PLANT ORGANS AND ORGANISMS 
159 
gametophyte. The nucleus of the mother-cell divides to form two 
sperm nuclei. Within the tube cell is found a tube nucleus embedded 
in protoplasm. Upon germinating the partition disappears and the 
thin endospore, carrying within it the protoplasm in which are 
embedded the tube nucleus and twos perm nuclei, penetrates through 
a deficiency of the exospore. The contents of the pollen grain at this 
stage is called the male gametophyte. 
Maturation of the Embryo Sac and Formation of the Female 
Gametophyte. — The nucleus of the megaspore or embryo sac under- 
goes division until eight daughter-nuclei are produced which are 
separated into the following groups: 
(а) Three of these nuclei occupy a position at the apex, the lower 
nucleus of the group being the egg or ovum, the other two nuclei 
being the synergids or assisting nuclei. 
(б) At the opposite end of the sac are three nuclei known as the 
antipodals which apparently have no special function. 
(c) The two remaining nuclei ( polar nuclei) form a group lying 
near the center of the embryo sac which unite to form a single 
endosperm nucleus from which, after fertilization, the endosperm or 
nourishing material is derived. This stage of the embryo sac con- 
stitutes the female gametophyte. 
Fertilization in Angiosperms. — After the pollen grain reaches the 
stigma, the viscid moisture of the stigma excites the outgrowth of the 
male gametophyte which bursts through the coats of the pollen grain 
forming a pollen tube. The pollen tube, carrying within its walls 
two sperm nuclei and one tube nucleus, penetrates through the loose 
cells of the style until it reaches the micropyle of the ovule, then 
piercing the nucellus, it enters the embryo sac. The tip of the tube 
breaks and one of the sperm nuclei unites with the egg to form the 
oospore. The oospore develops at once into an embryo or plantlet, 
which lies passive until the seed undergoes germination. The other 
sperm nucleus unites with the previously fused polar nuclei to form 
the endosperm nucleus which soon undergoes rapid division into a 
large number of nuclei that become scattered about through the pro- 
toplasm of the embryo sac. These accumulate protoplasm about 
them, cells walls are laid down, endosperm resulting. 
