FIXATION OF ATMOSPHERIC NITROGEN 



561 



with which the different organisms use the available energy, as will be 

 shown later. 



Sources of energy. The process of nitrogen fixation is an endothermic 

 reaction, which necessitates a large supply of energy for the organisms 

 concerned. This energy is derived primarily from carbohydrates and 

 allied compounds as well as from salts of organic acids. Glucose, 



TABLE 55 

 Nitrogen fixed per gram of energy material 



mannite and other simple carbohydrates and alcohols form the most 

 available sources of energy for nitrogen-fixing bacteria. It has been 

 found that polysaccharides, like celluloses, can also serve as valuable 

 sources of energy if they are first partially broken down by cellulose de- 

 composing organisms. 7 However, these results need still further con- 



7 McBeth, I. G. Cellulose as a source of energy for nitrogen-fixation. U. S. 

 Dept. Agr. Bur. PI. Ind. Circ. 131: 25-34. 1913; Pringsheim, 1906-1912 (p. 110); 



