radicleshad grown greatly. 

The following case proves that the action of the caustic by itself does notprevent the curvature of the radicle. Ten radicles were extendedhorizontally on and beneath a layer of damp friable peat-earth; and beforebeing extended their tips were touched with dry caustic on the upper side. Ten other radicles similarly placed were touched on the lower side; andthis would tend to make them bend from the cauterised side; and therefore, as now placed, upwards, or in opposition to geotropism. Lastly, tenuncauterised radicles were extended horizontally as controls. After 24 h. All the latter were geotropic; and the ten with their tips cauterised onthe upper side were equally geotropic; and we believe that they becamecurved downwards before the controls. The ten which had been cauterised onthe lower side presented a widely different appearance: No. 1, however, wasperpendicularly geotropic, but this was no real exception, for onexamination under the microscope, there was no vestige of a coloured markon the tip, and it was evident that by a mistake it had not been touchedwith the caustic. No. 2 was plainly geotropic, being inclined at about 45obeneath the horizon; No. 3 was slightly, and No. 4 only just perceptiblygeotropic; Nos. 5 and 6 were strictly horizontal; and the four remainingones were bowed upwards, in opposition to geotropism. In these four casesthe radius of the upward curvatures (according to Sachs' cyclometer) was 5mm. , 10 mm. , 30 mm. , and 70 mm. This curvature was distinct long before the24 h. Had elapsed, namely, after 8 h. 45 m. From the time when the lowersides of the tips were touched with the caustic. 

Phaseolus multiflorus. --Eight radicles, serving as controls, were extendedhorizontally, some in damp friable peat and some in damp air. They allbecame (temp 20o - 21o C. ) plainly geotropic in 8 h. 30 m. , for they thenstood at an average angle of 63o beneath the horizon. A rather greaterlength of the radicle is bowed downwards by geotropism than in the case ofVicia faba, [page 536]that is to say, rather more than 6 mm. As measured from the apex of theroot-cap. Nine other radicles were similarly extended, three in damp peatand six in damp air, and dry caustic was held transversely to their tipsduring 4 or 5 seconds. Three of their tips were afterwards examined: in (1)a length of 0. 68 mm. Was discoloured, of which the basal 0. 136 mm. Wasyellow, the apical part being black; in (2) the discoloration was 0. 65 mm. In length, of which the basal 0. 04 mm. Was yellow; in (3) the discolorationwas 0. 6 mm. In length, of which the basal 0. 13 mm. Was yellow. Thereforeless than 1 mm. Was affected by the caustic, but this sufficed almostwholly to prevent geotropic action; for after 24 h. One alone of the ninecauterised radicles became slightly geotropic, being now inclined at 10obeneath the horizon; the eight others remained horizontal, though one wascurved a little laterally. 

The terminal part (10 mm. In length) of the six cauterised radicles in thedamp air, had more than doubled in length in the 24 h. , for this part wasnow on an average 20. 7 mm. Long. The increase in length within the sametime was greater in the control specimens, for the terminal part had grownon an average from 10 mm. To 26. 6 mm. But as the cauterised radicles hadmore than doubled their length in the 24 h. , it is manifest that they hadnot been seriously injured by the caustic. We may here add that whenexperimenting on the effects of touching one side of the tip with caustic, too much was applied at first, and the whole tip (but we believe not morethan 1 mm. In length) of six horizontally extended radicles was killed, andthese continued for two or three days to grow out horizontally. 

Many trials were made, by coating the tips of horizontally extendedradicles with the before described thick grease. The geotropic curvature of12 radicles, which were thus coated for a length of 2 mm. , was delayedduring the first 8 or 9 h. , but after 24 h. Was nearly as great as that ofthe control specimens. The tips of nine radicles were coated for a lengthof 3 mm. , and after 7 h. 10 m. These stood at an average angle of 30obeneath the horizon, whilst the controls stood at an average of 54o. After24 h. The two lots differed but little in their degree of curvature. Insome other trials, however, there was a fairly well-marked difference after24 h. Between those with greased tips and the controls. The terminal partof eight control specimens increased in 24 h. From 10 mm. To a mean lengthof[page 537]24. 3 mm. , whilst the mean increase of those with greased tips was 20. 7 mm. The grease, therefore, slightly checked the growth of the terminal part, but this part was not much injured; for several radicles which had beengreased for a length of 2 mm. Continued to grow during seven days, and werethen only a little shorter than the controls. The appearance presented bythese radicles after the seven days was very curious, for the black greasehad been drawn out into the finest longitudinal striae, with dots andreticulations, which covered their surfaces for a length of from 26 to 44mm. , or of 1 to 1. 7 inch. We may therefore conclude that grease on the tipsof the radicles of this Phaseolus somewhat delays and lessens the geotropiccurvature of the part which ought to bend most. 

Gossypium herbaceum. --The radicles of this plant bend, through the actionof geotropism, for a length of about 6 mm. Five radicles, placedhorizontally in damp air, had their tips touched with caustic, and thediscoloration extended for a length of from 2/3 to 1 mm. They showed, after7 h. 45 m. And again after 23 h. , not a trace of geotropism; yet theterminal portion, 9 mm. In length, had increased on an average to 15. 9 mm. Six control radicles, after 7 h. 45 m. , were all plainly geotropic, two ofthem being vertically dependent, and after 23 h. All were vertical, ornearly so. 

Cucurbita ovifera. --A large number of trials proved almost useless, fromthe three following causes: Firstly, the tips of radicles which have grownsomewhat old are only feebly geotropic if kept in damp air; nor did wesucceed well in our experiments, until the germinating seeds were placed inpeat and kept at a rather high temperature. Secondly, the hypocotyls of theseeds which were pinned to the lids of the jars gradually became arched;and, as the cotyledons were fixed, the movement of the hypocotyl affectedthe position of the radicle, and caused confusion. Thirdly, the point ofthe radicle is so fine that it is difficult not to cauterise it either toomuch or too little. But we managed generally to overcome this latterdifficulty, as the following experiments show, which are given to provethat a touch with caustic on one side of the tip does not prevent the upperpart of the radicle from bending. Ten radicles were laid horizontallybeneath and on damp friable peat, and their tips were touched with causticon the upper side. After 8 h. All were plainly geotropic, three of themrectangularly; after 19 h. [page 538]all were strongly geotropic, most of them pointing perpendicularlydownwards. Ten other radicles, similarly placed, had their tips touchedwith caustic on the lower side; after 8 h. Three were slightly geotropic, but not nearly so much so as the least geotropic of the foregoingspecimens; four remained horizontal; and three were curved upwards inopposition to geotropism. After 19 h. The three which were slightlygeotropic had become strongly so. Of the four horizontal radicles, onealone showed a trace of geotropism; of the three up-curved radicles, oneretained this curvature, and the other two had become horizontal. 

The radicles of this plant, as already remarked, do not succeed well indamp air, but the result of one trial may be briefly given. Nine youngradicles between . 3 and . 5 inch in length, with their tips cauterised andblackened for a length never exceeding ½ mm. , together with eight controlspecimens, were extended horizontally in damp air. After an interval ofonly 4 h. 10 m. All the controls were slightly geotropic, whilst not one ofthe cauterised specimens exhibited a trace of this action. After 8 h. 35m. , there was the same difference between the two sets, but rather morestrongly marked. By this time both sets had increased greatly in length. The controls, however, never became much more curved downwards; and after24 h. There was no great difference between the two sets in their degree ofcurvature. 

Eight young radicles of nearly equal length (average . 36 inch) were placedbeneath and on peat-earth, and were exposed to a temp. Of 75o - 76o F. Their tips had been touched transversely with caustic, and five of themwere blackened for a length of about 0. 5 mm. , whilst the other three wereonly just visibly discoloured. In the same box there were 15 controlradicles, mostly about . 36 inch in length, but some rather longer andolder, and therefore less sensitive. After 5 h. , the 15 control radicleswere all more or less geotropic: after 9 h. , eight of them were bent downbeneath the horizon at various angles between 45o and 90o, the remainingseven being only slightly geotropic: after 25 h. All were rectangularlygeotropic. The state of the eight cauterised radicles after the sameintervals of time was as follows: after 5 h. One alone was slightlygeotropic, and this was one with the tip only a very little discoloured:after 9 h. The one just mentioned was rectangularly geotropic, and twoothers were slightly so, and these were the three which had been scarcely[page 539]affected by the caustic; the other five were still strictly horizontal. After 24 h. 40 m. The three with only slightly discoloured tips were bentdown rectangularly; the other five were not in the least affected, butseveral of them had grown rather tortuously, though still in a horizontalplane. The eight cauterised radicles which had at first a mean length of. 36 inch, after 9 h. Had increased to a mean length of . 79 inch; and after24 h. 40 m. To the extraordinary mean length of 2 inches. There was noplain difference in length between the five well cauterised radicles whichremained horizontal, and the three with slightly cauterised tips which hadbecome abruptly bent down. A few of the control radicles were measuredafter 25 h. , and they were on an average only a little longer than thecauterised, viz. , 2. 19 inches. We thus see that killing the extreme tip ofthe radicle of this plant for a length of about 0. 5 mm. , though it stopsthe geotropic bending of the upper part, hardly interferes with the growthof the whole radicle. 

In the same box with the 15 control specimens, the rapid geotropic bendingand growth of which have just been described, there were six radicles, about . 6 inch in length, extended horizontally, from which the tips hadbeen cut off in a transverse direction for a length of barely 1 mm. Theseradicles were examined after 9 h. And again after 24 h. 40 m. , and they allremained horizontal. They had not become nearly so tortuous as those abovedescribed which had been cauterised. The radicles with their tips cut offhad grown in the 24 h. 40 m. As much, judging by the eye, as the cauterisedspecimens. 

Zea mays. --The tips of several radicles, extended horizontally in damp air, were dried with blotting-paper and then touched in the first trial during 2or 3 seconds with dry caustic; but this was too long a contact, for thetips were blackened for a length of rather above 1 mm. They showed no signsof geotropism after an interval of 9 h. , and were then thrown away. In asecond trial the tips of three radicles were touched for a shorter time, and were blackened for a length of from 0. 5 to 0. 75 mm. : they all remainedhorizontal for 4 h. , but after 8 h. 30 m. One of them, in which theblackened tip was only 0. 5 mm. In length, was inclined at 21o beneath thehorizon. Six control radicles all became slightly geotropic in 4 h. , andstrongly so after 8 h. 30 m. , with the chief seat of curvature generallybetween 6 or 7 mm. From the apex. In the cauterised specimens, the terminalgrowing part, 10 mm. In length, increased during[page 540]the 8 h. 30 m. To a mean length of 13 mm. ; and in the controls to 14. 3 mm. 

In a third trial the tips of five radicles (exposed to a temp. Of 70o -71o) were touched with the caustic only once and very slightly; they wereafterwards examined under the microscope, and the part which was in any waydiscoloured was on an average . 76 mm. In length. After 4 h. 10 m. None werebent; after 5 h. 45 m. , and again after 23 h. 30 m. , they still remainedhorizontal, excepting one which was now inclined 20o beneath the horizon. The terminal part, 10 mm. In length, had increased greatly in length duringthe 23 h. 30 m. , viz. , to an average of 26 mm. Four control radicles becameslightly geotropic after the 4 h. 10 m. , and plainly so after the 5 h. 45m. Their mean length after the 23 h. 30 m. Had increased from 10 mm. To 31mm. Therefore a slight cauterisation of the tip checks slightly the growthof the whole radicle, and manifestly stops the bending of that part whichought to bend most under the influence of geotropism, and which stillcontinues to increase greatly in length. ]

Concluding Remarks. --Abundant evidence has now been given, showing thatwith various plants the tip of the radicle is alone sensitive togeotropism; and that when thus excited, it causes the adjoining parts tobend. The exact length of the sensitive part seems to be somewhat variable, depending in part on the age of the radicle; but the destruction of alength of from less than 1 to 1. 5 mm. (about 1/20th of an inch), in theseveral species observed, generally sufficed to prevent any part of theradicle from bending within 24 h. , or even for a longer period. The fact ofthe tip alone being sensitive is so remarkable a fact, that we will heregive a brief summary of the foregoing experiments. The tips were cut off 29horizontally extended radicles of Vicia faba, and with a few exceptionsthey did not become geotropic in 22 or 23 h. , whilst unmutilated radicleswere always bowed downwards in 8 or 9 h. It should be borne in mind thatthe mere act of cutting[page 541]off the tip of a horizontally extended radicle does not prevent theadjoining parts from bending, if the tip has been previously exposed for anhour or two to the influence of geotropism. The tip after amputation issometimes completely regenerated in three days; and it is possible that itmay be able to transmit an impulse to the adjoining parts before itscomplete regeneration. The tips of six radicles of Cucurbita ovifera wereamputated like those of Vicia faba; and these radicles showed no signs ofgeotropism in 24 h. ; whereas the control specimens were slightly affectedin 5 h. , and strongly in 9 h. 

