GAMETOPHYTE AND SPOROPHYTE. 



329 



nucleus with the egg nucleus it is called double fertilization. The 

 sperm nucleus is usually smaller than the egg nucleus, but often 

 grows to near or quite the size of the egg nucleus before union. 

 See figs. 394 and 395. 



657. Fertilization in plants is fundamentally the same as 

 in animals. In all the great groups of plants as represented by 

 spirogyra, cedogonium, vaucheria, peronospora, ferns, gymno- 



Fig. 392- 



Two- and four-celled stage of embryo-sac of lilium. The middle one shows 

 division of nuclei to form the four-celled stage. (Easter lily.) 



sperms, and in the angiosperms, fertilization, as we have seen, 

 consists in the fusion of a male nucleus with a female nucleus. 

 Fertilization, then, in plants is identical with that which takes 

 place in animals. 



658. Embryo. After fertilization the egg develops into a short 

 row of cells, the suspensor of the embryo. At the free end the em- 

 bryo develops. In figs. 397 and 398 is a young embryo of trillium. 



659. Endosperm, the mature female prothallium. During 

 the development of the embryo the endosperm nucleus divides 



