348 MICROBIOLOGY OF SOIL 



nitrogen. It is conceivable that strains of Ps. radicicola may be de- 

 veloped that would grow rapidly and yet possess but a feeble nitrogen- 

 fixing power. In other words, they would possess a high vegetative 

 power and a low physiological eflaciency. 



Mechanism of Fixation. — It is generally believed that the fixation 

 of nitrogen is accomplished by the bacteria within the tubercles. <. The 

 claim, at one time, advanced by Stoklasa, that the fixation is accom- 

 plished by the plants themselves with the aid of enzymes produced by 

 the bacteria in their roots, has been disproved. It is known that the 

 period of active nitrogen assimilation by the plants coincides with the 

 appearance of the bacteroids in the tubercles, and it is supposed that 

 the microorganisms fashion nitrogen compounds out of atmospheric 

 nitrogen by using the carbohydrates and organic acids in the plant 

 juices as a source of energy. The plants then seem to utilize the soluble 

 nitrogen compounds that pass out of the bacterial cells. It is further 

 supposed that bacteroid formation is an attempt on the part of the 

 microorganisms to adjust themselves to the drain caused by the 

 activities of the host plant. 



Variations and Specialization. — Apparent differences in bacteria 

 from different legumes were noted by Hellriegel. Some of his experi- 

 ments indicated that bacteria from clovers could not produce tubercles 

 on lupines and serradella. Analogous differences were found by 

 Nobbe and his associates, nevertheless they were finally led to conclude 

 that the root invasion of legumes is caused by a single species. How- 

 ever, continued association with any particular legume accomplished 

 in the end a certain modification, or specialization, as it were, of the 

 microorganisms, and they were then no longer able to invade the roots 

 of other legumes. Later, Hiltner and Stormer have been led to 

 modify this view and have arranged the tubercle bacteria in two 

 groups, possessing, according to them, well-defined inorphological and 

 physiological differences. One of these groups is included under the 

 species "Rhizobium radicicola" and the other under "Rhizobium 

 beyerinckii." The former comprises the organisms from lupines, serra- 

 della arid soy beans while the latter comprises all of the others. 



Relation to Environment. — Nitrogen fixation by leguminous 

 vegetation is readily influenced by soil conditions, particularly the 

 supply of lime and of other basic substances; the supply of organic 

 matter and the aeration of the soil. As to the first of these it is well 



