66 MORPHOLOGY AND LIFE HISTORY 



C. Descent. Just as one of the higher plants, such as a 

 maple tree, begins life as a single cell, and becomes 

 more and more complex as it matures, so the plant 

 kingdom as a whole, presents us with a series of organ- 

 isms of gradually increasing complexity from the sim- 

 plest one-celled forms to myriad-celled, complex forms. 

 This fact suggests that the entire plant kingdom, like 

 every individual plant, has had a developmental his- 

 tory, the more complex organisms being derived from 

 more simple ones by a series of gradual changes. This is 

 the theory of descent, or organic evolution. It teaches 

 us that all organisms are related to each other, and 

 is one explanation of why we so often find the same 

 organ appearing again and again, under various guises, 

 in plants externally unlike. 



D. Classification. The study of morphology and life 

 histories enables us to recognize relationships among 

 plants, and hence to build up a genealogical tree, 

 showing lines of descent. Thus we can arrange plants, 

 together with their nearest relatives, in groups; and 

 related groups, again, in larger groups of successively 

 higher orders. This gives us a rational basis for the 

 classification of plants, and this phase of plant study 

 is called systematic botany, for it makes possible 

 the arrangement of plants in a system, which en- 

 deavors to show how the plant kingdom, in all its diversity, 

 has developed, or evolved. This greatly simplifies our 

 study of plants, for the number of different plants is too 

 great for us to study every one; but if we recognize 

 that each plant more or less imperfectly illustrates 

 a group, then we can study an illustration of each 

 group, and thus get a more nearly adequate picture 

 of the kingdom of plants as a whole. The various 

 systematic groups are given in E below. 



