GROWTH 173 



The analogy is quite close between carbon and nitrogen. While the 

 completely oxidized and the completely reduced compounds are used 

 by some microorganisms, and the partly oxidized, partly reduced com- 

 pounds by many others, the elementary stage is least suited for 

 assimilation, though less energy would be required to build cells from 

 N than from HNO3. It seems very difficult for cells to break up the 

 affinity between the atoms of the same element. With oxygen, 

 however, the opposite seems to be true; at least this is the case in 

 fermentation, and we may well assume that it holds true for growth. 



Elementary oxygen is readily used by most living 

 beings. Oxygen fixation is the most common biological 

 reaction. There are exceptions, however, such as the 

 lactic organisms and some strictly anaerobic bacteria 

 which depend exclusively upon oxygen from organic 

 compounds, and cannot use elementary oxygen if it is 

 offered them. We may assume in these cases an analogy 

 to the nitrogen and carbon sources, i.e., the inability to 

 break up the oxygen molecule into oxygen atoms. 



There is no use in speaking about hydrogen sources, 

 because hydrogen is so common in water as well as in 

 practically all organic compounds that its source could 

 not be traced. Attention should be called only to the 

 hydrogen-fixing bacteria (Kaserer, 1906). 



(/) WASTE PRODUCTS OF CONSTRUCTION 



When prototrophic bacteria build their cells from 

 carbon dioxide as the only carbon source and from nitrates 

 as the only nitrogen source, the growth process is entirely 

 a process of reduction and condensation. The only part 

 of the building material that is not used is a good share 

 of the oxygen in these compounds. This oxygen is 

 most probably used for the oxidation of the energy 

 furnishing substrate and cannot be traced. 



Other organisms build their cells from very complex 

 material, such as amino acids, peptones, etc., and these 



