10 A TEXTBOOK OP BOTANY [Cii. II 



lives in the past. Tlie evidence seems to show lie^und 

 question that our present speeies of plants have descended 

 by gradual evolution from simpler and fewer species which 

 formerly existed, and which in turn were evolved from stdl 

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

 kind which throve in remotest antiquity. In the course of 

 this evolution, plants have diverged into the many grou])s, 

 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 huuilies, and 

 even persist to the present, often having lost completely 

 their original significance. It is the aim of l)otanists to 

 distinguish lietween those features whicli 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 i)lant life, and the latter as guides 

 to evolution and classification. Hence botanical study fails 

 most fundamentally into the two phases represented ])y the 

 two Parts of this l)ook. 



While the groups and classification of plants \nll 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 Pl.\nts, the most highly evolved 

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

 great majority of the trees, shrulis, and herbs constituting 

 the familiar land vegetation. Th(>y are distinguished not 

 only l)y the possession of flowers, which often are extremely 

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

 which account they are called scentifically Speriiatophytes. 

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

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

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

 plants. They are clearly descended froni the following 



