EVOLUTIONARY HISTORY OF PLANTS 261 
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 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 recently 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 ones, he would probably put some one of 
the green alge at the bottom and the angiosperms at the 
top of the series. 
338. The Oldest Angiosperms. — It is impossible to give 
any of the reasons for the statements of this section with- 
out making an unduly long chapter. Briefly, it may be 
stated that the monocotyledons are the simplest but per- 
haps the youngest angiosperms; the dicotyledons are 
higher in organization but came earlier. The descent and 
various relationships of the families of dicotyledons can 
be discovered by the study of the flower, fruit, and seed 
better than by the examination of the vegetative organs. 
The entire pedigree of the several families cannot be 
represented by arranging the names of the families in a 
straight line. It is, however, in a general way, as indicated 
