ALTERNATION OF GENERATIONS 



323 



than the nail of your little finger, called a prothallus (see Fig. 1 5 o). 

 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 ; /, the female gamete organ ; m, the male 

 gamete organ ; c, the fertilized egg. 6^ 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. 



