ALTERNATION OF GENERATIONS 



323 



than the nail of your little finger, called a prothallus (see Fig. 150). 

 Prothalli are often found growing on flowerpots in greenhouses. 



The prothallus corresponds to the gametophyte of the moss, while 

 the plant which is familiar to us as the fern is a sporophyte. The 



FIG. 151. Alternation of generations in the life history of the fern 



G, the gametophyte, or gamete-bearing plant ; f, the female gamete organ ; m, the male 

 gamete organ ; c, the fertilized egg. S, the sporophyte, or spore-bearing plant ; s, the 

 spores discharged by the spore-bearing organ. The spore develops into a gametophyte ; 

 the gametes (egg) always give rise to a sporophyte. The alternate generations repro- 

 duce in different ways, one by means of gametes, or sexually, the other by means of 

 spores, or asexually 



spore always gives rise to a prothallus, which bears gametes. When 

 fertilization has taken place, the zygote formed develops not into 

 another prothallus but into a sporophyte. The diagram in Fig. 151 

 shows us the alternation of generations in this group of plants. 



