e embryo-sac.

_Fertilization._

At the period of fertilization the embryo-sac lies in close proximity
to the opening of the micropyle, into which the pollen-tube has
penetrated, the separating cell-wall becomes absorbed, and the male or
sperm-cells are ejected into the embryo-sac. Guided by the synergidae
one male-cell passes into the oosphere with which it fuses, the two
nuclei uniting, while the other fuses with the definitive nucleus, or,
as it is also called, the endosperm nucleus. This remarkable
double fertilization as it has been called, although only recently
discovered, has been proved to take place in widely-separated
families, and both in Monocotyledons and Dicotyledons, and there is
every probability that, perhaps with variations, it is the normal
process in Angiosperms. After impregnation the fertilized oosphere
immediately surrounds itself with a cell-wall and becomes the oospore
which by a process of growth forms the embryo of the new plant. The
endosperm-nucleus divides rapidly to produce a cellular tissue which
fills up the interior of the rapidly-growing embryo-sac, and forms a
tissue, known as endosperm, in which is stored a supply of nourishment
for the use later on of the embryo. It has long been known that after
fertilization of the egg has taken place, the formation of endosperm
begins from the endosperm nucleus, and this had come to be regarded as
the recommencement of the development of a prothallium after a pause
following the reinvigorating union of the polar nuclei. This view is
still maintained by those who differentiate two acts of fertilization
within the embryo-sac, and regard that of the egg by the first
male-cell, as the true or generative fertilization, and that of the
polar nuclei by the second male gamete as a vegetative fertilization
which gives a stimulus to development in correlation with the other.
If, on the other hand, the endosperm is the product of an act of
fertilization as definite as that giving rise to the embryo itself,
we have to recognize that twin-plants are produced within the
embryo-sac--one, the embryo, which becomes the angiospermous plant,
the other, the endosperm, a short-lived, undifferentiated nurse to
assist in the nutrition of the former, even as the subsidiary embryos
in a pluri-embryonic Gymnosperm may facilitate the nutrition of the
dominant one. If this is so, and the endosperm like the embryo is
normally the product of a sexual act, hybridization w