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 
