196 FIXATION OR ASSIMILATION OF NITROGEN 



food from the leguminous plant as well as the carbo- 

 hydrates essential for the supply of energy needed in 

 the nitrogen fixing process. Starch is generally present in 

 the tissues of the nodules, especially in autumn and during 

 the resting period of the plant. It is possible that the 

 organisms in some cases live altogether as parasites with- 

 out offering anything in return for the nutrient substances 

 which they receive from their host, under which circum- 

 stances the plant may exhibit little or no increase in 

 nitrogen content. Usually, however, the nodules grow 

 in size, vast numbers of bacteroids are produced, and 

 large amounts of nitrogen are obtained by them from 

 the air, and used for the formation of nitrogenous com- 

 pounds, of which their bodies are mainly composed. 

 After a time the bacteroids become disorganized and 

 dissolved, the compounds in solution being absorbed 

 and utilized by the leguminous plant for its own 

 nutrition, and especially for the formation of its 

 seeds, the plant thus becoming parasitic upon the 

 bacteria. 



The facts of the assimilation of the free nitrogen of 

 the air are clear, but the manner in which the process is 

 carried out by the combination of plant and bacteria is 

 still obscure. There is apparently no increase of nitrogen 

 in the soil itself on which leguminous plants are growing, 

 the excess obtained on analysis of the soil after the growth 

 of a crop of beans or clover being due to the root residues 

 left in the ground after the plants are cut and harvested. 

 Nobbe and Hiltner and others have shown that the 

 gaseous nitrogen is absorbed by the nodules themselves, 

 and not by the other parts of the plant : plants grown in 

 water culture with the root nodules immersed gain no 

 nitrogen, and not until the latter are brought above the 



