ARE THERE DIFFERENT SPECIES DF TUB>:*CL 8ACTEXIA? IOI 
entrance of the bacteria. The growing pea, when it enters the 
nitrogen-starved stage (see page 97) becomes weakened and the 
tubercle organism readily penetrates its roots. Thus it happens 
that these tubercles form only upon plants that grow in soils some- 
what deficient in nitrogen, and thus under exactly the conditions 
where nitrogen assimilation is needed. Whenever the bacteria do 
succeed in entering the root they stimulate the plant to excessive 
growth which results in the formation of tubercles, and they them- 
selves become transformed into the bacterioids. 
Assimilation of Nitrogen. Just how the nitrogen is assimi- 
lated is also uncertain. It is possible that the legume thus stimu- 
lated can absorb the nitrogen directly from the air. A second 
possibility is that the bacteria assimilate the nitrogen, and that later 
the legume utilizes this extra supply that has been absorbed. It is 
impossible to decide between these views, at present, although, con- 
sidering that some bacteria are known to possess this power of 
nitrogen fixing while green plants do not have such power, the 
probability is in favor of the latter view. However this may be, it 
is certain that the legumes obtain from the growth of the tubercles 
an extra supply of nitrogen which is derived from the air. 
ARE THERE DIFFERENT SPECIES OF TUBERCLE 
BACTERIA? 
It is practically certain that nearly all soils contain bacteria 
capable of living in symbiosis with leguminous plants. Nearly 
all soils, except extremely sandy soils that support little or no 
vegetation, will support leguminous plants and develop tubercles 
on their roots. One can hardly examine the roots of legumes any- 
where without finding tubercles, a fact which shows that the bacteria 
in question are very widely distributed in nature. But are the 
bacteria all of the same species? A very large number of species 
of legumes with their tubercles can grow in most if not all soils. 
Are the bacteria that form tubercles upon the clover the same as 
those that form them upon the pea, or is there a different species 
of bacteria for the different species of legume ? It would not seem 
