Mosses, Ferns, and Horsetails. 299 



of these groups then moves toward the center of the 

 embryo sac (see Fig. 88), and the two fuse, forming one, 

 which by repeated division gives rise to the endosperm 

 tissue of the seed. A plasma membrane is organized 

 about each of the remaining nuclei of the two groups, 

 including with each some of the cytoplasm of the embryo 

 sac, thus forming definite cells. When a sperm cell leaves 

 the pollen tube, it is found to penetrate and fuse with one 

 of the three cells at the micropylar end, resulting in its 

 fertilization and subsequent division and ultimate forma- 

 tion of the embryo (0) within the seed. We may conclude 

 from this that the cell with which the sperm fused is the 

 egg, and that it, and the other cells resulting from the 

 germination of the embryo sac spore, including the endo- 

 sperm (/), constitute a gametophyte, which has no differen- 

 tiated archegonium. As has been said, the division of the 

 fertilized egg results in the formation of the embryo (o). 

 This is an early stage of a new sporophyte, and we may 

 say that, just as in the case of the ferns and mosses, the 

 fertilized egg marks the close of the gametophyte part 

 and the beginning of the sporophyte part of the life cycle 

 of Spermatophytes. The sporophyte part of the life cycle 

 continues through the germination of the seed and on 

 through the growth of the plant up to the formation of 

 pollen spore and embryo sac spore, which marks the 

 beginning of the gametophyte part of the life cycle. 



We see that the flowering plants have two sorts of game- 

 tophytes, — male, produced by the germination of the pollen 

 spore, and female, formed by the germination of the embryo 

 sac spore. The same thing is true of the Selaginellas 

 (standing in the scale of evolution between the ferns and 

 flowering plants), in which the microspores produce male, 

 and the macrospores female, gametophytes (see page 202). 



