GROWTH 163 



which would give a true measure of living matter because we do not 

 know enough about it. Cells may increase in weight by adsorbing 

 more water, or by depositing fat, without increasing the living matter. 

 They may increase in numbers without increase in weight. They 

 may increase in nitrogen content by storing reserve substances of 

 protein nature. 



In measuring growth, it should be kept in mind that all living cells 

 undergo endogenous catabolism. Therefore, the measured growth 

 is the difference between total growth and endogenous catabolism. 

 The latter, however, is usually so small as to be neglected without 

 appreciable error; but this may not be true at super-optimal 

 temperatures. 



Growth of cells requires material for cell construction; 

 it also requires a definite amount of energy to weld 

 together the building material which is of a simpler 

 nature than the living matter; finally, it requires a very 

 intricate building plan to have all the construction done 

 in the correct way and at the correct place. It is possible 

 to study the materials needed for cell construction though 

 it is rather difficult in some cases, on account of the 

 extremely small quantities needed; the advance of this 

 line of research depends primarily upon the improvement 

 of the methods of analytical chemistry. It is also 

 possible to study the sources and amounts of energy 

 available to the cells; the entire preceding section on 

 fermentation was devoted to this study. It does not 

 seem possible that we shall ever understand the actual 

 process of growing, the mechanism of the production of 

 new protoplasmic molecules, the principles which keep 

 order in the microcosmos of the cell, arranging all 

 molecules in their proper places and proper sequences 

 after they have been created. 



(6) BUILDING MATERIALS OF CELLS 



A study of growth should begin with the materials of 

 cell construction. Unfortunately, the requirements are 



