CHAPTER LII 

 PLANTS OF THE PAST 



The enormous numbers of plant species, genera, and families that are 

 now living on the earth; the almost unbelievable diversity of structural 

 patterns among the major and minor groups of plants; and the apparent 

 lack of relationship among the major, and even some of the minor, 

 groups that we attempt to discover in order to classify plants, should 

 be evident even from the brief review of the plant world in the pre- 

 ceding chapters. 



At the same time there is an astonishing similarity in the reproductive 

 processes and structures within any one of the major groups. Among 

 the more complex plants the very regular occurrence of the two phases 

 in the hfe cycles indicates a unity of origin, and often definite relation- 

 ships. In each phase of the life cycle, moreover, there are so many 

 fundamental similarities among the structural features and the sequences 

 of stages in development that we can rather truthfully picture and 

 describe the most important phenomena in the lives of many species, 

 genera, and families of plants on a few printed pages. Even more im- 

 pressive are the similarities, in both plants and animals, of such basic 

 phenomena as reduction division, sex, mitosis, nutrition, general compo- 

 sition of protoplasm, and a number of other cellular phenomena. 



The first paragraph of this chapter emphasizes the heterogeneous 

 character of the great assemblages of plants and the problems they 

 present when we attempt to discover relationships and classify them. 

 The second emphasizes the remarkable homogeneity of these same as- 

 semblages when we study their most fundamental characteristics such 

 as cell phenomena, reproduction, heredity, life cycles, and the sequences 

 of events during development. The solution of the problem of relation- 

 ships and origins would be almost insuperable if we were limited in 

 our search to the plants now living on the earth. 



Fortunately, some of the plants that lived a thousand, a million, and 

 even a billion years ago have left a record — though a meager one — 

 among the sedimentary deposits in ponds, lakes and shallow seas. Some 



719 



