94 A GRAIN OF BARLEY. 
are perforated by a canal-like entrance, the micropyle, m. At 
an early stage of the development of the flower there is one 
cell within the nucleus which enlarges considerably at the 
expense of the others. This cell, which is situated near the 
micropyle, is the emdbryo-sac, ¢, and by the time the anthers 
are ready to shed their grains of pollen it has considerably 
increased in magnitude, and has developed within itself, at the end 
towards the apex, a minute cell, the oosphere, z, which is fertilised 
by the pollen in the following manner. The pollen grains which 
have attached themselves to the stigma undergo a sort of 
germination, and protrude their contents in the form of 
extremely minute tubes, 4, 7, m, which find their way down 
the style, and along the walls of the ovary or ovule until they 
reach the micropyle. Through this aperture they enter the 
embryo-sac, and mingle their contents with the vosphere. 
This is the act of fertilization, which is immediately followed 
by a development of the oosphere by spontaneous division 
until it forms the embryo of the seed. Whilst this is going 
on the whole embryo-sac gradually enlarges, and its internal 
walls become lined with a thin layer of exdosperm, the material 
laid up for the future use of the young embryo. The endo- 
sperm generally extends inwards until the whole cavity of the 
embryo-sac is filled up. In the so-called exalbuminous seeds, 
such as peas, beans, walnuts, hazel-nuts, acorns, &c., we | 
find no endosperm when the fruit is fully ripened, the whole 
seed being entirely filled with the embryo. In all such cases, 
however, the seed contained endosperm in the earlier stages 
of its development, but this has been completely absorbed 
and replaced by the embryo before the period of ripening. 
The developed embryo-sac, with its embryo and endosperm, 
as it increases in size and replaces the nucleus, becomes 
closely united with the walls of the ovule, whose inner wall 
is, as a rule, completely absorbed. Some portion, however, 
of the outer integument of the ovule remains, and by its union 
with the wall of the enlarged embryo-sac, forms the /es¢fa, or 
integument of the seed. 