With plants belonging to six genera, the tips of the radicles were touchedtransversely with dry caustic; and the injury thus caused rarely extendedfor a greater length than 1 mm. , and sometimes to a less distance, asjudged by even the faintest discoloration. We thought that this would be abetter method of destroying the vegetative point than cutting it off; forwe knew, from many previous experiments and from some given in the presentchapter, that a touch with caustic on one side of the apex, far frompreventing the adjoining part from bending, caused it to bend. In all thefollowing cases, radicles with uncauterised tips were observed at the sametime and under similar circumstances, and they became, in almost everyinstance, plainly bowed downwards in one-half or one-third of the timeduring which the cauterised specimens were observed. With Vicia faba 19radicles were cauterised; 12 remained horizontal during 23-24 h. ; 6 becameslightly and 1 strongly geotropic. Eight of these radicles were afterwardsreversed, and again touched with caustic, and none of them became geotropicin 24 h. , whilst the reversed control specimens became strongly boweddownwards within this time. [page 542]With Pisum sativum, five radicles had their tips touched with caustic, andafter 32 h. Four were still horizontal. The control specimens were slightlygeotropic in 7 h. 20 m. , and strongly so in 24 h. The tips of 9 otherradicles of this plant were touched only on the lower side, and 6 of themremained horizontal for 24 h. , or were upturned in opposition togeotropism; 2 were slightly, and 1 plainly geotropic. With Phaseolusmultiflorus, 15 radicles were cauterised, and 8 remained horizontal for 24h. ; whereas all the controls were plainly geotropic in 8 h. 30 m. Of 5cauterised radicles of Gossypium herbaceum, 4 remained horizontal for 23 h. And 1 became slightly geotropic; 6 control radicles were distinctlygeotropic in 7 h. 45 m. Five radicles of Cucurbita ovifera remainedhorizontal in peat-earth during 25 h. , and 9 remained so in damp air during8 ½ h. ; whilst the controls became slightly geotropic in 4 h. 10 m. Thetips of 10 radicals of this plant were touched on their lower sides, and 6of them remained horizontal or were upturned after 19 h. , 1 being slightlyand 3 strongly geotropic. 

Lastly, the tips of several radicles of Vicia faba and Phaseolusmultiflorus were thickly coated with grease for a length of 3 mm. Thismatter, which is highly injurious to most plants, did not kill or stop thegrowth of the tips, and only slightly lessened the rate of growth of thewhole radicle; but it generally delayed a little the geotropic bending ofthe upper part. 

The several foregoing cases would tell us nothing, if the tip itself wasthe part which became most bent; but we know that it is a part distant fromthe tip by some millimeters which grows quickest, and which, under theinfluence of geotropism, bends most. We have no reason to suppose that thispart is injured by the death or injury of the tip; and it is certain[page 543]that after the tip has been destroyed this part goes on growing at such arate, that its length was often doubled in a day. We have also seen thatthe destruction of the tip does not prevent the adjoining part frombending, if this part has already received some influence from the tip. Aswith horizontally extended radicles, of which the tip has been cut off ordestroyed, the part which ought to bend most remains motionless for manyhours or days, although exposed at right angles to the full influence ofgeotropism, we must conclude that the tip alone is sensitive to this power, and transmits some influence or stimulus to the adjoining parts, causingthem to bend. We have direct evidence of such transmission; for when aradicle was left extended horizontally for an hour or an hour and a half, by which time the supposed influence will have travelled a little distancefrom the tip, and the tip was then cut off, the radicle afterwards becamebent, although placed perpendicularly. The terminal portions of severalradicles thus treated continued for some time to grow in the direction oftheir newly-acquired curvature; for as they were destitute of tips, theywere no longer acted on by geotropism. But after three or four days whennew vegetative points were formed, the radicles were again acted on bygeotropism, and now they curved themselves perpendicularly downwards. Tosee anything of the above kind in the animal kingdom, we should have tosuppose than an animal whilst lying down determined to rise up in someparticular direction; and that after its head had been cut off, an impulsecontinued to travel very slowly along the nerves to the proper muscles; sothat after several hours the headless animal rose up in the predetermineddirection. 

As the tip of the radicle has been found to be the[page 544]part which is sensitive to geotropism in the members of such distinctfamilies as the Leguminosae, Malvaceae, Cucurbitaceae and Gramineae, we mayinfer that this character is common to the roots of most seedling plants. Whilst a root is penetrating the ground, the tip must travel first; and wecan see the advantage of its being sensitive to geotropism, as it has todetermine the course of the whole root. Whenever the tip is deflected byany subterranean obstacle, it will also be an advantage that a considerablelength of the root should be able to bend, more especially as the tipitself grows slowly and bends but little, so that the proper downwardcourse may be soon recovered. But it appears at first sight immaterialwhether this were effected by the whole growing part being sensitive togeotropism, or by an influence transmitted exclusively from the tip. Weshould, however, remember that it is the tip which is sensitive to thecontact of hard objects, causing the radicle to bend away from them, thusguiding it along the lines of least resistance in the soil. It is again thetip which is alone sensitive, at least in some cases, to moisture, causingthe radicle to bend towards its source. These two kinds of sensitivenessconquer for a time the sensitiveness to geotropism, which, however, ultimately prevails. Therefore, the three kinds of sensitiveness must oftencome into antagonism; first one prevailing, and then another; and it wouldbe an advantage, perhaps a necessity, for the interweighing and reconcilingof these three kinds of sensitiveness, that they should be all localised inthe same group of cells which have to transmit the command to the adjoiningparts of the radicle, causing it to bend to or from the source ofirritation. 

Finally, the fact of the tip alone being sensitive to[page 545]the attraction of gravity has an important bearing on the theory ofgeotropism. Authors seem generally to look at the bending of a radicletowards the centre of the earth, as the direct result of gravitation, whichis believed to modify the growth of the upper or lower surfaces, in such amanner as to induce curvature in the proper direction. But we now know thatit is the tip alone which is acted on, and that this part transmits someinfluence to the adjoining parts, causing them to curve downwards. Gravitydoes not appear to act in a more direct manner on a radicle, than it doeson any lowly organised animal, which moves away when it feels some weightor pressure. [page 546]

CHAPTER XII. 

SUMMARY AND CONCLUDING REMARKS. 

Nature of the circumnutating movement--History of a germinating seed--Theradicle first protrudes and circumnutates--Its tip highly sensitive--Emergence of the hypocotyl or of the epicotyl from the ground under theform of an arch - Its circumnutation and that of the cotyledons--Theseedling throws up a leaf-bearing stem--The circumnutation of all the partsor organs--Modified circumnutation--Epinasty and hyponasty--Movements ofclimbing plants--Nyctitropic movements--Movements excited by light andgravitation--Localised sensitiveness--Resemblance between the movements ofplants and animals--The tip of the radicle acts like a brain. 

IT may be useful to the reader if we briefly sum up the chief conclusions, which, as far as we can judge, have been fairly well established by theobservations given in this volume. All the parts or organs in every plantwhilst they continue to grow, and some parts which are provided withpulvini after they have ceased to grow, are continually circumnutating. This movement commences even before the young seedling has broken throughthe ground. The nature of the movement and its causes, as far asascertained, have been briefly described in the Introduction. Why everypart of a plant whilst it is growing, and in some cases after growth hasceased, should have its cells rendered more turgescent and its cell-wallsmore extensile first on one side and then on another, thus inducingcircumnutation is not known. It would appear as if the changes in the cellsrequired periods of rest. [page 547]

In some cases, as with the hypocotyls of Brassica, the leaves of Dionaeaand the joints of the Gramineae, the circumnutating movement when viewedunder the microscope is seen to consist of innumerable small oscillations. The part under observation suddenly jerks forwards for a length of . 002 to. 001 of an inch, and then slowly retreats for a part of this distance;after a few seconds it again jerks forwards, but with many intermissions. The retreating movement apparently is due to the elasticity of theresisting tissues. How far this oscillatory movement is general we do notknow, as not many circumnutating plants were observed by us under themicroscope; but no such movement could be detected in the case of Droserawith a 2-inch object-glass which we used. The phenomenon is a remarkableone. The whole hypocotyl of a cabbage or the whole leaf of a Dionaea couldnot jerk forwards unless a very large number of cells on one side weresimultaneously affected. Are we to suppose that these cells steadily becomemore and more turgescent on one side, until the part suddenly yields andbends, inducing what may be called a microscopically minute earthquake inthe plant; or do the cells on one side suddenly become turgescent in anintermittent manner; each forward movement thus caused being opposed by theelasticity of the tissues?

Circumnutation is of paramount importance in the life of every plant; forit is through its modification that many highly beneficial or necessarymovements have been acquired. When light strikes one side of a plant, orlight changes into darkness, or when gravitation acts on a displaced part, the plant is enabled in some unknown manner to increase the always varyingturgescence of the cells on one side; so that the ordinary circumnutatingmovement is[page 548]modified, and the part bends either to or from the exciting cause; or itmay occupy a new position, as in the so-called sleep of leaves. Theinfluence which modifies circumnutation may be transmitted from one part toanother. Innate or constitutional changes, independently of any externalagency, often modify the circumnutating movements at particular periods ofthe life of the plant. As circumnutation is universally present, we canunderstand how it is that movements of the same kind have been developed inthe most distinct members of the vegetable series. But it must not besupposed that all the movements of plants arise from modifiedcircumnutation; for, as we shall presently see, there is reason to believethat this is not the case. 

Having made these few preliminary remarks, we will in imagination take agerminating seed, and consider the part which the various movements play inthe life-history of the plant. The first change is the protrusion of theradicle, which begins at once to circumnutate. This movement is immediatelymodified by the attraction of gravity and rendered geotropic. The radicle, therefore, supposing the seed to be lying on the surface, quickly bendsdownwards, following a more or less spiral course, as was seen on thesmoked glass-plates. Sensitiveness to gravitation resides in the tip; andit is the tip which transmits some influence to the adjoining parts, causing them to bend. As soon as the tip, protected by the root-cap, reaches the ground, it penetrates the surface, if this be soft or friable;and the act of penetration is apparently aided by the rocking orcircumnutating movement of the whole end of the radicle. If the surface iscompact, and cannot easily be penetrated, then[page 549]the seed itself, unless it be a heavy one, is displaced or lifted up by thecontinued growth and elongation of the radicle. But in a state of natureseeds often get covered with earth or other matter, or fall into crevices, etc. , and thus a point of resistance is afforded, and the tip can moreeasily penetrate the ground. But even with seeds lying loose on the surfacethere is another aid: a multitude of excessively fine hairs are emittedfrom the upper part of the radicle, and these attach themselves firmly tostones or other objects lying on the surface, and can do so even to glass;and thus the upper part is held down whilst the tip presses against andpenetrates the ground. The attachment of the root-hairs is effected by theliquefaction of the outer surface of the cellulose walls, and by thesubsequent setting hard of the liquefied matter. This curious processprobably takes place, not for the sake of the attachment of the radicles tosuperficial objects, but in order that the hairs may be brought into theclosest contact with the particles in the soil, by which means they canabsorb the layer of water surrounding them, together with any dissolvedmatter. 

After the tip has penetrated the ground to a little depth, the increasingthickness of the radicle, together with the root-hairs, hold it securely inits place; and now the force exerted by the longitudinal growth of theradicle drives the tip deeper into the ground. This force, combined withthat due to transverse growth, gives to the radicle the power of a wedge. Even a growing root of moderate size, such as that of a seedling bean, candisplace a weight of some pounds. It is not probable that the tip whenburied in compact earth can actually circumnutate and thus aid its downwardpassage, but the circumnutating movement will facilitate the tip enteringany lateral[page 550]or oblique fissure in the earth, or a burrow made by an earth-worm orlarva; and it is certain that roots often run down the old burrows ofworms. The tip, however, in endeavouring to circumnutate, will continuallypress against the earth on all sides, and this can hardly fail to be of thehighest importance to the plant; for we have seen that when little bits ofcard-like paper and of very thin paper were cemented on opposite sides ofthe tip, the whole growing part of the radicle was excited to bend awayfrom the side bearing the card or more resisting substance, towards theside bearing the thin paper. We may therefore feel almost sure that whenthe tip encounters a stone or other obstacle in the ground, or even earthmore compact on one side than the other, the root will bend away as much asit can from the obstacle or the more resisting earth, and will thus followwith unerring skill a line of least resistance. 

The tip is more sensitive to prolonged contact with an object than togravitation when this acts obliquely on the radicle, and sometimes evenwhen it acts in the most favourable direction at right angles to theradicle. The tip was excited by an attached bead of shellac weighing lessthan 1/200th of a grain (0. 33 mg. ); it is therefore more sensitive than themost delicate tendril, namely, that of Passiflora gracilis, which wasbarely acted on by a bit of wire weighing 1/50th of a grain. But thisdegree of sensitiveness is as nothing compared with that of the glands ofDrosera, for these are excited by particles weighing only 1/78740 of agrain. The sensitiveness of the tip cannot be accounted for by its beingcovered by a thinner layer of tissue than the other parts, for it isprotected by the relatively thick root-cap. It is remarkable that althoughthe radicle bends away, when one side of the tip is slightly touched[page 551]with caustic, yet if the side be much cauterised the injury is too great, and the power of transmitting some influence to the adjoining parts causingthem to bend, is lost. Other analogous cases are known to occur. 

