PLANT ORGANS AND ORGANISMS 



2OI 



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, 



mic 



A B C 



PIG. 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 (/); synergids (5); 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. 



