ALTERNATION OF GENERATIONS 



325 



The female gametophyte is still further simplified, for it never gets 

 out of the spore wall. It is nourished altogether by the parent plant, 

 and its activities are confined to the dividing up of the nucleus, finally 

 separating the portion of nucleus that is to act as the female gamete. 



FIG. 1 53. Alternation of generations in seed-bearing plants 



Gm, the male gametophyte, or pollen tube ; m, the male gamete, a nucleus at end of pollen 

 tube ; Gf, the female gametophyte, or embryo sac ; f, the female gamete, a nucleus in the 

 embryo sac ; c, the fertilized egg, or embryo sac ; S, young sporophyte, the embryo in 

 the seed ; S 2 , the mature sporophyte, a flower-bearing plant ; J X , the large spore, giving 

 rise to the female gametophyte, or the embryo sac. s z , the small spores, or pollen grains, 

 giving rise to the male gametophyte. The spores always give rise to gametophytes, and 

 the gametes (producing a fertilized egg) always give rise to sporophytes. Sporophytes 

 alternate with gametophytes, generation after generation 



Our common seed plants are accordingly seen to be sporophytes, or 

 spore-bearing plants. The alternation of generations of these plants 

 is illustrated by the diagram in Fig. 153. 