After a radicle has been deflected by some obstacle, geotropism directs thetip again to grow perpendicularly downwards; but geotropism is a feeblepower, and here, as Sachs has shown, another interesting adaptive movementcomes into play; for radicles at a distance of a few millimeters from thetip are sensitive to prolonged contact in such a manner that they bendtowards the touching object, instead of from it as occurs when an objecttouches one side of the tip. Moreover, the curvature thus caused is abrupt;the pressed part alone bending. Even slight pressure suffices, such as abit of card cemented to one side. Therefore a radicle, as it passes overthe edge of any obstacle in the ground, will through the action ofgeotropism press against it; and this pressure will cause the radicle toendeavour to bend abruptly over the edge. It will thus recover as quicklyas possible its normal downward course. 

Radicles are also sensitive to air which contains more moisture on one sidethan the other, and they bend towards its source. It is therefore probablethat they are in like manner sensitive to dampness in the soil. It wasascertained in several cases that this sensitiveness resides in the tip, which transmits an influence causing the adjoining upper part to bend inopposition to geotropism towards the moist object. We may therefore inferthat roots will be deflected from their downward course towards any sourceof moisture in the soil. 

Again, most or all radicles are slightly sensitive to light, and accordingto Wiesner, generally bend a little[page 552]from it. Whether this can be of any service to them is very doubtful, butwith seeds germinating on the surface it will slightly aid geotropism indirecting the radicles to the ground. * We ascertained in one instance thatsuch sensitiveness resided in the tip, and caused the adjoining parts tobend from the light. The sub-aërial roots observed by Wiesner were allapheliotropic, and this, no doubt, is of use in bringing them into contactwith trunks of trees or surfaces of rock, as is their habit. 

We thus see that with seedling plants the tip of the radicle is endowedwith diverse kinds of sensitiveness; and that the tip directs the adjoininggrowing parts to bend to or from the exciting cause, according to the needsof the plant. The sides of the radicle are also sensitive to contact, butin a widely different manner. Gravitation, though a less powerful cause ofmovement than the other above specified stimuli, is ever present; so thatit ultimately prevails and determines the downward growth of the root. 

The primary radicle emits secondary ones which project sub-horizontally;and these were observed in one case to circumnutate. Their tips are alsosensitive to contact, and they are thus excited to bend away from anytouching object; so that they resemble in these respects, as far as theywere observed, the primary radicles. If displaced they resume, as Sachs hasshown, their original sub-horizontal position; and this apparently is dueto diageotropism. The secondary radicles emit tertiary ones, but these, inthe case of the bean, are not affected by gravitation; consequently theyprotrude in all directions. Thus the general

* Dr. Karl Richter, who has especially attended to this subject ('K. Akad. Der Wissenschaften in Wien, ' 1879, p. 149), states that apheliotropism doesnot aid radicles in penetrating the ground. [page 553]

arrangement of the three orders of roots is excellently adapted forsearching the whole soil for nutriment. 

Sachs has shown that if the tip of the primary radicle is cut off (and thetip will occasionally be gnawed off with seedlings in a state of nature)one of the secondary radicles grows perpendicularly downwards, in a mannerwhich is analogous to the upward growth of a lateral shoot after theamputation of the leading shoot. We have seen with radicles of the beanthat if the primary radicle is merely compressed instead of being cut off, so that an excess of sap is directed into the secondary radicles, theirnatural condition is disturbed and they grow downwards. Other analogousfacts have been given. As anything which disturbs the constitution is aptto lead to reversion, that is, to the resumption of a former character, itappears probable that when secondary radicles grow downwards or lateralshoots upwards, they revert to the primary manner of growth proper toradicles and shoots. 

With dicotyledonous seeds, after the protrusion of the radicle, thehypocotyl breaks through the seed-coats; but if the cotyledons arehypogean, it is the epicotyl which breaks forth. These organs are at firstinvariably arched, with the upper part bent back parallel to the lower; andthey retain this form until they have risen above the ground. In somecases, however, it is the petioles of the cotyledons or of the first trueleaves which break through the seed-coats as well as the ground, before anypart of the stem protrudes; and then the petioles are almost invariablyarched. We have met with only one exception, and that only a partial one, namely, with the petioles of the two first leaves of Acanthus candelabrum. With Delphinium nudicaule the petioles of the two cotyledons are com-[page 554]pletely confluent, and they break through the ground as an arch; afterwardsthe petioles of the successively formed early leaves are arched, and theyare thus enabled to break through the base of the confluent petioles of thecotyledons. In the case of Megarrhiza, it is the plumule which breaks as anarch through the tube formed by the confluence of the cotyledon-petioles. With mature plants, the flower-stems and the leaves of some few species, and the rachis of several ferns, as they emerge separately from the ground, are likewise arched. The fact of so many different organs in plants of many kinds breakingthrough the ground under the form of an arch, shows that this must be insome manner highly important to them. According to Haberlandt, the tendergrowing apex is thus saved from abrasion, and this is probably the trueexplanation. But as both legs of the arch grow, their power of breakingthrough the ground will be much increased as long as the tip remains withinthe seed-coats and has a point of support. In the case of monocotyledonsthe plumule or cotyledon is rarely arched, as far as we have seen; but thisis the case with the leaf-like cotyledon of the onion; and the crown of thearch is here strengthened by a special protuberance. In the Gramineae thesummit of the straight, sheath-like cotyledon is developed into a hardsharp crest, which evidently serves for breaking through the earth. Withdicotyledons the arching of the epicotyl or hypocotyl often appears as ifit merely resulted from the manner in which the parts are packed within theseed; but it is doubtful whether this is the whole of the truth in anycase, and it certainly was not so in several cases, in which the archingwas seen to commence after the parts had wholly[page 555]escaped from the seed-coats. As the arching occurred in whatever positionthe seeds were placed, it is no doubt due to temporarily increased growthof the nature of epinasty or hyponasty along one side of the part. 

As this habit of the hypocotyl to arch itself appears to be universal, itis probably of very ancient origin. It is therefore not surprising that itshould be inherited, at least to some extent, by plants having hypogeancotyledons, in which the hypocotyl is only slightly developed and neverprotrudes above the ground, and in which the arching is of course now quiteuseless. This tendency explains, as we have seen, the curvature of thehypocotyl (and the consequent movement of the radicle) which was firstobserved by Sachs, and which we have often had to refer to as Sachs'curvature. 

The several foregoing arched organs are continually circumnutating, orendeavouring to circumnutate, even before they break through the ground. Assoon as any part of the arch protrudes from the seed-coats it is acted uponby apogeotropism, and both the legs bend upwards as quickly as thesurrounding earth will permit, until the arch stands vertically. Bycontinued growth it then forcibly breaks through the ground; but as it iscontinually striving to circumnutate this will aid its emergence in someslight degree, for we know that a circumnutating hypocotyl can push awaydamp sand on all sides. As soon as the faintest ray of light reaches aseedling, heliotropism will guide it through any crack in the soil, orthrough an entangled mass of overlying vegetation; for apogeotropism byitself can direct the seedling only blindly upwards. Hence probably it isthat sensitiveness to light resides in the tip of the cotyledons of theGramineae, and in[page 556]the upper part of the hypocotyls of at least some plants. 

As the arch grows upwards the cotyledons are dragged out of the ground. Theseed-coats are either left behind buried, or are retained for a time stillenclosing the cotyledons. These are afterwards cast off merely by theswelling of the cotyledons. But with most of the Cucurbitaceae there is acurious special contrivance for bursting the seed-coats whilst beneath theground, namely, a peg at the base of the hypocotyl, projecting at rightangles, which holds down the lower half of the seed-coats, whilst thegrowth of the arched part of the hypocotyl lifts up the upper half, andthus splits them in twain. A somewhat analogous structure occurs in Mimosapudica and some other plants. Before the cotyledons are fully expanded andhave diverged, the hypocotyl generally straightens itself by increasedgrowth along the concave side, thus reversing the process which caused thearching. Ultimately not a trace of the former curvature is left, except inthe case of the leaf-like cotyledons of the onion. 

The cotyledons can now assume the function of leaves, and decomposecarbonic acid; they also yield up to other parts of the plant the nutrimentwhich they often contain. When they contain a large stock of nutriment theygenerally remain buried beneath the ground, owing to the small developmentof the hypocotyl; and thus they have a better chance of escapingdestruction by animals. From unknown causes, nutriment is sometimes storedin the hypocotyl or in the radicle, and then one of the cotyledons or bothbecome rudimentary, of which several instances have been given. It isprobable that the extraordinary manner of germination of MegarrhizaCalifornica, [page 557]Ipomoea leptophylla and pandurata, and of Quercus virens, is connected withthe burying of the tuber-like roots, which at an early age are stocked withnutriment; for in these plants it is the petioles of the cotyledons whichfirst protrude from the seeds, and they are then merely tipped with aminute radicle and hypocotyl. These petioles bend down geotropically like aroot and penetrate the ground, so that the true root, which afterwardsbecomes greatly enlarged, is buried at some little depth beneath thesurface. Gradations of structure are always interesting, and Asa Grayinforms us that with Ipomoea Jalappa, which likewise forms huge tubers, thehypocotyl is still of considerable length, and the petioles of thecotyledons are only moderately elongated. But in addition to the advantagegained by the concealment of the nutritious matter stored within thetubers, the plumule, at least in the case of Megarrhiza, is protected fromthe frosts of winter by being buried. 

With many dicotyledonous seedlings, as has lately been described by DeVries, the contraction of the parenchyma of the upper part of the radicledrags the hypocotyl downwards into the earth; sometimes (it is said) untileven the cotyledons are buried. The hypocotyl itself of some speciescontracts in a like manner. It is believed that this burying process servesto protect the seedlings against the frosts of winter. 

Our imaginary seedling is now mature as a seedling, for its hypocotyl isstraight and its cotyledons are fully expanded. In this state the upperpart of the hypocotyl and the cotyledons continue for some time tocircumnutate, generally to a wide extent relatively to the size of theparts, and at a rapid rate. But seedlings profit by this power of movementonly when it is modified, especially by the action of light and[page 558]gravitation; for they are thus enabled to move more rapidly and to agreater extent than can most mature plants. Seedlings are subjected to asevere struggle for life, and it appears to be highly important to themthat they should adapt themselves as quickly and as perfectly as possibleto their conditions. Hence also it is that they are so extremely sensitiveto light and gravitation. The cotyledons of some few species are sensitiveto a touch; but it is probable that this is only an indirect result of theforegoing kinds of sensitiveness, for there is no reason to believe thatthey profit by moving when touched. 

Our seedling now throws up a stem bearing leaves, and often branches, allof which whilst young are continually circumnutating. If we look, forinstance, at a great acacia tree, we may feel assured that every one of theinnumerable growing shoots is constantly describing small ellipses; as iseach petiole, sub-petiole, and leaflet. The latter, as well as ordinaryleaves, generally move up and down in nearly the same vertical plane, sothat they describe very narrow ellipses. The flower-peduncles are likewisecontinually circumnutating. If we could look beneath the ground, and oureyes had the power of a microscope, we should see the tip of each rootletendeavouring to sweep small ellipses or circles, as far as the pressure ofthe surrounding earth permitted. All this astonishing amount of movementhas been going on year after year since the time when, as a seedling, thetree first emerged from the ground. 

Stems are sometimes developed into long runners or stolons. Thesecircumnutate in a conspicuous manner, and are thus aided in passing betweenand over surrounding obstacles. But whether the circumnutating movement hasbeen increased for this special purpose is doubtful. [page 559]

We have now to consider circumnutation in a modified form, as the source ofseveral great classes of movement. The modification may be determined byinnate causes, or by external agencies. Under the first head we see leaveswhich, when first unfolded, stand in a vertical position, and graduallybend downwards as they grow older. We see flower-peduncles bending downafter the flower has withered, and others rising up; or again, stems withtheir tips at first bowed downwards, so as to be hooked, afterwardsstraightening themselves; and many other such cases. These changes ofposition, which are due to epinasty or hyponasty, occur at certain periodsof the life of the plant, and are independent of any external agency. Theyare effected not by a continuous upward or downward movement, but by asuccession of small ellipses, or by zigzag lines, --that is, by acircumnutating movement which is preponderant in some one direction. 

Again, climbing plants whilst young circumnutate in the ordinary manner, but as soon as the stem has grown to a certain height, which is differentfor different species, it elongates rapidly, and now the amplitude of thecircumnutating movement is immensely increased, evidently to favour thestem catching hold of a support. The stem also circumnutates rather moreequally to all sides than in the case of non-climbing plants. This isconspicuously the case with those tendrils which consist of modifiedleaves, as these sweep wide circles; whilst ordinary leaves usuallycircumnutate nearly in the same vertical plane. Flower-peduncles whenconverted into tendrils have their circumnutating movement in like mannergreatly increased. 

We now come to our second group of circumnu-[page 560]tating movements--those modified through external agencies. The so-calledsleep or nyctitropic movements of leaves are determined by the dailyalternations of light and darkness. It is not the darkness which excitesthem to move, but the difference in the amount of light which they receiveduring the day and night; for with several species, if the leaves have notbeen brightly illuminated during the day, they do not sleep at night. Theyinherit, however, some tendency to move at the proper periods, independently of any change in the amount of light. The movements are insome cases extraordinarily complex, but as a full summary has been given inthe chapter devoted to this subject, we will here say but little on thishead. Leaves and cotyledons assume their nocturnal position by two means, by the aid of pulvini and without such aid. In the former case the movementcontinues as long as the leaf or cotyledon remains in full health; whilstin the latter case it continues only whilst the part is growing. Cotyledonsappear to sleep in a larger proportional number of species than do leaves. In some species, the leaves sleep and not the cotyledons; in others, thecotyledons and not the leaves; or both may sleep, and yet assume widelydifferent positions at night. 

