290 INTRODUCTION 



paratively well developed. In addition to the many kinds of 

 plants now living, many other kinds once existed but are now 

 known only by their fossils. 



Among the kinds of plants, including both the living and fossil 

 forms, there are almost all degrees of complexity, ranging from 

 the simplest unicellular plants to the most highly developed 

 Flowering Plants. Although varying widely in complexity, the 

 various kinds of plants are evidently related as a study of their 

 structures and habits reveals. Scientists believe that the living 

 forms have come from previously existing forms and hence are 

 related through a common ancestry. They have originated 

 through the process known as evolution, which assumes that the 

 first plants on earth were extremely simple and from these simple 

 forms the more complex forms arose. In response to a changing 

 environment or due to changes arising wholly within, the simple 

 forms gave rise to more complex forms, which in turn gave rise to 

 forms still more complex. Thus through slow changes involving 

 millions of years the highly developed forms were evolved. 



Most generally evolution results in the origin of organisms with 

 tissues and organs better differentiated and thus better adapted 

 to perform special functions, but in some cases evolution moves 

 backward, resulting in the origin of organisms with tissues and 

 organs fewer and less differentiated. Thus through regressive 

 evolution the Bacteria and Fungi are thought to have arisen 

 from the Algae. Progressive evolution has not been direct 

 from the simplest to the highest organisms, but has been along 

 many lines which, although usually progressive, have been 

 more or less divergent and this accounts for many kinds of 

 organisms among both plants and animals. Animals and plants 

 can be distinguished in their higher forms on the basis of loco- 

 motion, methods of getting food, character of the skeleton, and 

 so on, but in their simpler forms animals and plants are not easily 

 distinguished, and this fact suggests that plants and animals 

 arose as diverging lines from the same preexisting organism. A 

 diagram of evolution in plants looks like a tree with many 

 branches. The trunk represents the main line and the branches 

 the diverging lines of evolution. The lowest branches with their 

 sub-branches represent the groups of the simplest plants and 

 their relationships. The groups of Seed Plants and their rela- 

 tionships are represented by the topmost branches, and the 



