592 PRINCIPLES OF SOIL MICROBIOLOGY 



lism alone. It was, therefore, suggested 131 that the organism actually 

 utilizes the energy derived from the exothermic nitrogen fixation for 

 metabolic processes. 



Of the three possible processes by which the nitrogen can be fixed, 

 namely, (a) reduction, (6) oxidation and (c) direct union with organic 

 compounds, the first is the most plausible, especially in view of the 

 fact that a great many microorganisms assimilate the nitrogen in the 

 form of ammonia. Whiting 132 could not demonstrate any ammonia, 

 nitrites or nitrates within the plants, his work tends to confirm the direct 

 organic combination theory. Some evidence was previously obtained 133 

 on the direct union of the free nitrogen with some organic compound in- 

 side the bacterial cell; this compound was believed 134 to be glycogen and 

 carbamic acid as the first product of combination. 



Once the nitrogen has been fixed in the bacterial cells, it may be trans- 

 ferred to the host when the bacterial cells (so-called bacteroids) are 

 decomposed and the contents absorbed by the plants, 135 or when the ni- 

 trogen has been secreted by the bacterial cells in a form which the plant 

 then utilizes. 136 This removal of the products of bacterial growth by the 

 plant was believed to stimulate further nitrogen fixation. The compo- 

 sition of the nitrogenous substance of the bacterial secretions is still 

 unknown, except that it is believed to be protein-like in nature. 



The formation of a bacteriophage in the nodules of the leguminous 

 plants has been established. 137 This bacteriophage dissolves the bac- 

 teria and thus makes their contents available to the plant. It was found 

 not only in the nodules but also in the roots and stems of the plants, 

 and not in the leaves, also in garden and field soils, but not in prairie 

 soils. The bacteriophage is specific in its lytic action and attacks only 

 those bacteria which form the nodules in the roots of the specific plants. 

 The bacteriophage can resist, according to species, a temperature of 60° 



132 whiting, A. L., and Schoonover, W. R. Nitrogen fixation by cowpeas and 

 nodule bacteria. Soil Sci., 10: 411-420. 1920. 



133 Gerlach and Vogel, 1903 (p. 379). 



134 Heinze, B. tlber die Stickstoffassimilation durch niedere Organismen. 

 Landw. Jahrb., 35: 889-910. 1906. 



135 Nobbe and Hiltner, 1900 (p. 127). 



136 Golding, J. The importance of the removal of the products of growth 

 in the assimilation of nitrogen by the organisms of the root nodules of leguminous 

 plants. Jour. Agr. Sci., 1 : 59-G4. 1905. 



137 Gerretsen, F. C., Gryns, A., Sack, J., and Sohngen, N. L. Das Vorkom- 

 men eines Bakteriophagen in den Wurzelknollchen der Leguminosen. Centrbl. 

 Bakt. II, 60: 311-316. 1923. 



