HISTORY OF THE PLANT KINGDOM 303 



occupies a less and less important share in the life of the 

 plant as one goes higher in the scale of plant life.i In 

 the case of the ^ock^^-eed, for instance, the sexual genera- 

 tion is the plant. Among mosses the sexual generation is 

 still xery prominent in the life of the plant. Ordinary 

 ferns show us the sexual generation existing only as a 

 tiny independent organism, living on food materials which 

 it derives from the earth and air. In the Salvinia it is 

 reduced to microscopic size and is wholly dependent on 

 the parent plant for support. Among seed-plants the 

 sexual generation is so short-lived, so microscopic, and so 

 largely enclosed by the tissues of the flower that it is 

 comparatively hard to demonstrate that it exists. 



The fact that the life history of so many of the classes 

 of plants embraces a sexual stage, in which an egg-cell is 

 fertilized by some sort of specialized cell produced wholly 

 for use in fei-tilization, tends strongly to show the common 

 origin of the plants of all such classes. We have reason 

 to believe, from the evidence afforded by fossils, that plants 

 which have only a sexual generation are among the oldest 

 on the earth. It is therefore likely that those plants which 

 spend the least portion of their entire life in the sexual 

 condition were among the latest to appear. Then, too, 

 those -nhicli have the least developed sexual generation 

 are among the latest of plants. Judged by these tests, 

 the angiosperms must be the most recently developed of 

 all plants. 



If one were to attempt to arrange all the classes of ex- 

 isting plants in a sort of branching series, to show the way 

 in which the higher plants have actually descended from 



1 A good manv plants of low organization, however, are not known to pass 

 thr'ough any sexual stage. 



