AGE CYCLE IN PLANTS AND LOWER ANIMALS 255 



limits. And, finally, there are certainly very clear indications that 

 a general decrease in rate of metabolism, doubtless interrupted by 

 greater or less increases in rate accompanying the various reproduc- 

 tions, occurs from the early vegetative stages to the stage of gamete 

 formation. There seems, in short, to be adequate ground for the 

 conclusion that the life cycle of the plant is not fundamentally 

 different from that of the animal and that senescence does occur 

 in the plant, not only in certain cells, organs, or tissues and in the 

 phytoids which make up most plants, but in the plant as a whole. 

 The slight degree of individuation in plants makes possible frequent 

 reproduction, so that senescence is not a continuous or nearly con- 

 tinuous process, as in the higher animals, but may be interrupted 

 repeatedly, or may even be compensated for an indefinite length 

 of time by periodic reproduction and rejuvenescence, such as has 

 been shown in Part II to occur in some of the lower animals. 



INDIVIDUATION, AGAMIC REPRODUCTION, AND THE AGE CYCLE IN 



THE LOWER ANIMALS 



Experimental evidence on the relation between agamic repro- 

 duction and rejuvenescence in various animals was presented in 

 chap, vi, and only certain points of more general significance remain 

 to be considered. The occurrence of agamic reproduction in the 

 lower animals, as in the plants, is commonly either the result of 

 growth or decreased dominance, and often the same reproductive 

 process may be brought about in both ways. The variety of forms 

 of reproduction is less than in the plant, but in various protozoa 

 growth and division occur under the usual conditions, while under 

 others, apparently such as decrease metabolism, the body may 

 break up into small independent cells, which are usually known as 

 spores. Such fragmentations of the body may apparently result 

 either from a physiological senescence or from a decrease in meta- 

 bolic activity due to external conditions. Fragmentation often 

 occurs during encystment and is preceded or accompanied by 

 complete dedifferentiation of the original individual. These cases 

 in fact constitute some of the strongest evidence for the occurrence 

 of dedifferentiation in animals. Often, particularly in the sporozoa, 

 which are parasitic and show a very low degree of individuation, 



