Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III THE LOWER PLANTS IN endeavoring to trace the pedigree of the vegetable kingdom, we can rely on the record of the past only to a limited extent. While many plants have left perfectly recognizable fossil remains, the record is extremely fragmentary. This is especially true of the delicate and more perishable plants, such as the seaweeds and mosses. Nevertheless most important results have been obtained from the careful study of fossil plant remains. Comparative Morphology as a Guide to Relationships.—On the assumption that all plants are more or less closely related, a comparison of the structures of the living forms affords a clue to the degree of relationship, and hence the great stress which is laid upon the importance of Comparative Morphology. In view, however, of the ready response of plant structures to changes in environment, great caution must be exercised in distinguishing true homologies from similarities in structure due to response to similar conditions. We have already pointed out, for instance, that leaf-like organs have developed in plants of widely separate origin, e.g., seaweeds, mosses, and the higher land plants. These leaves are in no sense homologous organs and do not point to any close relationship between the plants which possess them. A proper study of comparative morphology must take into account all the organs of the forms compared, but it also is evident that some of these organs are much more important in heredity than others. It is also necessary to distinguish between structures which are readily affected by external conditions and those which show evidence of being more permanent in character. The reproductive parts are as a rule much more stable than the vegetative organs, and are rightly considered to hold the first place in in...