THE LIFE CYCLE 39 



stances once formed are not as readily or as rapidly decomposed 

 and eliminated. 



It is evident that synthesis of proteid molecules is a factor of 

 great importance in growth, since proteids form the chief constitu- 

 ents of protoplasm, but there is no reason to believe, as various 

 authorities have maintained, that the metabolic process consists 

 wholly or chiefly in the synthesis and decomposition of proteid 

 molecules. All the facts indicate that much of the energy of the 

 organism comes from substances other than proteids, and that pro- 

 teid synthesis is only one of many chemical transformations occur- 

 ring in the organism. 



Moreover, according to physico-chemical laws, the accumulation 

 of colloids and other substances as a substratum in the organism or 

 in the cell must depend upon what we may call their physiological 

 stability. A physiologically stable substance is one which, when 

 once formed, cannot readily escape from the living cell or organism 

 under the existing conditions, unless it undergoes chemical change, 

 and which, under the usual physiological conditions, does not under- 

 go this change or undergoes it less readily than other substances. 

 Physiological stability depends then, not only on the constitution 

 of the substance concerned, but also and probably to a large extent 

 on the conditions to which it is subjected. Different substances 

 differ in stability under the same conditions, and the same substance 

 may differ very greatly in stability under different conditions. 

 Moreover, physiological stability does not necessarily imply com- 

 plete chemical stability. There is good reason to believe that 

 many substances in the cell are undergoing more or less continuous 

 partial chemical breakdown and reconstitution, but so long as they 

 do not undergo complete breakdown and elimination they consti- 

 tute parts of the cell which are relatively stable physiologically. 

 In most plants, for example, proteid molecules once formed never 

 undergo decomposition to the point where the nitrogen which they 

 contain is eliminated in any form, yet there can be no doubt that 

 these proteids, or some of them, take part in the chemical reactions 

 within the cell and that their molecules are often partially decom- 

 posed and reconstituted. They are then physiologically, though 

 not necessarily chemically, stable constituents of the plant cell. 



