KINETICS OF GROWTH 97 



space per se cannot be considered seriously as an important factor in 

 growth. 



Thus the best accessible evidence indicates that the growth rate is 

 not affected by the concentration of nutrients between very wide limits, 

 and that the total yield, over an equally wide range, is proportional to 

 the amount of food. This situation carries important implications which 

 will next be considered. 



Let me reiterate that growth consists in part of the transformation of 

 foodstuffs into cellular constituents. This, in turn, depends upon 

 metabolism. In Chapter III Thimann has presented a number of cases 

 in which growth responses are strictly paralleled by metabolic responses 

 following upon the exposure of plants to auxin solutions. It is therefore 

 of the greatest significance that the rate of metabolism, as has been 

 repeatedly ascertained, is generally unaffected by concentration dif- 

 ferences of the substrate down to extremely low levels." And it is not 

 surprising that growth would follow the same pattern. 



In many respects the relation between metabolism and growth is 

 quite obvious. "No growth without metabolism" was a dictum as 

 familiar since the latter part of the nineteenth century as the phrase "No 

 thought without phosphorus" had been earlier. The reason for this has 

 become apparent as the concepts of thermodynamics were developed. 

 The growing cell must synthesize cellular constituents, and for these 

 syntheses energy is required. 



As a first approximation the relation between growth and metabolism 

 could thus be understood as one of energy linkage ; the metabolic activi- 

 ties of the cell provide the energy necessary for the performance of 

 synthetic reactions. Based upon this concept, various investigations have 

 been conducted aimed at measuring the "energetic yield" of growth 

 processes. By determinations of the amount of cell material that could 

 be synthesized from a known quantity of substrate, of the caloric value 

 of the cell material, and of the energy liberated by oxidations accom- 

 panying the observed growth, data were assembled which permitted the 

 computation of such "energetic yields." In subsequent studies of the 

 same general nature more precise relations were calculated by distin- 

 guishing between a "metabolism for upkeep" and a "metabolism for 

 growth" (20, 2i). 



However useful and important this sort of information may have 

 been, recent trends have brought about a considerable change in the 



2 Some interesting cases have been found in which this is not true. These, however, 

 present problems which are outside the scope of this discussion. 



