BACTERIA FIXING ATMOSPHERIC NITROGEN 133 



Lohnis and Hansen and Shunk observed the two distinct types of 

 flagellation referred to above. In the single flagellate types (monotri- 

 chous) , the flagellum is not strictly polar but is usually attached to the 

 corner. However, organisms obtained from nodules of different species 

 of plants belonging to one genus have the same type of flagellation. 



Physiology of nodule bacteria. The different strains of Bad . radicicola 

 are strictly aerobic. 



Maltose, sucrose, glucose and mannite offer the best sources of 

 carbon; lactose, dextrin and glycerol can also be utilized. According 

 to Beijerinck, separate carbon and nitrogen sources (asparagine, 

 ammonium sulfate, sodium or potassium nitrate) are required. 

 Laurent 147 first showed that the organism can be cultivated on nitrogen- 

 free media, containing 0.1 per cent KH 2 P0 4 , 0.01 per cent MgS0 4 and 

 5 to 10 per cent of an available energy source. When grown on such 

 a medium, it will fix atmospheric nitrogen. 148 



The presence of nitrates in the medium and in the soil diminishes 

 nitrogen-fixation by the organism. This has been demonstrated by 

 Nobbe and Richter 149 and others, and it was found to be due not to any 

 injurious influence of the nitrate but to the fact that the plant, capable of 

 obtaining its nitrogen from the soil, represses the development of the 

 nodules. 



A condition is found here very similar to the influence of nitrates 

 upon nitrogen fixation by non-symbiotic bacteria. Prucha 150 found 

 that the addition of KN0 3 , Ca(N0 3 ) 2 , NH 4 C1, or peptone to sandy 

 soil, at the rate of 0.25 gram of the salts to 300 grams air-dry soil, had an 

 inhibiting effect on nodule development of Canada field pea, while 

 MgS0 4 , KH 2 P0 4 , Ca (H 2 P0 4 ) 2 and tannic acid, especially in low 

 concentrations, had a beneficial effect. 



The optimum reaction for the growth of the bacteria is pH 5.5 to 7.0, 

 depending on the nature of the plant, with limiting reactions of pH 

 3.2 to 5.0 on the acid side, and pH 9.0 to 10.0 on the alkaline. The 

 optimum temperature is 25° to 28°C. with 0° and 50° as the limits. 



147 Laurent, E. Sur le microbe des nodosit6s des Lcgumineuses. Compt. 

 Rend. Acad. Sci. Ill: 754. 1890; Ann. Inst. Past. 4: 722. 1890; 5: 105-139. 

 1891. 



148 Fred, 1910 (p. 120). 



149 Nobbe, F., and Richter, L. Uber den Einfluss des Nitratstickstoffs und der 

 Humussubstanzen auf den Impfungserfolg bei Leguminosen. Landw. Vers. 56: 

 441-448. 1902; 59: 167-174. 1904. 



150 p ruc ha, M. J. Physiological studies of Bacillus radicicola of Canada field 

 pea. Cornell Univ. Agr. Exp. Sta. Mem. 5, 1915. 



