464 



SPERMATOPHYTES (SEED PLANTS) 



region enclosed within the cell membrane of the germinating 

 megaspore. In this position nuclear division follows until there 

 are four nuclei at each end. The megaspore has now become the 

 female gametophyte consisting of eight nuclei, four at the 

 micropylar and four at the opposite end, known as the chalazal 

 or antipodal end of the embryo sac. After the stage with eight 

 nuclei is reached, then the organization of the female gameto- 

 phyte begins as shown in Figure 110. A nucleus called polar 



nucleus from each end of the 

 embryo sac moves toward 

 the center of the sac until the 

 two come in contact. Some- 

 times they fuse soon after 

 coming in contact to form 

 the primary endosperm nu- 

 cleus, but often they remain 

 in contact until fertilization 

 and then fuse at the same 

 time they fuse with the 

 sperm to form the endo- 

 sperm nucleus. The three 

 nuclei and adjacent cyto- 

 plasm at the micropylar end 

 are organized into three 

 naked cells, the inner one 

 being the egg and the other 

 two the synergids. The three 

 nuclei at the antipodal end 

 and known as antipodals 

 usually disappear early, but 

 in some Angiosperms they 

 become organized with the adjacent cytoplasm into cells that 

 seem to have an absorptive function. The female gametophyte 

 is now organized and ready for fertilization. When compared 

 with the female gametophyte of the Pine, its remarkable reduc- 

 tion in number of cells, the absence of archegonia, and the forma- 

 tion of a nucleus for providing endosperm are notable features. 

 Male Gametophyte and Fertilization. On the stigma the 

 pollen grain develops a tube which by means of enzymes eats its 

 way through the stigma, style, and ovule into the embryo sac. 



FIG. 410. Organization of the female 

 gametophyte in Red Clover. At the left, 

 a section through the nucellus, showing 

 eight nuclei of the female gametophyte 

 with four nuclei at each end of the em- 

 bryo sac. At the right, the gametophyte 

 fully organized, showing the antipodals 

 at a, the polars at p, the egg at e, and 

 the synergids at s. 



