THE EVOLUTIONARY HISTORY OF PLANTS 303 
development the macrospore produces an endosperm which 
is really a small cellular prothallium, concealed in the ovule. 
The microspore contains vestiges of a minute prothallium. 
In the angiosperms the macrospore and its prothallium 
are still less developed, and the 
microspore, or pollen grain, has 
lost all traces of a prothallium 
and is merely an antheridium 
which contains two generative 
cells.1 These are most easily 
seen in the pollen grain, but 
sometimes they are plainly visi- 
ble in the pollen tube (Fig. 164). 
Phanerogams are distinguished 
from all other plants by their 
power of producing seeds, or 
enclosed macrosporangia, with 
embryos. 
375. The Law of Biogenesis 
and the Relationships of the Great = rie. 217.— Longitudinal Section 
Groups of Plants. —Onsumming —-{urongh Fertilued ovmie or 
up Sects. 372-374 it is evident — p, pollen grains; ¢, pollen tubes ; 
that the sexual generation in peice aa 
general occupies a less and less _nucleus; ¢, embryo sac filled 
important share in the life of the ba rae 
plant as one goes higher in the scale of plant life.? In the 
case of the rockweed, for instance, the sexual generation 
is the plant. Among mosses and liverworts the sexual 
1 Sometimes only one generative cell escapes from the pollen grain into the 
pollen tube, and there it divides into two cells. 
2 A good many plants of low organization, however, are not known to pass 
through any sexual stage. 