Although the nyctitropic movements of leaves and cotyledons are wonderfullydiversified, and sometimes differ much in the species of the same genus, yet the blade is always placed in such a position at night, that its uppersurface is exposed as little as possible to full radiation. We cannot doubtthat this is the object gained by these movements; and it has been provedthat leaves exposed to a clear sky, with their blades compelled to remainhorizontal, suffered much more from the cold than others which were allowedto assume[page 561]their proper vertical position. Some curious facts have been given underthis head, showing that horizontally extended leaves suffered more atnight, when the air, which is not cooled by radiation, was prevented fromfreely circulating beneath their lower surfaces; and so it was, when theleaves were allowed to go to sleep on branches which had been renderedmotionless. In some species the petioles rise up greatly at night, and thepinnae close together. The whole plant is thus rendered more compact, and amuch smaller surface is exposed to radiation. 

That the various nyctitropic movements of leaves result from modifiedcircumnutation has, we think, been clearly shown. In the simplest cases aleaf describes a single large ellipse during the 24 h. ; and the movement isso arranged that the blade stands vertically during the night, andreassumes its former position on the following morning. The course pursueddiffers from ordinary circumnutation only in its greater amplitude, and inits greater rapidity late in the evening and early on the followingmorning. Unless this movement is admitted to be one of circumnutation, suchleaves do not circumnutate at all, and this would be a monstrous anomaly. In other cases, leaves and cotyledons describe several vertical ellipsesduring the 24 h. ; and in the evening one of them is increased greatly inamplitude until the blade stands vertically either upwards or downwards. Inthis position it continues to circumnutate until the following morning, when it reassumes its former position. These movements, when a pulvinus ispresent, are often complicated by the rotation of the leaf or leaflet; andsuch rotation on a small scale occurs during ordinary circumnutation. Themany diagrams showing the movements of sleeping and non-sleeping leaves andcoty-[page 562]ledons should be compared, and it will be seen that they are essentiallyalike. Ordinary circumnutation is converted into a nyctitropic movement, firstly by an increase in its amplitude, but not to so great a degree as inthe case of climbing plants, and secondly by its being rendered periodic inrelation to the alternations of day and night. But there is frequently adistinct trace of periodicity in the circumnutating movements ofnon-sleeping leaves and cotyledons. The fact that nyctitropic movementsoccur in species distributed in many families throughout the whole vascularseries, is intelligible, if they result from the modification of theuniversally present movement of circumnutation; otherwise the fact isinexplicable. 

In the seventh chapter we have given the case of a Porlieria, the leafletsof which remained closed all day, as if asleep, when the plant was keptdry, apparently for the sake of checking evaporation. Something of the samekind occurs with certain Gramineae. At the close of this same chapter, afew observations were appended on what may be called the embryology ofleaves. The leaves produced by young shoots on cut-down plants of MelilotusTaurica slept like those of a Trifolium, whilst the leaves on the olderbranches on the same plants slept in a very different manner, proper to thegenus; and from the reasons assigned we are tempted to look at this case asone of reversion to a former nyctitropic habit. So again with Desmodiumgyrans, the absence of small lateral leaflets on very young plants, makesus suspect that the immediate progenitor of this species did not possesslateral leaflets, and that their appearance in an almost rudimentarycondition at a somewhat more advanced age is the result of reversion to atrifoliate predecessor. However this may be, the rapid circumnutating or[page 563]gyrating movements of the little lateral leaflets, seem to be dueproximately to the pulvinus, or organ of movement, not having been reducednearly so much as the blade, during the successive modifications throughwhich the species has passed. 

We now come to the highly important class of movements due to the action ofa lateral light. When stems, leaves, or other organs are placed, so thatone side is illuminated more brightly than the other, they bend towards thelight. This heliotropic movement manifestly results from the modificationof ordinary circumnutation; and every gradation between the two movementscould be followed. When the light was dim, and only a very little brighteron one side than on the other, the movement consisted of a succession ofellipses, directed towards the light, each of which approached nearer toits source than the previous one. When the difference in the light on thetwo sides was somewhat greater, the ellipses were drawn out into astrongly-marked zigzag line, and when much greater the course becamerectilinear. We have reason to believe that changes in the turgescence ofthe cells is the proximate cause of the movement of circumnutation; and itappears that when a plant is unequally illuminated on the two sides, thealways changing turgescence is augmented along one side, and is weakened orquite arrested along the other sides. Increased turgescence is commonlyfollowed by increased growth, so that a plant which has bent itself towardsthe light during the day would be fixed in this position were it not forapogeotropism acting during the night. But parts provided with pulvinibend, as Pfeffer has shown, towards the light; and here growth does notcome into play any more than in the ordinary circumnutating movements ofpulvini. [page 564]

Heliotropism prevails widely throughout the vegetable kingdom, butwhenever, from the changed habits of life of any plant, such movementsbecome injurious or useless, the tendency is easily eliminated, as we seewith climbing and insectivorous plants. 

Apheliotropic movements are comparatively rare in a well-marked degree, excepting with sub-aërial roots. In the two cases investigated by us, themovement certainly consisted of modified circumnutation. 

The position which leaves and cotyledons occupy during the day, namely, more or less transversely to the direction of the light, is due, accordingto Frank, to what we call diaheliotropism. As all leaves and cotyledons arecontinually circumnutating, there can hardly be a doubt thatdiaheliotropism results from modified circumnutation. From the fact ofleaves and cotyledons frequently rising a little in the evening, it appearsas if diaheliotropism had to conquer during the middle of the day a widelyprevalent tendency to apogeotropism. 

Lastly, the leaflets and cotyledons of some plants are known to be injuredby too much light; and when the sun shines brightly on them, they moveupwards or downwards, or twist laterally, so that they direct their edgestowards the light, and thus they escape being injured. Theseparaheliotropic movements certainly consisted in one case of modifiedcircumnutation; and so it probably is in all cases, for the leaves of allthe species described circumnutate in a conspicuous manner. This movementhas hitherto been observed only with leaflets provided with pulvini, inwhich the increased turgescence on opposite sides is not followed bygrowth; and we can understand why this should be so, as the movement isrequired only for a temporary purpose. It would manifestly be dis-[page 565]advantageous for the leaf to be fixed by growth in its inclined position. For it has to assume its former horizontal position, as soon as possibleafter the sun has ceased shining too brightly on it. 

The extreme sensitiveness of certain seedlings to light, as shown in ourninth chapter, is highly remarkable. The cotyledons of Phalaris becamecurved towards a distant lamp, which emitted so little light, that a pencilheld vertically close to the plants, did not cast any shadow which the eyecould perceive on a white card. These cotyledons, therefore, were affectedby a difference in the amount of light on their two sides, which the eyecould not distinguish. The degree of their curvature within a given timetowards a lateral light did not correspond at all strictly with the amountof light which they received; the light not being at any time in excess. They continued for nearly half an hour to bend towards a lateral light, after it had been extinguished. They bend with remarkable precision towardsit, and this depends on the illumination of one whole side, or on theobscuration of the whole opposite side. The difference in the amount oflight which plants at any time receive in comparison with what they haveshortly before received, seems in all cases to be the chief exciting causeof those movements which are influenced by light. Thus seedlings broughtout of darkness bend towards a dim lateral light, sooner than others whichhad previously been exposed to daylight. We have seen several analogouscases with the nyctitropic movements of leaves. A striking instance wasobserved in the case of the periodic movements of the cotyledons of aCassia; in the morning a pot was placed in an obscure part of a room, andall the cotyledons rose up closed; another pot had stood in the sunlight, and[page 566]the cotyledons of course remained expanded; both pots were now placed closetogether in the middle of the room, and the cotyledons which had beenexposed to the sun, immediately began to close, while the others opened; sothat the cotyledons in the two pots moved in exactly opposite directionswhilst exposed to the same degree of light. 

We found that if seedlings, kept in a dark place, were laterallyilluminated by a small wax taper for only two or three minutes at intervalsof about three-quarters of an hour, they all became bowed to the pointwhere the taper had been held. We felt much surprised at this fact, anduntil we had read Wiesner's observations, we attributed it to theafter-effects of the light; but he has shown that the same degree ofcurvature in a plant may be induced in the course of an hour by severalinterrupted illuminations lasting altogether for 20 m. , as by a continuousillumination of 60 m. We believe that this case, as well as our own, may beexplained by the excitement from light being due not so much to its actualamount, as to the difference in amount from that previously received; andin our case there were repeated alternations from complete darkness tolight. In this, and in several of the above specified respects, light seemsto act on the tissues of plants, almost in the same manner as it does onthe nervous system of animals. There is a much more striking analogy of the same kind, in thesensitiveness to light being localised in the tips of the cotyledons ofPhalaris and Avena, and in the upper part of the hypocotyls of Brassica andBeta; and in the transmission of some influence from these upper to thelower parts, causing the latter to bend towards the light. This influenceis also trans-[page 567]mitted beneath the soil to a depth where no light enters. It follows fromthis localisation, that the lower parts of the cotyledons of Phalaris, etc. , which normally become more bent towards a lateral light than theupper parts, may be brightly illuminated during many hours, and will notbend in the least, if all light be excluded from the tip. It is aninteresting experiment to place caps over the tips of the cotyledons ofPhalaris, and to allow a very little light to enter through minute orificeson one side of the caps, for the lower part of the cotyledons will thenbend to this side, and not to the side which has been brightly illuminatedduring the whole time. In the case of the radicles of Sinapis alba, sensitiveness to light also resides in the tip, which, when laterallyilluminated, causes the adjoining part of the root to bendapheliotropically. 

Gravitation excites plants to bend away from the centre of the earth, ortowards it, or to place themselves in a transverse position with respect toit. Although it is impossible to modify in any direct manner the attractionof gravity, yet its influence could be moderated indirectly, in the severalways described in the tenth chapter; and under such circumstances the samekind of evidence as that given in the chapter on Heliotropism, showed inthe plainest manner that apogeotropic and geotropic, and probablydiageotropic movements, are all modified forms of circumnutation. 

Different parts of the same plant and different species are affected bygravitation in widely different degrees and manners. Some plants and organsexhibit hardly a trace of its action. Young seedlings which, as we know, circumnutate rapidly, are eminently sensitive; and we have seen thehypocotyl of Beta bending[page 568]upwards through 109o in 3 h. 8 m. The after-effects of apogeotropism lastfor above half an hour; and horizontally-laid hypocotyls are sometimes thuscarried temporarily beyond an upright position. The benefits derived fromgeotropism, apogeotropism, and diageotropism, are generally so manifestthat they need not be specified. With the flower-peduncles of Oxalis, epinasty causes them to bend down, so that the ripening pods may beprotected by the calyx from the rain. Afterwards they are carried upwardsby apogeotropism in combination with hyponasty, and are thus enabled toscatter their seeds over a wider space. The capsules and flower-heads ofsome plants are bowed downwards through geotropism, and they then burythemselves in the earth for the protection and slow maturation of theseeds. This burying process is much facilitated by the rocking movement dueto circumnutation. 

In the case of the radicles of several, probably of all seedling plants, sensitiveness to gravitation is confined to the tip, which transmits aninfluence to the adjoining upper part, causing it to bend towards thecentre of the earth. That there is transmission of this kind was proved inan interesting manner when horizontally extended radicles of the bean wereexposed to the attraction of gravity for 1 or 1 ½ h. , and their tips werethen amputated. Within this time no trace of curvature was exhibited, andthe radicles were now placed pointing vertically downwards; but aninfluence had already been transmitted from the tip to the adjoining part, for it soon became bent to one side, in the same manner as would haveoccurred had the radicle remained horizontal and been still acted on bygeotropism. Radicles thus treated continued to grow out horizontally fortwo or three days, until a new tip was[page 569]re-formed; and this was then acted on by geotropism, and the radicle becamecurved perpendicularly downwards. 

