278 MICROBIOLOGY OF SOIL. 



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

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

 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. However, 

 continued association with any particular legume accomplished in the 

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

 organisms, and they were then no longer able to invade the roots of other 

 legume genera. Latterly, Hiltner and Stormer have been led to modify 

 this view and have arranged the tubercle bacteria in two groups, possess- 

 ing, according to them, well defined morphological 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, serradella and 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 known that all legumes, 

 with the exception of lupines and serradella, are stimulated in their 

 growth by generous applications of lime. The top dressing of lawns 

 with lime, marl or wood ashes encourages the appearance of white clover; 

 an adequate supply of lime makes possible the successful growing of 

 alfalfa in almost any soil, while the leguminous vegetation of limestone 

 soils is proverbially vigorous. The favorable influence of lime is due to the 



