468 SPERMATOPHYTES (SEED PLANTS) 
sac develcp like buds and form embryos, in which case, of course, 
there is no fertilization. Polyembryony may also be a source of 
annoyance to plant breeders, for if plants that are used in cross- 
ing develop polyembryonous seeds, the offspring arising from 
these seeds may develop from embryos that were formed by the 
budding of the nucellus, in which case the embryos have only the 
characteristics of the mother plant. For example, in crossing 
different strains of Tobacco, in some cases the plants arising 
from the seeds obtained by crossing are not hybrids but like the 
mother plant. Some think this may be due to parthenogenesis 
and others attribute it to polyembryony. 
Endosperm. — While the embryo is developing, the endosperm 
nucleus is dividing and its accompanying cytoplasm is increasing. 
The free nuclei at first form in a chain around the wall and then 
multiply towards the center. Cell walls are finally formed and in 
these cells food is stored. In some Angiosperms the endosperm 
is taken up by the embryo almost as rapidly as formed and stored 
in the cotyledons, while in other Angiosperms most of the en- 
dosperm remains outside of the embryo until the seed germi- 
nates. 
Since the endosperm nucleus contains the contents of a sperm, 
the character of the endosperm of a seed is often determined by 
the sperm. Thus, as in case of Corn where the endosperm re- 
mains outside of the embryo, the color and other characteristics 
of the endosperm are often like the pollen parent and not at 
all like those of the mother parent. This feature called xenia 
has already been referred to. In some seeds, in addition to the 
formation of endosperm, the portion of the nucellus remaining 
becomes stored with food and forms what is known as peri- 
sperm. 
Seed Coat.— As the embryo and endosperm develop, the 
ovule enlarges rapidly, and at the same time the embryo sac de- 
stroys much or all of the nucellus and frequently a part or all 
of the inner integument. Consequently the seed coat consists 
chiefly of the outer integument, which is usually very much mod- 
ified for protection. 
It is obvious that the seeds of Angiosperms differ considerably 
from the seeds of Gymnosperms, for the female gametophyte of 
Angiosperms is soon destroyed after fertilizatidn by the develop- 
ing embryo and endosperm, and consequently there is no gameto- 
