324 ELEMENTARY BIOLOGY 



In some plants related to the ferns the two kinds of gametes are 

 borne on two different individuals; that is, each individual gameto- 

 phyte is either male or female. In such species each spore therefore 

 gives rise either to a male plant or to a female plant, as is the case 

 with the moss. It is impossible in such cases to find any difference 

 between the spores that give rise to male plants and the spores that 

 develop into female plants. 



375. Heterospory. But there are other plants related to the ferns 

 in which two different kinds of spores are produced, a large spore 

 and a small spore. In such species the large spore always develops 



h Gf Is Gf 



Gm Gm 



FIG. 152. Heterospory 



Plants producing spores of two sizes, Is and ss, give rise to two distinct forms of sexual, 

 or gamete-bearing, individuals, female and male, Gf and Gm. The gametes, f and m, 

 unite to form the zygote, z, which develops into the spore-bearing plant, 6". There is 

 an alternation between the sexual (gametophyte) and the asexual (sporophyte) generation ; 

 and there is a differentiation between male and female gametophytes, and, finally, a differ- 

 entiation between two types of spores. The next step would be to have two kinds of 

 sporophytes, S, one bearing large spores and the other bearing small spores ; and, 

 indeed, there are plants in which this condition is found 



into a female gametophyte, while the small spore always develops into 

 a male gametophyte. There are thus two kinds of spores as well as 

 two kinds of gametes (see Fig. 152). 



376. Alternation of generations in seed plants. The pollen grain 

 corresponds to a small spore ; that is, one that gives rise to a male 

 gametophyte. The embryo sac is really a large spore, one that can 

 give rise to a female gametophyte. In seed plants the small spore is 

 scattered, as in ferns and mosses ; but the large spore remains in the 

 spore case the ovule. The male gametophyte is a very much sim- 

 pler organism than we have found in mosses or ferns ; it is, in fact, 

 the simple pollen tube. It is incapable of nourishing itself, but lives 

 in part on the nourishment stored up in the pollen grain and in part 

 on material absorbed from the stigma. The only distinct organ that 

 it has is the divided nucleus that acts as a gamete. 



