AGE CYCLE IN PLANTS AND LOWER ANIMALS 259 



direction of specialization without any characteristic morpho- 

 logical differentiation, and there is every reason to believe that 

 changes in the opposite direction, if not very great, do not neces- 

 sarily involve changes in the visible morphological features of the 

 cell. Since senescence and rejuvenescence are processes which 

 concern the dynamic activity of the cell, changes in this activity 

 must be the chief criterion for the occurrence of age changes, 

 although morphological changes, when they occur, may be of value 

 as indications of the changes in activity. 



SENESCENCE AS A CONDITION OF REPRODUCTION AND 



REJUVENESCENCE 



Agamic reproduction of one kind or another unquestionably 

 occurs in the plants and lower animals in consequence of the decrease 

 or elimination of dominance, i.e., the physiological disintegration 

 of the individual may result in the reconstitution of new individuals. 

 Moreover, decrease or elimination of dominance may result from 

 decrease in rate of metabolism as well as from growth, and 

 finally a decrease in rate of metabolism occurs in senescence. It is 

 possible, therefore, that agamic reproduction with the accompa- 

 nying rejuvenescence may occur simply as the result of senescence. 

 The fragmentation of Planaria velata (pp. 130-33) is undoubtedly 

 a case of this sort, and it is probable that this relation between 

 senescence and reproduction is very general. In fact, the forma- 

 tion of spores in plants and in the protozoa, of gemmules in the 

 sponges and statoblasts in the bryozoa, and various other reproduc- 

 tive processes, which are not directly connected with growth, are 

 probably very often simply the result of senescence of the indi- 

 vidual concerned, although they may of course appear when the 

 rate of metabolism is lowered by external conditions. The forma- 

 tion or development of new buds in many perennial plants often 

 results from decrease in activity of the dominant growing tip, and 

 this decrease is probably very frequently due to senescence. The 

 formation of buds or the development of buds already formed on 

 the leaves of various plants may likewise result from senescence of 

 the leaf or plant. During the earlier stages of senescence disinte- 

 gration of the individual may be prevented by the development and 



