FIXATION OF ATMOSPHERIC NITROGEN 591 



injury; the nitrogen-fixation by the bacterium is beneficial to the plant 

 and is, to a certain extent, correlated with the vegetative energy of the 

 bacterium. This stimulates the growth energy of the plants. The 

 equilibrium established is between the growth energy of the plant, 

 which enables it to utilize the nitrogen made available by the bacterium, 

 and the vegetative energy of the bacterium. When this stage is reached, 

 the growth of the plant continues uninterrupted. The vegetative 

 energy of the nodule bacteria is increased by repeated physiological 

 adaptation to the host plant, namely by repeated passage through the 

 host plant. 



Chemistry of nitrogen- fixation by symbiotic bacteria. In the presence of 

 an abundance of available nitrogen in the soil, the leguminous plants 

 utilize that nitrogen and do not depend on the activities of the 

 bacteria. 12S Alkali nitrates in concentrations of 1 : 10,000 and ammo- 

 nium salts in 1:2,000 repress nodule formation. 129 The addition of 5 

 mgm. nitrogen as KN0 3 per liter of medium was sufficient to prevent 

 the penetration of the bacteria into the roots of the plants in water 

 cultures; this action was much less in sand and hardly obtained in soil. 

 In some cases small amounts of nitrogenous substances were found to 

 stimulate plant growth and nodule formation. 130 



The average amount of nitrogen fixed by a good crop of a legume, 

 under favorable conditions, may be taken as 200 pounds per acre. If 

 the energy need of the organisms is 100 parts of carbohydrate for every 

 part of nitrogen fixed, as in the «ase of the non-symbiotic bacteria, the 

 symbiotic bacteria would require 20,000 pounds of carbohydrate per 

 acre for the fixation of the favorable amount of nitrogen. This would 

 have to be supplied by the growing plant which is hardly imaginable 

 since it would amount to two to four times as much as the total crop 

 itself. We must assume that the organism uses the energy supplied by 

 the plant much more efficiently than the non-symbiotic bacteria or 

 that the process of nitrogen-fixation by the legume bacteria is exo- 

 thermic. 131 In the second case, the energy liberated is so small that it 

 would hardly be sufficient to cover the need of the bacteria for metabo- 



128 Wohltmann and Bergene. Die Knollchen-Bakterien in ihrer Abhangigkeit 

 von Boden und Dungung. Jour. Landw., 50: 377-395. 1902. 



129 Marchal, E. Influence des sels mineraux nutritifs sur la production des 

 nodosites chez le pois. Compt. Rend. Acad. Sci., 133: 1032-1033. 1901. 



130 Hiltner, L. Uber die Ursachen, welche die Grosse, Zahl, Stellung und 

 Wirkung der Wurzelknollchen der Leguminosen bedingen. Arb. biol. Anst, K. 

 Ges. Amt., 11: 177-222. 1900. 



131 Christiansen-Weniger, 1923 (p. 570). 



