38 SENESCENCE AND REJUVENESCENCE 



term growth to one or the other of the particular processes as some 

 authors incline to do. Growth results primarily from the ability 

 of the cell to synthesize certain substances which, once formed, 

 remain as relatively permanent constituents of the cell. Under 

 different conditions the nutritive substances necessary, the course 

 of synthesis, and the substances formed must differ widely, but 

 growth is a complex organic process rather than this or that par- 

 ticular chemical reaction. 



The nature of growth and reduction. — -The question why the 

 organism grows is one of great interest, and while we cannot at 

 present answer it fully, we can at least reach certain provisional 

 conclusions. On the basis of the chemical hjrpo thesis of the labile 

 proteid molecule, growth remains a mystery. We cannot conceive 

 how these labile molecules are able to build up others like them- 

 selves. Reduction, however, is readily enough accoimted for 

 as the result of breakdown of the labile molecules. But if we 

 regard the organism as a complex of reactions in a coUoid sub- 

 stratum, the problem of growth assumes a different form and is 

 open to attack. Certain aspects of the problem require brief con- 

 sideration from this point of view. 



The reversibiHty of the growth process leads us at once to ask 

 whether or to what extent reversible chemical reactions are con- 

 cerned. If we could regard growth and reduction as the two 

 terms of a reversible chemical reaction it would simplify our con- 

 ceptions very greatly. Unfortunately, however, this seems to be 

 impossible. Reversible chemical reactions are undoubtedly con- 

 cerned in the synthesis and breakdown of the various molecules 

 which make up protoplasm, but the growth-reduction process is 

 something more than such a reaction. Apparently the course of 

 synthesis and of breakdown and the character of the end products 

 may differ widely. Many or all of the component reactions in 

 growth and reduction may be reversible, but it does not by any 

 means follow that reduction is a reversal in the chemical sense of 

 growth. During a considerable part of Hfe under the usual condi- 

 tions the synthesis of certain substances overbalances their break- 

 down, they accumulate in the organism, and growth occurs. 

 Evidently conditions in the organism are such that certain sub- 



