PLANT ORGANS AND ORGANISMS 
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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 micropyleof 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, 
Fig. 97.—A, Immature angiospermous ovule; B, same, after embryo-sac ( e.s). 
has matured to form the female gametophyte; nucellus ( nuc); outer integument 
(o. int ); inner integument ( i. int); embryo sac (e.s.); micropyle ( mic); chalaza 
(ch); funiculus (f); synergids (s); ovum ( o); polar nuclei ( p); antipodals (a); C, 
fertilized and matured angiospermous ovule (seed). Note that the nucellus 
(nuc) has been pushed out by the encroachment of the embryo sac, in which 
endosperm has formed by the fusion of the two polar nuclei with the second 
sperm nucleus from the pollen tube which has later divided to form numerous 
nuclei scattered about in the protoplasm of the embryo sac and accumulated 
protoplasm and laid down walls, within which nourishment was stored; embryo 
(em) from fertilized ovum; testa (t) from outer integument; tegmen (te) from 
maturation of inner integument; micropyle (mic); hilum or scar (h), after funicu¬ 
lus became detached. 
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. 