It has now been shown that the following important classes of movement allarise from modified circumnutation, which is omnipresent whilst growthlasts, and after growth has ceased, whenever pulvini are present. Theseclasses of movement consist of those due to epinasty and hyponasty, --thoseproper to climbing plants, commonly called revolving nutation, --thenyctitropic or sleep movements of leaves and cotyledons, --and the twoimmense classes of movement excited by light and gravitation. When we speakof modified circumnutation we mean that light, or the alternations of lightand darkness, gravitation, slight pressure or other irritants, and certaininnate or constitutional states of the plant, do not directly cause themovement; they merely lead to a temporary increase or diminution of thosespontaneous changes in the turgescence of the cells which are already inprogress. In what manner, light, gravitation, etc. , act on the cells is notknown; and we will here only remark that, if any stimulus affected thecells in such a manner as to cause some slight tendency in the affectedpart to bend in a beneficial manner, this tendency might easily beincreased through the preservation of the more sensitive individuals. Butif such bending were injurious, the tendency would be eliminated unless itwas overpoweringly strong; for we know how commonly all characters in allorganisms vary. Nor can we see any reason to doubt, that after the completeelimination of a tendency to bend in some one direction under a certainstimulus, the power to bend in a directly[page 570]opposite direction might gradually be acquired through natural selection. *

Although so many movements have arisen through modified circumnutation, there are others which appear to have had a quite independent origin; butthey do not form such large and important classes. When a leaf of a Mimosais touched it suddenly assumes the same position as when asleep, but Bruckehas shown that this movement results from a different state of turgescencein the cells from that which occurs during sleep; and as sleep-movementsare certainly due to modified circumnutation, those from a touch can hardlybe thus due. The back of a leaf of Drosera rotundifolia was cemented to thesummit of a stick driven into the ground, so that it could not move in theleast, and a tentacle was observed during many hours under the microscope;but it exhibited no circumnutating movement, yet after being momentarilytouched with a bit of raw meat, its basal part began to curve in 23seconds. This curving movement therefore could not have resulted frommodified circumnutation. But when a small object, such as a fragment of abristle, was placed on one side of the tip of a radicle, which we know iscontinually circumnutating, the induced curvature was so similar to themovement caused by geotropism, that we can hardly doubt that it is due tomodified circumnutation. A flower of a Mahonia was cemented to a stick, andthe stamens exhibited no signs of circumnutation under the microscope, yetwhen they were lightly touched they suddenly moved towards the pistil. Lastly, the curling of the extremity of a tendril when

* See the remarks in Frank's 'Die wagerechte Richtung von Pflanzentheilen'(1870, pp. 90, 91, etc. ), on natural selection in connection withgeotropism, heliotropism, etc. [page 571]

touched seems to be independent of its revolving or circumnutatingmovement. This is best shown by the part which is the most sensitive tocontact, circumnutating much less than the lower parts, or apparently notat all. *

Although in these cases we have no reason to believe that the movementdepends on modified circumnutation, as with the several classes of movementdescribed in this volume, yet the difference between the two sets of casesmay not be so great as it at first appears. In the one set, an irritantcauses an increase or diminution in the turgescence of the cells, which arealready in a state of change; whilst in the other set, the irritant firststarts a similar change in their state of turgescence. Why a touch, slightpressure or any other irritant, such as electricity, heat, or theabsorption of animal matter, should modify the turgescence of the affectedcells in such a manner as to cause movement, we do not know. But a touchacts in this manner so often, and on such widely distinct plants, that thetendency seems to be a very general one; and if beneficial, it might beincreased to any extent. In other cases, a touch produces a very differenteffect, as with Nitella, in which the protoplasm may be seen to recede fromthe walls of the cell; in Lactuca, in which a milky fluid exudes; and inthe tendrils of certain Vitaceae, Cucurbitaceae, and Bignoniaceae, in whichslight pressure causes a cellular outgrowth. 

Finally it is impossible not to be struck with the resemblance between theforegoing movements of plants and many of the actions performedunconsciously by the lower animals. ** With plants an

* For the evidence on this head, see the 'Movements and Habits of ClimbingPlants, ' 1875, pp. 173, 174. 

** Sachs remarks to nearly the same effect: "Dass sich die le-[[page 572]]bende Pflanzensubstanz derart innerlich differenzirt, dass einzelne Theilemit specifischen Energien ausgerüstet sind, ähnlich, wie die verschiedenenSinnesnerven des Thiere" ('Arbeiten des Bot. Inst. In Würzburg, ' Bd. Ii. 1879, p. 282). [page 572]

astonishingly small stimulus suffices; and even with allied plants one maybe highly sensitive to the slightest continued pressure, and another highlysensitive to a slight momentary touch. The habit of moving at certainperiods is inherited both by plants and animals; and several other pointsof similitude have been specified. But the most striking resemblance is thelocalisation of their sensitiveness, and the transmission of an influencefrom the excited part to another which consequently moves. Yet plants donot of course possess nerves or a central nervous system; and we may inferthat with animals such structures serve only for the more perfecttransmission of impressions, and for the more complete intercommunicationof the several parts. 

We believe that there is no structure in plants more wonderful, as far asits functions are concerned, than the tip of the radicle. If the tip belightly pressed or burnt or cut, it transmits an influence to the upperadjoining part, causing it to bend away from the affected side; and, whatis more surprising, the tip can distinguish between a slightly harder andsofter object, by which it is simultaneously pressed on opposite sides. If, however, the radicle is pressed by a similar object a little above the tip, the pressed part does not transmit any influence to the more distant parts, but bends abruptly towards the object. If the tip perceives the air to bemoister on one side than on the other, it likewise transmits an influenceto the upper adjoining part, which bends towards the source of moisture. When the tip is excited by light (though[page 573]in the case of radicles this was ascertained in only a single instance) theadjoining part bends from the light; but when excited by gravitation thesame part bends towards the centre of gravity. In almost every case we canclearly perceive the final purpose or advantage of the several movements. Two, or perhaps more, of the exciting causes often act simultaneously onthe tip, and one conquers the other, no doubt in accordance with itsimportance for the life of the plant. The course pursued by the radicle inpenetrating the ground must be determined by the tip; hence it has acquiredsuch diverse kinds of sensitiveness. It is hardly an exaggeration to saythat the tip of the radicle thus endowed, and having the power of directingthe movements of the adjoining parts, acts like the brain of one of thelower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the severalmovements. 

[page 574]

INDEX. 

ABIES--AMPHICARPOEA. 

A. 

Abies communis, effect of killing or injuring the leading shoot, 187-- pectinata, effect of killing or injuring the leading shoot, 187--, affected by Aecidium elatinum, 188

Abronia umbellata, its single, developed cotyledon, 78--, rudimentary cotyledon, 95--, rupture of the seed coats, 105

Abutilon Darwinii, sleep of leaves and not of cotyledons, 314--, nocturnal movement of leaves, 323

Acacia Farnesiana, state of plant when awake and asleep, 381, 382--, appearance at night, 395--, nyctitropic movements of pinnae, 402--, the axes of the ellipses, 404-- lophantha, character of first leaf, 415-- retinoides, circumnutation of young phyllode, 236

Acanthosicyos horrida, nocturnal movement of cotyledon 304

Acanthus candelabrum, inequality in the two first leaves, 79--, petioles not arched, 553-- latifolius, variability in first leaves 79-- mollis, seedling, manner of breaking through the ground, 78, 79--, circumnutation of young leaf, 249, 269-- spinosus, 79--, movement of leaves, 249

Adenanthera pavonia, nyctitropic movements of leaflets, 374

Aecidium elatinum, effect on the lateral branches of the silver fir, 188

Aesculus hippocastanum, movements of radicle, 28, 29--, sensitiveness of apex of radicle, 172-174

Albizzia lophantha, nyctitropic movements of leaflets, 383--, of pinnae, 402

Allium cepa, conical protuberance on arched cotyledon, 59--, circumnutation of basal half of arched cotyledon, 60--, mode of breaking through ground, 87--, straightening process, 101-- porrum, movements of flower-stems, 226

Alopecurus pratensis, joints affected by apogeotropism, 503

Aloysia citriodora, circumnutation of stem, 210

Amaranthus, sleep of leaves, 387-- caudatus, nocturnal movement of cotyledons, 307

Amorpha fruticosa, sleep of leaflets, 354

Ampelopsis tricuspidata, hyponastic movement of hooked tips, 272-275

Amphicarpoea monoica, circumnutation and nyctitropic movements of leaves, 365--, effect of sunshine on leaflets, 445--, geotropic movements of, 520[page 575]

ANODA--BRASSICA

Anoda Wrightii, sleep of cotyledons, 302, 312--, of leaves, 324--, downward movement of cotyledons, 444

Apheliotropism, or negative heliotropism, 5, 419, 432

Apios graveolens, heliotropic movements of hypocotyl, 422-424-- tuberosa, vertical sinking of leaflets at night, 368

Apium graveolens, sleep of cotyledons, 305--, petroselinum, sleep of cotyledons, 304

Apogeotropic movements effected by joints or pulvini, 502

Apogeotropism, 5, 494; retarded by heliotropism, 501; concluding remarkson, 507

Arachis hypogoea, circumnutation of gynophore, 225--, effects of radiation on leaves, 289, 296--, movements of leaves, 357-- rate of movement, 404--, circumnutation of vertically dependent young gynophores, 519--, downward movement of the same, 519

Arching of various organs, importance of, to seedling plants, 87, 88;emergence of hypocotyls or epicotyls in the form of an, 553

Asparagus officinalis, circumnutation of plumules, 60-62. --, effect of lateral light, 484

Asplenium trichomanes, movement in the fruiting fronds, 257, n. 

Astragalus uliginosus, movement of leaflets, 355

Avena sativa, movement of cotyledons, 65, 66. --, sensitiveness of tip of radicle to moist air, 183--, heliotropic movement and circumnutation of cotyledon, 421, 422--, sensitiveness of cotyledon to a lateral light, 477--, young sheath-like cotyledons strongly apogeotropic, 499

Avena sativa, movements of oldish cotyledons, 499, 500

Averrhoa bilimbi, leaf asleep, 330--, angular movements when going to sleep, 331-335--, leaflets exposed to bright sunshine, 447

Azalea Indica, circumnutation of stem, 208

B. 

Bary, de, on the effect of the Aecidium on the silver fir, 188

Batalin, Prof. , on the nyctitropic movements of leaves, 283; on the sleepof leaves of Sida napoea, 322; on Polygonum aviculare, 387; on the effectof sunshine on leaflets of Oxalis acetosella, 447

Bauhinia, nyctitropic movements, 373--, movements of petioles of young seedlings, 401--, appearance of young plants at night, 402

Beta vulgaris, circumnutation of hypocotyl of seedlings, 52--, movements of cotyledons, 52, 53--, effect of light, 124--, nocturnal movement of cotyledons, 307--, heliotropic movements of, 420--, transmitted effect of light on hypocotyl, 482--, apogeotropic movement of hypocotyl, 496

Bignonia capreolata, apheliotropic movement of tendrils, 432, 450

Bouché on Melaleuca ericaefolia, 383

Brassica napus, circumnutation of flower-stems, 226

Brassica oleracea, circumnutation of seedling, 10--, of radicle, 11--, geotropic movement of radicle, 11[page 576]

Brassica oleracea, movement of buried and arched hypocotyl, 13, 14, 15--, conjoint circumnutation of hypocotyl and cotyledons, 16, 17, 18--, of hypocotyl in darkness, 19--, of a cotyledon with hypocotyl secured to a stick, 19, 20--, rate of movement, 20--, ellipses described by hypocotyls when erect, 105--, movements of cotyledons, 115--, -- of stem, 202--, -- of leaves at night, 229, 230--, sleep of cotyledons, 301--, circumnutation of hypocotyl of seedling plant, 425--, heliotropic movement and circumnutation of hypocotyls, 426--, effect of lateral light on hypocotyls, 479-482--, apogeotropic movement of hypocotyls, 500, 501

Brassica rapa, movements of leaves, 230

Brongniart, A. , on the sleep of Strephium floribundum, 391

Bruce, Dr. , on the sleep of leaves in Averrhoa, 330

Bryophyllum (vel Calanchoe) calycinum, movement of leaves, 237

C. 

Camellia Japonica, circumnutation of leaf, 231, 232

Candolle, A. De, on Trapa natans, 95; on sensitiveness of cotyledons, 127

Canna Warscewiczii, circumnutation of plumules, 58, 59--, of leaf, 252

Cannabis sativa, movements of leaves, 250--, nocturnal movements of cotyledons, 307Cannabis sativa, sinking of the young leaves at night, 444

Cassia, nyctitropic movement of leaves, 369

Cassia Barclayana, nocturnal movement of leaves, 372--, slight movement of leaflets, 401-- calliantha, uninjured by exposure at night, 289, n. --, nyctitropic movement of leaves, 371-- circumnutating movement of leaves, 372-- corymbosa, cotyledons sensitive to contact, 126--, nyctitropic movement of leaves, 369-- floribunda, use of sleep movements, 289--, effect of radiation on the leaves at night, 294--, circumnutating and nyctitropic movement of a terminal leaflet, 372, 373--, movements of young and older leaves, 400-- florida, cotyledons sensitive to contact, 126--, sleep of cotyledons, 308-- glauca, cotyledons sensitive to contact, 126--, sleep of cotyledons, 308-- laevigata, effect of radiation on leaves, 289, n. -- mimosoides, movement of cotyledons. 116--, sensitiveness of, 126--, sleep of, 308--, nyctitropic movement of leaves, 372--, effect of bright sunshine on cotyledons, 446-- neglecta, movements of, 117--, effect of light, 124--, sensitiveness of cotyledons, 126-- nodosa, non-sensitive cotyledons, 126--, do not rise at night, 308-- pubescens, non-sensitive cotyledons, 126[page 577]

CASSIA--CRINUM

Cassia pubescens, uninjured by exposure at night, 293--, sleep of cotyledons, 308--, nyctitropic movement of leaves, 371--, circumnutating movement of leaves, 372--, nyctitropic movement of petioles, 400--, diameter of plant at night, 402-- sp. (?) movement of cotyledons, 116-- tora, circumnutation of cotyledons and hypocotyls, 34, 35, 109, 308--, effect of light, 124, 125--, sensitiveness to contact, 125--, heliotropic movement and circumnutation of hypocotyl, 431--, hypocotyl of seedling slightly heliotropic, 454--, apogeotropic movement of old hypocotyl, 497--, movement of hypocotyl of young seedling, 510

Caustic (nitrate of silver), effect of, on radicle of bean, 150, 156; onthe common pea, 160. 

