192 SENESCENCE AND REJUVENESCENCE 



of the same tree.' In such cases the periodicity may perhaps be 

 associated with the alternate accumulation and removal of sub- 

 stance. It is also possible that periods which appear superficially 

 to be seasonal may be at least often of this character. Schimper 

 beheved that an internally determined periodicity might occur 

 independently of cHmatic and other conditions. Klebs, however, 

 denies the existence of such periodicity, yet at the same time he 

 regards the accumulation of organic substances, which as products 

 of enzyme activity inhibit or retard further activity, as a factor in 

 bringing about quiescent periods. If such substances are produced 

 more rapidly than they are used, they must accumulate, and it seems 

 probable that, at least sometimes, an internally determined perio- 

 dicity may result. 



The view that the formation of the gametes or sex cells is essen- 

 tially a process of differentiation and senescence and the early 

 stages of embryonic development a process of rejuvenescence has 

 already been mentioned and will be discussed more fully in later 

 chapters. The cycle of changes in the egg is somewhat similar to 

 that in the gland cell, with the difference that in the egg the yolk 

 becomes a source of energy and substance for growth. 



If the point of view advanced here is correct, then the age cycle 

 in the strictest sense is merely one of many periodicities or cycles 

 in organisms, some longer, some shorter, which result from the rela- 

 tions existing between the chemical reactions of metabolism and 

 the substratum in which they occur. The distinction between an 

 age cycle and other cycles is but Uttle more than a matter of con- 

 venience or custom. The changes which fall into the category of 

 what we are accustomed to call age changes are merely those in 

 which the more stable and less rapidly changing features of the 

 organism are involved. Various other cycles of different length 

 differ mainly in that less stable and more rapidly changing condi- 

 tions in the substratum are concerned. Whether we call one cycle 

 an age cycle and another something else is of little importance, except 

 as regards convenience. From the cycle of fatigue and recovery 

 at one extreme, to the cycle of senescence and rejuvenescence 

 in the stricter sense at the other, there are many intermedi- 



' See, for example, Schimper, '98, pp. 260-81; Klebs, '11; Volkens, '12; Simon, '14. 



