452 



SENESCENCE AND REJUVENESCENCE 



The general resemblance of the growth process to an autocata- 

 lytic reaction is self-evident: in the first place one result of growth 



is an increase in the amount of protoplasm, 

 and the greater the amount of protoplasm 

 the greater the amount of growth in a given 

 time. Or more specifically, assuming what 

 is undoubtedly true, that growth is dependent 

 directly or indirectly upon the presence of 

 certain enzymes, then it is evident that 

 greater amounts of growth are possible as 

 growth proceeds, for the necessary enzymes 

 are one of the products of growth. 



Doubtless certain reactions concerned in 

 growth are autocatalytic reactions, but it 

 seems obvious that growth is very much more 

 than an autocatalytic reaction and that 

 certain processes which do not follow the 

 laws of autocatalysis are much more impor- 

 tant in relation to the more conspicuous 

 characteristics of growth than those which 

 do or seem to. Growth produces other sub- 

 stances besides active protoplasm or enzymes, 

 viz., substances which play little or no part 

 in bringing about further growth, but form 



140 



130 



1 20 



no 



100 



80 



70 



60 



50 



40 



30 



20 



10 



Years 123 



FIG. 201. Curve of human growth from birth to three years, drawn from the 

 percentage increments of weight in Table XIII : each vertical interval indicated on 

 the axis of ordinates indicates an increment of 10 per cent in weight, each horizontal 

 interval on the axis of abscissae, three months. 



more or less stable structural constituents of the organism. 

 As growth proceeds, the proportion of these substances to the total 



