ARE THERE DIFFERENT SPECIES OF TUBERCLE BACTERIA? 103 



other varieties of closely related legumes are unable to produce 

 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 pro- 

 duces tubercles at once. But just what this means is not so evident. 

 It certainly means that different legumes demand different varieties 

 of tubercle bacteria. 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 it is of 

 importance to know 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 they assume 

 slightly different physiological relations. They can accommodate 

 themselves to growth in one or another legume, and having become 

 especially adapted to one species, they do not so readily develop in 

 the root of a second species, but, allowed to develop in the soil in 

 which the latter plants are growing, they adapt themselves in time 

 to the new plant. In other words, experiments indicate that there 

 is probably only one species of tubercle bacteria, and that this 

 species assumes different physiological characters under the influence 

 of the different conditions in which it grows. It may adapt itself 

 especially for growth in one leguminous plant and consequently 

 lose its ability to develop well in others; but if a new legume is 

 planted in the same soil, a slow change of physiological characters 

 takes place, and the soil organism becomes in time adapted to the 

 new leguminous plant. This conclusion is clearly in complete 

 harmony with the fact that the soil may at any time contain the 

 organisms which will support one species of legume luxuriantly, 

 while another species will have only a scanty growth. The matter 

 of practical importance is that a soil may support one species of 

 legume luxuriantly, with abundant tubercle production, while 