Cells, table of the measurement of, in the pulvini of Oxalis corniculata, 120; changes in, 547

Centrosema, 365

Ceratophyllum demersum, movements of stem, 211

Cereus Landbeckii, its rudimentary cotyledons, 97-- speciossimus, circumnutation of stem, 206, 207

Cerinthe major, circumnutation of hypocotyl, 49--, of cotyledons, 49--, ellipses described by hypocotyls when erect, 107-- effect of darkness, 124

Chatin, M. , on Pinus Nordmanniana, 389

Chenopodium album, sleep of leaves but not of cotyledons, 314, 319

Chenopodium album, movement of leaves, 387

Chlorophyll injured by bright light, 446

Ciesielski, on the sensitiveness of the tip of the radicles, 4, 523

Circumnutation, meaning explained, 1; modified, 263-279; and heliotropism, relation between, 435; of paramount importance to every plant, 547

Cissus discolor, circumnutation of leaf, 233

Citrus aurantium, circumnutation of epicotyl, 28--, unequal cotyledons, 95

Clianthus Dampieri, nocturnal movement of leaves, 297

Coboea scandens, circumnutation of, 270

Cohn, on the water secreted by Lathraea squamaria, 86, n. ; on the movementof leaflets of Oxalis, 447

Colutea arborea, nocturnal movement of leaflets, 355

Coniferae, circumnutation of, 211Coronilla rosea, leaflets asleep, 355

Corylus avellana, circumnutation of young shoot, emitted from the epicotyl, 55, 56--, arched epicotyl, 77

Cotyledon umbilicus, circumnutation of stolons, 219, 220

Cotyledons, rudimentary, 94-98; circumnutation of, 109-112; nocturnalmovements, 111, 112; pulvini or joints of, 112-122; disturbed periodicmovements by light, 123; sensitiveness of, to contact, 125; nyctitropicmovements of, 283, 297; list of cotyledons which rise or sink at night, 300; concluding remarks on their movements, 311

Crambe maritima, circumnutation of leaves, 228, 229

Crinum Capense, shape of leaves, 253[page 578]

CRINUM--DESMODIUM

Crinum Capense, circumnutation of, 254

Crotolaria (sp. ?), sleep of leaves, 340

Cryptogams, circumnutation of, 257-259

Cucumis dudaim, movement of cotyledons, 43, 44--, sleep of cotyledons, 304

Cucurbita aurantia, movement of hypocotyl, 42--, cotyledons vertical at night, 304--, ovifera, geotropic movement of radicle, 38, 39--, circumnutation of arched hypocotyl, 39--, of straight and vertical hypocotyl, 40--, movements of cotyledons, 41, 42, 115, 124--, position of radicle, 89--, rupture of the seed-coats, 102--, circumnutation of hypocotyl when erect, 107, 108--, sensitiveness of apex of radicle, 169-171--, cotyledons vertical at night, 304--, not affected by apogeotropism, 509--, tips cauterised transversely, 537

Curvature of the radicle, 193

Cycas pectinata, circumnutation of young leaf, whilst emerging from theground, 58--, first leaf arched, 78--, circumnutation of terminal leaflets, 252

Cyclamen Persicum, movement of cotyledon, 46--, undeveloped cotyledons, 78, 96--, circumnutation of peduncle, 225--, --, of leaf, 246, 247--, downward apheliotropic movement of a flower-peduncle, 433-435

Cyclamen Persicum, burying of the pods, 433

Cyperus alternifolius, circumnutation of stem, 212--, movement of stem, 509

Cytisus fragrans, circumnutation of hypocotyl, 37--, sleep of leaves, 344, 397--, apogeotropic movement of stem, 494-496

D. 

Dahlia, circumnutation of young leaves, 244-246

Dalea alopecuroides, leaflets depressed at night, 354

Darkness, effect of, on the movement of leaves, 407

Darlingtonia Californica, its leaves or pitchers apheliotropic, 450, n. 

Darwin, Charles, on Maurandia semperflorens, 225; on the Swedish turnip, 230, n. ; movements of climbing plants, 266, 271; the heliotropic movementof the tendrils of Bignonia capreolata, 433; revolution of climbing plants, 451; on the curling of a tendril, 570--, Erasmus, on the peduncles of Cyclamens, 433--, Francis, on the radicle of Sinapis alba, 486; on Hygroscopic seeds, 489, n. 

Datura stramonium, nocturnal movement of cotyledons, 298

Delpino, on cotyledons of Chaerophyllum and Corydalis, 96, n. 

Delphinium nudicaule, mode of breaking through the ground, 80--, confluent petioles of two cotyledons, 553

Desmodium gyrans, movement of leaflets, 257, n. --, position of leaves at night, 285--, sleep of leaves, not of cotyledons, 314--, circumnutation and nycti-[page 579]

DESMODIUM--EUCALYPTUS

tropic movement of leaves, 358-360Desmodium gyrans, movement of lateral leaflets, 361--, jerking of leaflets, 362-- nyctitropic movement of petioles, 400, 401--, diameter of plant at night, 402--, lateral movement of leaves, 404--, zigzag movement of apex of leaf, 405--, shape of lateral leaflet, 416--, vespertilionis, 364, n. 

Deutzia gracilis, circumnutation of stem, 205

Diageotropism, 5; or transverse-geotropism, 520

Diaheliotropism, 5; or Transversal-Heliotropismus of Frank, 419; influencedby epinasty, 439; by weight and apogeotropism, 440

Dianthus caryophyllus, 230--, circumnutation of young leaf, 231, 269

Dicotyledons, circumnutation widely spread among, 68

Dionoea, oscillatory movements of leaves, 261, 271

Dionoea muscipula, circumnutation of young expanding leaf, 239, 240--, closure of the lobes and circumnutation of a full-grown leaf, 241--, oscillations of, 242-244

Diurnal sleep, 419

Drosera Capensis, structure of first-formed leaves, 414-- rotundifolia, movement of young leaf, 237, 238--, of the tentacles, 239--, sensitiveness of tentacles, 261--, shape of leaves, 414--, leaves not heliotropic, 450--, leaves circumnutate largely, 454--, sensitiveness of 570

Duchartre on Trephrosia cariboea, 354; on the nyctitropic movement of theCassia, 369

Duval-Jouve, on the movements of Bryophyllum calycinum, 237; of the narrowleaves of the Gramineae, 413

Dyer, Mr. Thiselton, on the leaves of Crotolaria, 340; on Cassiafloribunda, 369, n. , on the absorbent hairs on the buried flower-heads ofTrifolium subterraneum, 517

E. 

Echeveria stolonifera, circumnutation of leaf, 237

Echinocactus viridescens, its rudimentary cotyledons, 97

Echinocystis lobata, movements of tendrils, 266--, apogeotropism of tendrils, 510

Elfving, F. , on the rhizomes of Sparganium ramosum, 189; on thediageotropic movement in the rhizomes of some plants, 521

Elymus arenareus, leaves closed during the day, 413

Embryology of leaves, 414

Engelmann, Dr. , on the Quercus virens, 85

Epinasty, 5, 267

Epicotyl, or plumule, 5; manner of breaking through the ground, 77; emergesfrom the ground under the form of an arch, 553

Erythrina caffra, sleep of leaves, 367-- corallodendron, movement of terminal leaflet, 367-- crista-galli, effect of temperature on sleep of leaves, 318--, circumnutation and nyctitropic movement of terminal leaflets, 367

Eucalyptus resinifera, circumnutation of leaves, 244[page 580]

EUPHORBIA--GYMNOSPERMS

Euphorbia jacquineaeflora, nyctitropic movement of leaves, 388

F. 

Flahault, M. , on the rupture of seed-coats, 102-104, 106

Flower-stems, circumnutation of, 223-226

Fragaria Rosacea, circumnutation of stolon, 214-218

Frank, Dr. A. B. , the terms Heliotropism and Geotropism, first used by him, 5, n. ; radicles acted on by geotropism, 70, n. ; on the stolons of Fragaria, 215; periodic and nyctitropic movements of leaves, 284; on the root-leavesof plants kept in darkness, 443; on pulvini, 485; on natural selection inconnection with geotropism, heliotropism, etc. , 570--, on Transversal-Heliotropismus, 419

Fuchsia, circumnutation of stem, 205, 206

G. 

Gazania ringens, circumnutation of stem, 208 Genera containing sleepingplants, 320, 321

Geotropism, 5; effect of, on the primary radicle, 196; the reverse ofapogeotropism, 512: effect on the tips of radicles, 543

Geranium cinereum, 304-- Endressii, 304-- Ibericum, nocturnal movement of cotyledons, 298-- Richardsoni, 304-- rotundifolium, nocturnal movement of cotyledon, 304, 312-- subcaulescens, 304

Germinating seed, history of a, 548

Githago segetum, circumnutation of hypocotyl, 21, 108--, burying of hypocotyl, 109--, seedlings feebly illuminated, 124, 128--, sleep of cotyledon, 302--, -- leaves 321

Glaucium luteum, circumnutation of young leaves, 228

Gleditschia, sleep of leaves, 368

Glycine hispida, vertical sinking of leaflets, 366

Glycyrrhiza, leaflets depressed at night, 355

Godlewski, Emil, on the turgescence of the cells, 485

Gooseberry, effect of radiation, 284

Gossypium (var. Nankin cotton), circumnutation of hypocotyl, 22--, movement of cotyledon, 22, 23--, sleep of leaves, 324--, arboreum (?), sleep of cotyledons, 303--, Braziliense, nocturnal movement of leaves, 324--, sleep of cotyledons, 303-- herbaceum, sensitiveness of apex of radicle, 168--, radicles cauterised transversely, 537-- maritimum, nocturnal movement of leaves, 324

Gravitation, movements excited by, 567

Gray, Asa, on Delphinium nudicaule, 80; on Megarrhiza Californica, 81; onthe movements in the fruiting fronds of Aesplenium trichomanes, 257; on theAmphicarpoea monoica, 520; on the Ipomoea Jalappa, 557

Grease, effect of, on radicles and their tips, 182, 185

Gressner, Dr. H. , on the cotyledons of Cyclamen Persicum, 46, 77; onhypocotyl of the same, 96

Gymnosperms, 389[page 581]

HABERLANDT--IPOMOEA

H. 

Haberlandt, Dr. , on the protuberance on the hypocotyl of Allium, 59; theimportance of the arch to seedling plants, 87; sub-aërial and subterraneancotyledons, 110, n. ; the arched hypocotyl, 554

Haematoxylon Campechianum, nocturnal movement of leaves, 368, 369

Hedera helix, circumnutation of stem, 207

Hedysarum coronarium, nocturnal movements of leaves, 356

Helianthemum prostratum, geotropic movement of flower-heads, 518

Helianthus annuus, circumnutation of hypocotyl, 45--, arching of hypocotyl, 90--, nocturnal movement of cotyledons, 305

Heliotropism, 5; uses of, 449; a modified form of circumnutation, 490

Helleborus niger, mode of breaking through the ground, 86

Hensen, Prof. , on roots in worm-burrows, 72

Henslow, Rev. G. , on the cotyledons of Phalaris Canariensis, 62

Hofmeister, on the curious movement of Spirogyra, 3, 259, n. ; of the leavesof Pistia stratiotes, 255; of cotyledons at night, 297; of petals, 414-- and Batalin on the movements of the cabbage, 229

Hooker, Sir J. , on the effect of light on the pitchers of Sarracenia, 450

Hypocotyl, 5; manner of breaking through the ground, 77; emerges under theform of an arch, 553

Hypocotyls and Epicotyls, circumnutation and other movements when arched, 98; power of straightening themselves, 100; rupture of the seed-coats, 102-106; illustration of, 106; circumnutation when erect, 107; when indark, 108

Hyponasty, 6, 267

I. Iberis umbellata, movement of stem, 202. 

Illumination, effect of, on the sleep of leaves, 398

Imatophyllum vel Clivia (sp. ?), movement of leaves, 255

Indigofera tinctoria, leaflets depressed at night, 354

Inheritance in plants, 407, 491

Insectivorous and climbing plants not heliotropic, 450; influence of lighton, 488

Ipomoea bona nox, arching of hypocotyl, 90--, nocturnal position of cotyledons, 306, 312-- coerulea vel Pharbitis nil, circumnutation of seedlings, 47--, movement of cotyledons, 47-49, 109--, nocturnal movements of cotyledons, 305--, sleep of leaves, 386--, sensitiveness to light, 451--, the hypocotyledonous stems heliotropic, 453-- coccinea, position of cotyledons at night, 306, 312-- leptophylla, mode of breaking through the ground, 83, 84--, arching of the petioles of the cotyledons, 90--, difference in sensitiveness to gravitation in different parts, 509--, extraordinary manner of germination, 557[page 582]

IPOMOEA--LOTUS

Ipomoea pandurata, manner of germination, 84, 557-- purpurea (vel Pharbitis hispida), nocturnal movement of cotyledons, 305, 312--, sleep of leaves, 386--, sensitiveness to light, 451--, the hypocotyledonous stems heliotropic, 453

Iris pseudo-acorus, circumnutation of leaves, 253

Irmisch, on cotyledons of Ranunculus Ficaria, 96

Ivy, its stems heliotropic, 451

K. 

