AGE CYCLE IN PLANTS AND LOWER ANIMALS 253 



spore, reconstitutional changes are again involved, at least in the 

 case of zoospores, which show more or less morphological differen- 

 tiation, and here again some degree of rejuvenescence must occur. 

 Bearing all the facts in mind, it is not difficult to understand how 

 it is that, under proper conditions, spore formation may continue 

 through an indefinite number of generations without any appre- 

 ciable progressive senescence of the stock. 



In the mosses, ferns, and seed plants where alternation of genera- 

 tions occurs, the fertilized egg gives rise to the asexual generation 

 or sporophyte which may show extensive and long-continued 

 vegetative reproduction, but sooner or later gives rise to spores. 

 The spore in turn gives rise to the sexual generation or gametophyte, 

 which also may show vegetative reproduction, but which finally 

 produces gametes; that is, sexual reproductive cells. 



In the mosses and ferns spore formation is very evidently a 

 process belonging to the later stages of development of the sporo- 

 phyte and it is, as in the algae and fungi, a process of disintegration 

 of an individual or part into independent cells. In the fern, for 

 example, the spores are formed only when the frond has completed 

 or largely completed its growth. In the seed plants the gameto- 

 phyte generation is so reduced that spore formation is closely con- 

 nected with the formation of gametes, and there is much evidence 

 to be considered in later chapters which indicates that gamete for- 

 mation belongs to a more advanced stage of the life cycle than the 

 various agamic processes. In these plants, as in the algae and fungi, 

 spore formation is, then, in general a process belonging to more 

 advanced stages than vegetative reproduction. 



Among algae and fungi there is apparently complete rejuvenes- 

 cence between the formation of the spore and the development of a 

 new plant from it, for it usually gives rise to a new plant like that 

 from which it arose, while in the plants with alternation of genera- 

 tions the spore gives rise to an individual of different character 

 from that which produced it. Evidently it has become different 

 in its developmental capacity from the egg. The simplest concep- 

 tion of this change is that the spore is in these forms a specialized 

 cell which does not entirely lose its specialization in reproduction. 

 In the seed plants, where the gametophytes do not lead an 



