88 Plant Life and Evolution 



which they enter, and one of them fertilizes the egg 

 in precisely the same way as happens in the per- 

 manently aquatic algae. 



After the egg is fertilized it becomes invested with 

 a cell-wall, but it does not enter a resting state as 

 is the case with the green algae. Instead of this, 

 the egg cell grows and undergoes repeated cell- 

 divisions, so that a cellular body, the embryo, re- 

 sults (Fig. 9, D). The embryo, by further growth, 

 finally develops into a plant which is entirely differ- 

 ent in appearance from the one which bears the 

 archegonium, and which is called the gametophyte 

 or sexual plant. The plant developed from the em- 

 bryo does not become independent, but remains at- 

 tached to the sexual plant, upon which it lives in a 

 parasitic fashion. Sooner or later it gives rise to 

 special reproductive cells, which are produced by 

 cell-division and are hence non-sexual in their na- 

 ture. These are called " spores," and the plant 

 bearing them is known as the " sporophyte," or non- 

 sexual plant. 



Alternation of Generations. The sporophyte as- 

 sumes greater and greater importance in the course 

 of the evolution of the archegoniates, while the 

 gametophyte becomes less and less conspicuous. 

 From the spores produced by the sporophyte there 

 arise new gametophytes. This alternation of the 

 sexual plant or gametophyte with the neutral one, 

 or sporophyte, produced as the result of fertilization, 

 constitutes the frequently discussed " alternation 



