THE FLOWER 223 
process is known as fertilization, and the product as the fertilized 
egg. The fertilized egg develops into an embryo, which is found 
in the seed, and the embryo in turn, after the germination of 
the seed, grows into a mature plant. 
Endosperm. ‘The male nucleus 
which does not fertilize the egg 
moves to the center of the embryo 
sac and fuses with the two polar nu- 
clei to form a single nucleus known 
as the endosperm nucleus. ‘This en- 
dosperm nucleus rapidly undergoes 
successive divisions and forms a 
mass of tissue, the endosperm, which 
fills the embryo sac and surrounds 
the embryo. The endosperm absorbs 
nourishment from the surrounding 
tissue and passes it on to the embryo. 
The endosperm may be entirely ab- 
sorbed by the developing embryo 
before the seed becomes mature, or 
it may remain in the seed and sur- 
round the embryo in the mature seed 
and be absorbed only during the 
germination of the seed (Fig. 269). 
THE NUCLEUS 
Since the essential part of ferti- 
lization consists in the fusion of 
the male nucleus with the egg (or, 
more particularly, the male nucleus 

Fia.228. Diagram of section of 
ovary, showing growth of pollen 
tubes toward embryo sac 
Within the ovary isasingle ovule ; 
in the center of the ovule is the 
‘embryo sac surrounded by the 
nucellus and this by two integu- 
ments. A pollen tube has grown 
from the pollen grain on the 
stigma and has entered the ovule 
through the micropyle 
with the egg nucleus), it is important that the structure of 
the nucleus should be studied in connection with fertilization. 
Resting nucleus. The nucleus when not in a state of division is 
called a resting nucleus. This designation is somewhat unfortu- 
nate, as the nucleus of an ordinary cell must be carrying on many 
activities even though no morphological changes may be evident. 