Kerner on the bending down of peduncles, 414

Klinostat, the, an instrument devised by Sachs to eliminate geotropism, 93

Kraus, Dr. Carl, on the underground shoots of Triticum repens, 189; onCannabis sativa, 250, 307, 312; on the movements of leaves, 318L. 

Lactuca scariola, sleep of cotyledons, 305

Lagenaria vulgaris, circumnutation of seedlings, 42--, of cotyledons, 43--, cotyledons vertical at night, 304

Lathraea squamaria, mode of breaking through the ground, 85--, quantity of water secreted, 85, 86, n. 

Lathyrus nissolia, circumnutation of stem of young seedling, 33--, ellipses described by, 107, 108

Leaves, circumnutation of, 226-262; dicotyledons, 226-252; monocotyledons, 252-257; nyctitropism of, 280; their temperature affected by their positionat night, 294; nyctitropic or sleep movements, 315, 394; periodicity oftheir movements inherited, 407; embryology of, 414; so-called diurnalsleep, 445

Leguminosae, sleep of cotyledons, 308; sleeping species, 340

Le Maout and Decaisne, 67

Lepidium sativum, sleep of cotyledons, 302

Light, movements excited by 418, 563; influence on most vegetable tissues, 486; acts on plant as on the nervous system of animals, 487

Lilium auratum, circumnutation of stem, 212--, apogeotropic movement of stem, 498, 499

Linnaeus, 'Somnus Plantarum', 280; on plants sleeping, 320; on the leavesof Sida abutilon, 324; on Oenothera mollissima, 383

Linum Berendieri, nocturnal movement of cotyledons, 298-- usitatissimum, circumnutation of stem, 203

Lolium perenne, joints affected by apogeotropism, 502

Lonicera brachypoda, hooking of the tip, 272--, sensitiveness to light, 453

Loomis, Mr. , on the movements in the fruiting fronds of Aspleniumtrichomanes, 257

Lotus aristata, effect of radiation on leaves, 292-- Creticus, leaves awake and asleep, 354-- Gebelii, nocturnal movement of cotyledons, 308--, leaflets provided with pulvini, 353-- Jacobaeus, movements of cotyledons, 35, 109--, pulvini of, 115[page 583]

LOTUS--MELILOTUS

Lotus Jacobaeus, movements at night, 116, 121, 124--, development of pulvini, 122--, sleep of cotyledons, 308, 313--, nyctitropic movement of leaves, 353-- major, sleep of leaves, 353-- perigrinus, movement of leaflets, 353

Lunularia vulgaris, circumnutation of fronds, 258

Lupinus, 340-- albifrons, sleep of leaves, 344-- Hartwegii, sleep of leaves, 341-- luteus, circumnutation of cotyledons, 38, 110--, effect of darkness, 124

Lupinus, position of leaves when asleep, 341--, different positions of leaves at night, 343--, varied movements of leaves and leaflets, 395-- Menziesii, sleep of leaves, 343-- mutabilis, sleep of leaves, 343-- nanus, sleep of leaves, 343-- pilosus, sleep of leaves, 340, 341-- polyphyllus, sleep of leaves, 343-- pubescens, sleep of leaves by day and night, 342--, position of petioles at night, 343--, movements of petioles, 401-- speciosus, circumnutation of leaves, 236

Lynch, Mr. R. , on Pachira aquatica, 95, n. ; sleep movements of Averrhoa, 330

M. 

Maranta arundinacea, nyctitropic movement of leaves, 389-391--, after much agitation do not sleep, 319

Marsilia quadrifoliata, effect of radiation at night, 292--, circumnutation and nyctitropic movement of leaflets, 392-394--, rate of movement, 404

Martins, on radiation at night, 284, n. 

Masters, Dr. Maxwell, on the leading shoots of the Coniferae, 211

Maurandia semperflorens, circumnutation of peduncle, 225Medicago maculata, nocturnal position of leaves, 345-- marina, leaves awake and asleep, 344

Meehan, Mr. , on the effect of an Aecidium on Portulaca oleracea, 189

Megarrhiza Californica, mode of breaking through the ground, 81--, germination described by Asa Gray, 82--, singular manner of germination, 83, 556

Melaleuca ericaefolia, sleep of leaves, 383

Melilotus, sleep of leaves, 345-- alba, sleep of leaves, 347-- coerulea, sleep of leaves, 347-- dentata, effect of radiation at night, 295-- elegans, sleep of leaves, 347-- gracilis, sleep of leaves, 347-- infesta, sleep of leaves, 347-- Italica, leaves exposed at night, 291--, sleep of leaves, 347-- macrorrhiza, leaves exposed at night, 292--, sleep of leaves, 347-- messanensis, sleep of leaves on full-grown and young plants, 348, 416-- officinalis, effect of exposure of leaves at night, 290, 296--, nocturnal movement of leaves, 346, 347--, circumnutation of leaves, 348--, movement of petioles, 401[page 584]

MELILOTUS--NEPTUNIA

Melilotus parviflora, sleep of leaves, 347-- Petitpierreana, leaves exposed at night, 291, 296--, sleep of leaves, 347-- secundiflora, sleep of leaves, 347-- suaveolens, leaves exposed at night, 291--, sleep of leaves, 347-- sulcata, sleep of leaves, 347-- Taurica, leaves exposed at night, 291--, sleep of leaves, 347, 415

Methods of observation, 6

Mimosa albida, cotyledons vertical at night, 116--, not sensitive to contact, 127--, sleep of cotyledons, 308--, rudimentary leaflets, 364--, nyctitropic movements of leaves, 379, 380--, circumnutation of the main petiole of young leaf, 381--, torsion, or rotation of leaves and leaflets, 400--, first true leaf, 416--, effect of bright sunshine on basal leaflets, 445-- marginata, nyctitropic movements of leaflets, 381-- pudica, movement of cotyledons, 105--, rupture of the seed-coats, 105--, circumnutation of cotyledons, 109--, pulvini of, 113, 115--, cotyledons vertical at night, 116--, hardly sensitive to contact, 127--, effect of exposure at night, 293--, nocturnal movement of leaves, 297--, sleep of cotyledons, 308--, circumnutation and nyctitropic movement of main petiole, 374-378--, of leaflets, 378

Mimosa albida, circumnutation and nyctitropic movement of pinnae, 402--, number of ellipses described in given time, 406--, effect of bright sunshine on leaflets, 446

Mirabilis jalapa and longiflora, nocturnal movements of cotyledons, 307--, nyctitropic movement of leaves, 387

Mohl, on heliotropism in tendrils, stems, and twining plants, 451

Momentum-like movement, the accumulated effects of apogeotropism, 508

Monocotyledons, sleep of leaves, 389

Monotropa hypopitys, mode of breaking through the ground, 86

Morren, on the movements of stamens of Sparmannia and Cereus, 226

Müller, Fritz, on Cassia tora, 34; on the circumnutation of Linumusitatissimum, 203; movements of the flower-stems of an Alisma, 226

Mutisia clematis, movement of leaves, 246--, leaves not heliotropic, 451

N. 

Natural selection in connection with geotropism, heliotropism, etc. , 570

Nephrodium molle, circumnutation of very young frond, 66--, of older frond, 257--, slight movement of fronds, 509

Neptunia oleracea, sensitiveness to contact, 128--, nyctitropic movement of leaflets, 374--, of pinnae, 402[page 585]

NICOTIANA--OXALIS

Nicotiana glauca, sleep of leaves, 385, 386--, circumnutation of leaves, 386

Nobbe, on the rupture of the seed-coats in a seedling of Martynia, 105

Nolana prostrata, movement of seedlings in the dark, 50--, circumnutation of seedling, 108

Nyctitropic movement of leaves, 560

Nyctitropism, or sleep of leaves, 281; in connection with radiation, 286;object gained by it, 413

O. 

Observation, methods of, 6

Oenothera mollissima, sleep of leaves, 383

Opuntia basilaris, conjoint circumnutation of hypocotyl and cotyledon, 44--, thickening of the hypocotyl, 96--, circumnutation of hypocotyl when erect, 107--, burying of, 109

Orange, seedling, circumnutation of, 510

Orchis pyramidalis, complex movement of pollinia, 489

Oxalis acetosella, circumnutation of flower-stem, 224--, effects of exposure to radiation at night, 287, 288, 296--, circumnutation and nyctitropic movement in full-grown leaf, 326--, circumnutation of leaflet when asleep, 327--, rate of circumnutation of leaflets, 404--, effect of sunshine on leaflets, 447--, circumnutation of peduncle, 506Oxalis acetosella, seed-capsules, only occasionally buried, 518-- articulata, nocturnal movements of cotyledons, 307-- (Biophytum) sensitiva, rapidity of movement of cotyledons during theday, 26--, pulvinus of, 113--, cotyledons vertical at night, 116, 118-- bupleurifolia, circumnutation of foliaceous petiole, 328--, nyctitropic movement of terminal leaflet, 329-- carnosa, circumnutation of flower-stem, 223--, epinastic movements of flower-stem, 504--, effect of exposure at night, 288, 296--, movements of the flower-peduncles due to apogeotropism and otherforces, 503-506-- corniculata (var. Cuprea), movements of cotyledons, 26--, rising of cotyledons, 116--, rudimentary pulvini of cotyledons, 119--, development of pulvinus, 122--, effect of dull light, 124--, experiments on leaves at night, 288-- floribunda, pulvinus of cotyledons, 114--, nocturnal movement, 118, 307, 313-- fragrans, sleep of leaves, 324-- Ortegesii, circumnutation of flower-stems, 224--, sleep of large leaves, 327--, diameter of plant at night, 402--, large leaflets affected by bright sunshine, 447 -- Plumierii, sleep of leaves, 327-- purpurea, exposure of leaflets at night, 293-- rosea, circumnutation of cotyledons, 23, 24[page 586]

OXALIS--PHASEOLUS

Oxalis rosea, pulvinus of, 113--, movement of cotyledons at night, 117, 118, 307--, effect of dull light, 124--, non-sensitive cotyledons, 127-- sensitiva, movement of cotyledons, 109, 127, 128--, circumnutation of flower-stem, 224--, nocturnal movement of cotyledons, 307, 312--, sleep of leaves, 327-- tropoeoloides, movement of cotyledons at night, 118, 120-- Valdiviana, conjoint circumnutation of cotyledons and hypocotyl, 25--, cotyledons rising vertically at night, 114, 115, 117, 118--, non-sensitive cotyledons, 127--, nocturnal movement of cotyledon, 307, 312--, sleep of leaves and not of cotyledons, 315--, movements of leaves, 327

P. 

Pachira aquatica, unequal cotyledons, 95, n. 

Pancratium littorale, movement of leaves, 255

Paraheliotropism, or diurnal sleep of leaves, 445

Passiflora gracilis, circumnutation and nyctitropic movement of leaves, 383, 384--, apogeotropic movement of tendrils, 510--, sensitiveness of tendrils, 550Pelargonium zonale, circumnutation of stem, 203--, and downward movement of young leaf, 232, 233, 269

Petioles, the rising of beneficial to plant at night, 402

Petunia violacea, downward movement and circumnutation of very young leaf, 248, 249, 269. 

Pfeffer, Prof. , on the turgescence of the cells, 2; on pulvini of leaves, 113, 117; sleep movements of leaves, 280, 283, 284; nocturnal rising ofleaves of Malva, 324; movements of leaflets in Desmodium gyrans, 358; onPhyllanthus Niruri, 388; influence of a pulvinus on leaves, 396; periodicmovements of sleeping leaves, 407, 408; movements of petals, 414; effect ofbright sunshine on leaflets of Robinia, 445; effect of light on partsprovided with pulvini, 363

Phalaris Canariensis, movements of old seedlings, 62--, circumnutation of cotyledons, 63, 64, 108--, heliotropic movement and circumnutation of cotyledon towards a dimlateral light, 427--, sensitiveness of cotyledon to light, 455--, effect of exclusion of light from tips of cotyledons, 456--, manner of bending towards light, 457--, effects of painting with Indian ink, 467--, transmitted effects of light, 469--, lateral illumination of tip, 470--, apogeotropic movement of the sheath-like cotyledons, 497--, change from a straight upward apogeotropic course to circumnutation, 499--, apogeotropic movement of cotyledons, 500

Phaseolus Hernandesii, nocturnal movement of leaves and leaflets, 368--, caracalla, 93--, nocturnal movement of leaves, 368--, effect of bright sunshine on leaflets, 446[page 587]

PHASEOLUS--QUERCUS

Phaseolus multiflorus, movement of radicles, 29--, of young radicle, 72--, of hypocotyl, 91, 93--, sensitiveness of apex of radicle, 163-167--, to moist air, 181--, cauterisation and grease on the tips, 535--, nocturnal movement of leaves, 368--, nyctitropic movement of the first unifoliate leaves, 397-- Roxburghii, effect of bright sunshine on first leaves, 445--, vulgaris, 93--, sleep of leaves, 318--, vertical sinking of leaflets at night, 368

Phyllanthus Niruri, sleep of leaflets, 388-- linoides, sleep of leaves, 387

Pilocereus Houlletii, rudimentary cotyledons, 97

Pimelia spectabilis, sleep of leaves, 387

Pincers, wooden, through which the radicle of a bean was allowed to grow, 75

Pinus austriaca, circumnutation of leaves, 251, 252-- Nordmanniana, nyctitropic movement of leaves, 389-- pinaster, circumnutation of hypocotyl, 56--, movement of two opposite cotyledons, 57--, circumnutation of young leaf, 250, 251--, epinastic downward movement of young leaf, 270

Pistia stratiotes, movement of leaves, 255

Pisum sativum, sensitiveness of apex of radicle, 158--, tips of radicles cauterised transversely, 534

Plants, sensitiveness to light, 449; hygroscopic movements of, 489

Plants, climbing, circumnutation of, 264; movements of, 559--, mature, circumnutation of, 201-214

Pliny on the sleep-movements of plants, 280

Plumbago Capensis, circumnutation of stem, 208, 209

Poinciana Gilliesii, sleep of leaves, 368

Polygonum aviculare, leaves vertical at night, 387-- convolvulus, sinking of the leaves at night, 318

Pontederia (sp. ?), circumnutation of leaves, 256

Porlieria hygrometrica, circumnutation and nyctitropic movements of petioleof leaf, 335, 336--, effect of watering, 336-338--, leaflets closed during the day, 413

Portulaca oleracea, effect of Aecidium on, 189

Primula Sinensis, conjoint circumnutation of hypocotyl and cotyledon, 45, 46

Pringsheim on the injury to chlorophyll, 446

Prosopis, nyctitropic movements of leaflets, 374Psoralea acaulis, nocturnal movements of leaflets, 354

Pteris aquilina, rachis of, 86

Pulvini, or joints; of cotyledons, 112-122; influence of, on the movementsof cotyledons, 313; effect on nyctitropic movements, 396

Q. 

