366 SENESCENCE AND REJUVENESCENCE 



possibility for the lower plant, and gamete formation is a feature 

 of its later stages, but since physiological progression may be 

 experimentally accelerated, retarded, or inhibited by controlling the 

 relation between progression and regression, the life cycle does not 

 appear as a definite, uniform, internally determined sequence of 

 events such as occurs in the higher animals. 



CONDITIONS OF GAMETE FORMATION IN MOSSES AND FERNS 



In mosses and ferns the life cycle is complicated by an alternation 

 of sporophyte and gametophyte generations, each of which possesses 

 a characteristic different structure. The gametophyte produces 

 sexual organs in which the gametes develop, and the gametes after 

 fertilization give rise to the sporophyte which produces asexual 

 spores, and these produce another gametophyte generation. In 

 mosses the gametophyte is the vegetative generation, and the sporo- 

 phyte does not lead an independent life but develops upon the 

 gametophyte. In the ferns, on the other hand, both the sporo- 

 phyte, the fern plant, and the gametophyte, the prothallium, lead 

 an independent vegetative life. 



In both mosses and ferns the production of sexual organs and 

 gametes on the gametophyte occurs only after a certain period of 

 vegetative activity which may vary in length with external con- 

 ditions; in other words, gamete formation seems to be characteristic 

 of a certain physiological condition which does not exist in the early 

 life of the gametophyte but arises only later. This condition evi- 

 dently corresponds to the condition of sexual maturity in the higher 

 animals. Moreover, after producing sex organs and gametes the 

 gametophyte dies, except where parts of it produce new gameto- 

 phytes asexually. 



Vegetative agamic reproduction in the gametophytes occurs 

 very widely and in a great variety of forms among both mosses and 

 ferns and leads directly to the formation of new gametophyte indi- 

 viduals. In certain species, or under certain external conditions, 

 vegetative reproduction of the gametophyte may continue indefi- 

 nitely, and sex organs and gametes do not appear or appear very 

 rarely. This is conspicuously the case in many of the so-called 

 true mosses, the Bryales, in which the degree of individuation in the 

 gametophyte is evidently very slight, and vegetative reproduction 



