HISTORY OF THE PLANT KINGDOM 3038 
occupies a less and less important share in the life of the 
plant as one goes higher in the scale of plant life In 
the case of the rockweed, for instance, the sexual genera- 
tion is the plant. Among mosses the sexual 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 
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 fertilization, 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 which 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 many plants of low organization, however, are not known to pass 
through any sexual stage. 
