254 SENESCENCE AND REJUVENESCENCE 



independent vegetative life, but are usually so much reduced that 

 only a few cell divisions occur between the spore and the formation of 

 gametes, the specialization of this reproductive process is evident, 

 but in the mosses where the sporophyte is merely a sporogonium — 

 a spore case without an independent vegetative life — and the game- 

 tophyte is the vegetative form, it is not so clear. If, however, we 

 consider the whole cycle from the fertilized egg of one generation to 

 that of the next, it is at once evident that in the mosses the process 

 of spore formation comes relatively early in this cycle, in the ferns 

 at a more advanced stage, and in the seed plants at a still more 

 advanced stage. In this connection it is of considerable interest 

 to note that the amount of agamic reproduction in the gametophyte 

 varies according to the point in the life cycle at which the gameto- 

 phyte appears. In the mosses, where the sporophyte shows almost 

 no vegetative activity before spore formation, the gametophyte, 

 which is the moss plant, usually shows extensive, often indefinite, 

 vegetative reproduction, and in many cases various, more or less 

 specialized, forms of agamic reproduction occur. In the ferns where 

 the sporophyte — the fern plant — shows extensive vegetative growth 

 and reproduction there is usually but little and in many cases no 

 agamic reproduction in the prothallium which represents the 

 gametophyte. And, finally, in the seed plants, where the whole 

 vegetative life of the plant occurs in the sporophyte stage, the 

 gametophyte does not as a rule reproduce gametophytes asexually. 

 In other words, the earlier in the life cycle the gametophyte appears, 

 the less its specialization and the more conspicuous its vegetative 

 activity and reproduction. 



All these facts indicate very clearly that a real life cycle with 

 progressive development and specialization exists in the plants, 

 but this life cycle is complicated by the occurrence of various forms 

 of agamic reproduction, and the regressive and reconstitutional 

 changes involved in the new individuations which occur in these 

 reproductions may balance the progressive changes and so retard 

 or prevent indefinitely the progressive advance of the plant in the 

 life cycle. And since external conditions influence individuation 

 and agamic reproduction, it is often possible to control experi- 

 mentally the developmental progress of the plant within very wide 



