1 82 SENESCENCE AND REJUVENESCENCE 



parts, i.e., these parts become somewhat younger, even though 

 nutrition is present. 



The results of the experiments together with the results of 

 observation in nature constitute an adequate foundation for the 

 conclusion that a greater or less degree of rejuvenescence must be 

 associated with agamic reproduction. As we have seen in the case 

 of Pennaria (pp. 148-51), it may be less in the more speciahzed 

 than in the less specialized types of reproduction and it must 

 differ in degree with various other conditions, but wherever recon- 

 stitutional or reductional changes are involved we must expect 

 to find some degree of rejuvenescence. 



The persistence of the embryonic condition in the growing tip 

 and meristematic tissues of the higher plants and in the growing 

 regions of many of the lower animals shows, however, that under 

 certain conditions growth may continue over long periods of time 

 without any very great degree of, and in many cases perhaps 

 without any, senescence. So far as we know, the long-continued 

 persistence of the embryonic condition in rapidly growing tissues 

 is always associated with a high frequency of cell or nuclear division, 

 and the experiments on the infusoria (see pp. 137-42) indicate 

 that at least in these forms some degree of rejuvenescence occurs 

 in connection with cell division. There is every reason to believe 

 that in nuclear and cell division in general, as in other forms of 

 reproduction, some degree of change in the direction of rejuvenes- 

 cence occurs. Whether this balances the changes which occur 

 between successive cell divisions depends upon the frequency of 

 division, the rate of growth, and various other conditions. Where 

 a balance is attained or approached, differentiation and senescence 

 do not occur, or proceed slowly; otherwise they proceed more or 

 less rapidly, according to conditions. 



The only possible conclusion in view of all the facts seems to be 

 that senescence is associated with the productive and progressive 

 phases, and rejuvenescence with the reductive and regressive 

 phases, of the life cycle. 



THE NATURE OF SENESCENCE AND REJUVENESCENCE 



The theories of senescence that have been advanced fall mainly 

 into two groups. Those of the one group regard the phenomena 



