Physiolog)' of Nodule Bacteria 219 



sensitive types. The alt alia organism is most sensitive to acidity, and 

 the kipine organism most resistant. 



The Hmiting temporatm-cs for the growth of nodule bacteria are 

 0^ and 50^C. The thermal death point is at 60-62^ and the opti- 

 mum varies between 18° and 28° C. The bacteria are not injured by 

 diffused sunlight and can readily withstand direct sunlight. Drying 

 is injiuious but not fully destructive. As a result of direct and rapid 

 drying of soil, the numbers of RJiizobiiim diminish rapidly, as de- 

 termined by the plate method. The organism can persist in the soil 

 for several \ears, even in absence of the host plant; however, it is 

 seldom found in soils where specific plants have not grown. Rhi- 

 zohium is not found in manures. The bacteria move through the 

 soil very slowly, and are largely distributed by seed, soil, and ground 

 waters. 



The rate of photosynthesis of the plant and the available supply 

 of combined nitrogen have an important effect upon nitrogen fixa- 

 tion. When photosynthesis is suppressed by permanent darkness, 

 nitrogen fixation ceases and the bacteria become parasitic upon the 

 host plant. With moderately rapid photosynthesis, nitrogen fixation 

 reaches its maximum and may even exceed the rate of protein syn- 

 thesis, so that excretion of combined nitrogen takes place. At an 

 excessive rate of photosynthesis, nitrogen fixation is again depressed. 

 Optimum fixation depends on a balance between the supplies of 

 carboh)xlrate and nitrogen; under these conditions the fixation process 

 is stimulated by nitrate, which otherwise is inhibitive. In the fixa- 

 tion of nitrogen, nitrate has two functions: (a) it counteracts the 

 deformation of the root hairs, which is necessary for entrance of the 

 bacteria, thus reducing the number of nodules; (b) it affects the 

 activities of the nodules already formed by reducing the volume of 

 bacterial tissue and by influencing the carbohydrate-nitrogen bal- 

 ance in the host plant. 



The fixation of nitrogen depends on certain relations between the 

 bacteria and the host plant. The bacteria may produce on one host 

 plant nodules of abnormal nature and yield little or no fixed nitro- 

 gen; in a host of a different species, normal nodule formation and 

 nitrogen fixation may occur. Clovers may show this phenomenon 

 of "host-plant specificity," a concept which has recently come into 

 prominence and which takes the place of the separation of bacterial 

 strains into the "effective" and the "ineffective." Chen and Thornton 

 (1940) showed that nodules produced by "ineffective" bacteria con- 



