358 Harris M. Benedict 



not hold for plant cells (Nemec, 1905). The formation of adventitious 

 tissue, of secondary meristems, of budding and mature leaves, has shown 

 the possibility of de-differentiation in specialized plant cells. 



The writer's results attack Minot's theory in an even more vital point, 

 since they indicate that senility occurs even in meristematic tissue and 

 is not dependent on differentiation. Furthermore, Minot's own figures 

 regarding the relative rapidity of the advance of senile degeneration, 

 showing that its onset is most rapid at the very beginning, when differ- 

 entiation is least, argues against the theory that specialization is the primary 

 cause of senility. 



Accumulation of katabolic products 



Child (1912), in an extremely interesting paper, states his opinion as 

 follows: " My experiments led me to the conclusion that senescence 

 in its simplest terms consists in a decrease in the rate of metabolism 

 determined by the gradual accumulation of relatively inactive structural 

 obstacles to metabolism which in turn are the necessary consequence of 

 continued metabolism under constant or relatively constant conditions 

 and in the presence of nutritive material." 



Hertwig, from his study of the " depression stages " in various protozoa, 

 comes to the conclusion that senescence is due to the accumulation of 

 materials in the nucleus until it becomes inert. He considers that a 

 large amount of material must be eliminated from such an engorged 

 nucleus before it can again become active. Child (1913), from his study 

 of a planarian, which reproduces only asexually, and from a long series 

 of regeneration experiments, has come to the conclusion that when such 

 an accumulation occurs a condition of senility is reached, and rejuvenes- 

 cence is restored only if the cell can rid itself of the accumulated inert 

 substances. He is of the opinion, further, that hunger, removal of a 

 part of the body, encystment, and asexual reproduction, are as efficient 

 in this respect as sexual reproduction. He says, as regards the plants, 

 there can be no doubt that the same is true. 



Hertwig (1906) disagrees in part with this conclusion. He thinks that 

 hunger, encystment, and special protozoan methods of nuclear destruc- 

 tion, are temporary patchwork repairs which, in forms capable of sexual 

 reproduction, postpone senility but cannot produce the complete rejuvenes- 

 cence that results from sexual reproduction. The evidence obtained from 



