304 FOUNDATIONS OF BOTANY 
generation is still very 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 com- 
paratively 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 fertilization, tends strongly to show the com- 
mon 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 which spend the least portion of their entire life in 
the sexual condition were among the latest of plants to 
appear. Then, too, those which have the least developed 
sexual generation are among the latest of plants. Judged 
by these tests the angiosperms must be the most receaae 
developed of all plants. 
If one were to attempt to arrange all the classes of 
existing plants in a sort of branching series to show the 
way in which the higher plants have actually descended 
from the lower, he would probably put some one of the 
green algz at the bottom and the angiosperms at the top 
of the series. 
376. The Oldest Angiosperms. — It is impossible to give 
any of the reasons for the statements of this section 