Quercus (American sp. ), circumnutation of young stem, 53, 54-- robur, movement of radicles, 54, 55--, sensitiveness of apex of radicle, 174-176[page 588]

QUERCUS--SACHS

Quercus virens, manner of germination, 85, 557

R. 

Radiation at night, effect of, on leaves, 284-286

Radicles, manner in which they penetrate the ground, 69-77; circumnutationof 69; experiments with split sticks, 74; with wooden pincers, 75;sensitiveness of apex to contact and other irritants, 129; of Vicia faba, 132-158; various experiments, 135-140; summary of results, 143-151; powerof an irritant on, compared with geotropism, 151-154; sensitiveness of tipto moist air, 180; with greased tips, 185; effect of killing or injuringthe primary radicle, 187-191; curvature of, 193; affected by moisture, 198;tip alone sensitive to geotropism, 540; protrusion and circumnutation in agerminating seed, 548; tip highly sensitive, 550; the tip acts like thebrain of one of the lower animals, 573--, secondary, sensitiveness of the tips in the bean, 154; becomevertically geotropic, 186-191

Ramey on the movements of the cotyledons of Mimosa pudica, and ClianthusDampieri at night, 297

Ranunculus Ficaria, mode of breaking through the ground, 86, 90--, single cotyledon, 96--, effect of lateral light, 484

Raphanus sativa, sensitiveness of apex of radicle, 171--, sleep of cotyledons, 301

Rattan, Mr. , on the germination of the seeds of Megarrhiza Californica, 82

Relation between circumnutation and heliotropism, 435

Reseda odorata, hypocotyl of seedling slightly heliotropic, 454

Reversion, due to mutilation, 190Rhipsalis cassytha, rudimentary cotyledons, 97

Ricinus Borboniensis, circumnutation of arched hypocotyl, 53

Robinia, effect of bright sunshine on its leaves, 445-- pseudo-acacia, leaflets vertical at night, 355

Rodier, M. , on the movements of Ceratophyllum demersum, 211

Royer, Ch. , on the sleep-movements of plants, 281, n. ; on the sleep ofleaves, 318; the leaves of Medicago maculata, 345; on Wistaria Sinensis, 354

Rubus idaeus (hybrid) circumnutation of stem, 205--, apogeotropic movement of stem, 498

Ruiz and Pavon, on Porlieria hygrometrica, 336

S. 

SACHS on "revolving nutation, " 1; intimate connection between turgescenceand growth, 2, n. ; cotyledon of the onion, 59; adaptation of root-hairs, 69; the movement of the radicle, 70, 72, 73; movement in the hypocotyls ofthe bean, etc. , 91; sensitiveness of radicles, 131, 145, 198; sensitivenessof the primary radicle in the bean, 155; in the common pea, 156; effect ofmoist air, 180; of killing or injuring the primary radicle, 186, 187;circumnutation of flower-stems, 225; epinasty, 268; movements of leafletsof Trifolium incarnatum, 350; action of light in modifying the periodicmovements of leaves, 418; on geotropism and heliotropism, 436, n. ; onTropaeolum majus, 453;[page 589]

SARRACENIA--STAPELIA

on the hypocotyls slightly heliotropic, and stems strongly apheliotropic ofthe ivy, 453; heliotropism of radicles, 482; experiments on tips ofradicles of bean, 523, 524; curvature of the hypocotyl, 555; resemblancebetween plants and animals, 571

Sarracenia purpurea, circumnutation of young pitcher, 227

Saxifraga sarmentosa, circumn utation of an inclined stolon, 218

Schrankia aculeata, nyctitropic movement of the pinnae, 381, 403-- uncinata, nyctitropic movements of leaflets, 381

Securigera coronilla, nocturnal movements of leaflets, 352

Seed-capsules, burying of, 513

Seed-coats, rupture of, 102-106

Seedling plants, circumnutating movements of, 10Selaginella, circumnutation of 258-- Kraussii (?), circumnutation of young plant, 66

Sida napoea, depression of leaves at night, 322--, no pulvinus, 322-- retusa, vertical rising of leaves, 322-- rhombifolia, sleep of cotyledons, 308--, sleep of leaves, 314--, vertical rising of leaves, 322--, no pulvinus, 322--, circumnutation and nyctitropic movements of leaf of young plant, 322--, nyctitropic movement of leaves, 397

Siegesbeckia orientalis, sleep of leaves, 319, 384

Sinapis alba, hypocotyl bending towards the light, 461--, transmitted effect of light on radicles, 482, 483, 567--, growth of radicles in darkness, 486

Sinapis nigra, sleep of cotyledons, 301

Smilax aspera, tendrils apheliotropic, 451

Smithia Pfundii, non-sensitive cotyledons, 127--, hyponastic movement of the curved summit of the stem, 274-276--, cotyledons not sleeping at night, 308--, vertical movement of leaves, 356-- sensitiva, sensitiveness of cotyledons to contact, 126--, sleep of cotyledons, 308

Sophora chrysophylla, leaflets rise at night, 368

Solanum dulcamara, circumnutating stems, 266-- lycopersicum, movement of hypocotyl, 50--, of cotyledons, 50--, effect of darkness, 124--, rising of cotyledons at night, 306--, heliotropic movements of hypocotyl, 421--, effect of an intermittent light, 457--, rapid heliotropism, 461-- palinacanthum, circumnutation of arched hypocotyl, 51, 100--, of cotyledon, 51--, ellipses described by hypocotyl when erect, 107--, nocturnal movement of cotyledons, 306

Sparganium ramosum, rhizomes of, 189

Sphaerophysa salsola, rising of leaflets, 355

Spirogyra princeps, movements of, 259, n. 

Stahl, Dr. , on the effect of Aecidium on shoot, 189; on the influence oflight on swarm-spores, 488, n. 

Stapelia sarpedon, circumnutation of hypocotyl, 46, 47[page 590]

STAPELIA--TRITICUM

Stapelia sarpedon, minute cotyledons, 97

Stellaria media, nocturnal movement of leaves, 297

Stems, circumnutation of, 201-214

Stolons, or Runners, circumnutation of, 214-222, 558

Strasburger, on the effect of light on spores of Haematococcus, 455, n. ;the influence of light on the swarm-spores, 488. 

Strawberry, stolons of the, circumnutate, but not affected by moderatelight, 454

Strephium floribundum, circumnutation and nyctitropic movement of leaves, 391, 392

T. 

Tamarindus Indica, nyctitropic movement of leaflets, 374

Transversal - heliotropismus (of Frank) or diaheliotropism, 438

Trapa natans, unequal cotyledons, 95, n. 

Tecoma radicans, stems apheliotropic, 451

Tephrosia caribaea, 354

Terminology, 5

Thalia dealbata, sleep of leaves, 389--, lateral movement of leaves, 404

Trichosanthes anguina, action of the peg on the radicle, 104--, nocturnal movement of cotyledons, 304

Trifolium, position of terminal leaflets at night, 282-- globosum, with hairs protecting the seed-bearing flowers, 517-- glomeratum, movement of cotyledons, 309-- incarnatum, movement of cotyledons, 309-- Pannonicum, shape of first true leaf, 350, 415Trifolium pratense, leaves exposed at night, 293-- repens, circumnutation of flower-stem, 225--, circumnutating and epinastic movements of flower-stem, 276-279--, nyctitropic movement of leaves, 349--, circumnutation and nyctitropic movements of terminal leaflets, 352, 353--, sleep movements, 349-- resupinatum, no pulvini to cotyledons, 118--, circumnutation of stem, 204--, effect of exposure at night, 295--, cotyledons not rising at night, 118, 309--, circumnutation and nyctitropic movements of terminal leaflets, 351, 352-- strictum, movements of cotyledons at night, 116, 118--, nocturnal and diurnal movements of cotyledons, 309-311, 313--, movement of the left-hand cotyledon, 316-- subterraneum, movement of flower-heads, 71--, of cotyledons at night, 116, 118, 309--, circumnutation of flower-stem, 224, 225--, circumnutation and nyctitropic movements of leaves, 350--, number of ellipses in 24 hours, 405--, burying its flower-heads, 513, 514--, downward movement of peduncle, 515--, circumnutating movement of peduncle, 516

Trigonella Cretica, sleep of leaves, 345

Triticum repens, underground shoots of, become apogeotropic, 189[page 591]

TRITICUM--WILSON

Triticum vulgare, sensitiveness of tips of radicle to moist air, 184

Tropaeolum majus (?), sensitiveness of apex of radicle to contact, 167--, circumnutation of stem, 204--, influence of illumination on nyctitropic movements, 338-340, 344--, heliotropic movement and circumnutation of epicotyl of a youngseedling, 428, 429--, of an old internode towards a lateral light, 430--, stems of very young plants highly heliotropic, of old plants slightlyapheliotropic, 453--, effect of lateral light, 484-- minus (?), circumnutation of buried and arched epicotyl, 27

U. 

Ulex, or gorse, first-formed leaf of, 415

Uraria lagopus, vertical sinking of leaflets at night, 365

V. Vaucher, on the burying of the flower-heads of Trifolium subterraneum, 513;on the protection of seeds, 517

Verbena melindres (?), circumnutation of stem, 210--, apogeotropic movement of stem, 495

Vicia faba, circumnutation of radicle, 29, 30--, of epicotyl, 31-33--, curvature of hypocotyl, 92--, sensitiveness of apex of radicle, 132-134--, of the tips of secondary radicles, 154--, of the primary radicle above the apex, 155-158--, various experiments, 135-143--, summary of results, 143-151--, power of an irritant on, compared with that of geotropism, 151-154Vicia faba, circumnutation of leaves, 233-235--, circumnutation of terminal leaflet, 235--, effect of apogeotropism, 444--, effect of amputating the tips of radicles, 523--, regeneration of tips, 526--, short exposure to geotropic action, 527--, effects of amputating the tips obliquely, 528--, of cauterising the tips, 529--, of grease on the tips, 534

Vines, Mr. , on cell growth, 3

Vries, De, on turgescence, 2; on epinasty and hyponasty, 6, 267, 268; theprotection of hypocotyls during winter, 557; stolons apheliotropic, 108;the nyctitropic movement of leaves, 283; the position of leaves influencedby epinasty, their own weight and apogeotropism, 440; apogeotropism inpetioles and midribs, 443; the stolons of strawberries, 454; the joints orpulvini of the Gramineae, 502

W. 

Watering, effect of, on Porlieria hygrometrica, 336-338

Wells, 'Essay on Dew, ' 284, n. 

Wiesner, Prof. , on the circumnutation of the hypocotyl, 99, 100; on thehooked tip of climbing stems, 272; observations on the effect of brightsunshine on chlorophyll in leaves, 446; the effects of an intermittentlight, 457; on aërial roots, 486; on special adaptations, 490

Wigandia, movement of leaves, 248

Williamson, Prof. , on leaves of Drosera Capensis, 414

Wilson, Mr. A. S. , on the movements of Swedish turnip leaves, 230, 298

Winkler on the protection of seedlings, 108

Wistaria Sinensis, leaflets depressed at night, 354--, circumnutation with lateral light, 452

Z. 

Zea mays, circumnutation of cotyledon, 64Zea mays, geotropic movement of radicles, 65--, sensitiveness of apex of radicle to contact, 177-179--, secondary radicles, 179--, heliotropic movements of seedling, 64, 421--, tips of radicles cauterised, 539

Zukal, on the movements of Spirulina, 259, n. 

THE END.