10 A TEXTBOOK OF BOTANY [Ch. II 



lives in the past. The evidence seems to show beyond 

 question that our present species of plants have descended 

 by gradual evolution from simpler and fewer species which 

 formerly existed, and which in turn were evolved from still 

 simpler and fewer kinds, — back, it is possible, to a single 

 kind which throve in remotest antiquity. In the course of 

 this evolution, plants have diverged into the many groups, 

 and groups within groups, expressed in our schemes of classi- 

 fication. Thus also various features originally distinctive 

 of one species came to prevail through whole families, and 

 even persist to the present, often having lost completely 

 their original significance. It is the aim of botanists to 

 distinguish between those features which have merely a tem- 

 porary functional significance and those which are deeply 

 fixed in heredity. They use the former in the interpreta- 

 tion of the phenomena of plant life, and the latter as guides 

 to evolution and classification. Hence botanical study falls 

 most fundamentally into the two phases represented by the 

 two Parts of this book. 



While the groups and classification of plants will receive 

 full treatment in Part II, some general knowledge of the more 

 important of such facts is essential to an understanding of 

 Part I. The main groups, with their essential character- 

 istics, are the following. 



1. The Flowering Plants, the most highly evolved 

 and therefore often called the "higher plants," comprise the 

 great majority of the trees, shrubs, and herbs constituting 

 the familiar land vegetation. They are distinguished not 

 only by the possession of flowers, which often are extremely 

 inconspicuous, but also, and especially, by their seeds, on 

 which account they are called scientifically Spermatophytes, 

 that is, "seed plants." While mostly they dwell on the land 

 with roots in the ground, and make their food in their 

 green leaves, some live in water, and some upon other 

 plants. They are clearly descended from the following 



