I5O AGRICULTURAL BACTERIOLOGY. 



Such facts as these have led to the suggestion that the 

 different species of leguminous plants demand different spe- 

 cies of bacteria for the proper production of their root no- 

 dules, and that there are different species of this />'. radicicola, 

 each capable of living with a different species of legume, and 

 incapable of producing tubercles upon others. It has not 

 been very easy to determine whether this conclusion is correct, 

 although experiments designed to answer the question have 

 been multiplied. In general it is believed to-day that the con- 

 clusion that the tubercle bacteria consist of a large number of 

 species, each characteristic of a distinct species of legume, is 

 not borne out by the facts. 



It is, nevertheless, true that certain varieties of legume will 

 grow in soils with an abundant production of tubercles, while 

 other varieties of closely related legumes are unable to pro- 

 duce an abundant crop of tubercles in the same soil. This 

 is evidently due to the lack of microorganisms especially 

 appropriate to the legume in question, since inoculation with 

 proper soil infusion produces tubercles at once. But just 

 what this means is not so evident. It certainly means that 

 different legumes demand different varieties of tubercle bac- 

 teria. Whether these different varieties are distinct species is, 

 of course, a fruitless question, inasmuch as we do not know 

 what we mean by a species among bacteria. But the question 

 is of importance whether these types are quite distinct, or, 

 whether they are simply physiological varieties of the same 

 general species. If the former is true we should expect them 

 to remain distinct, but if the latter is true we might expect the 

 soil bacteria to be capable of adaptation, by cultivation, to 

 different legumes. On the whole the evidence is decidedly in 

 favor of the latter view, and indicates that the different tubercle 

 bacteria are probably all one general species, but that, under 

 different conditions, this species assumes slightly different 

 physiological relations. It can accommodate itself to growth 



