108 RECLAIMING LOST NEFROGEN 
of the bacterioids in the filament. The appearance of the pouch- 
like threads and the bacterioids has been a puzzle that has not 
yet been wholly explained. 
It is evident that the tubercle bacteria must exist in the soil. 
But it is only recently that cultures capable of producing the 
nodules have actually isolated from soil. Itis now claimed that 
if the right culture media are used to study soil, B. radicicola 
can be shown to be quite numerous. 
The Production of Tubercles by the Bacteria.—Just how the 
bacteria produce the tubercle is not known. Twbercles, galls, or 
tumors are not infrequently produced in plants by bacteria and 
molds, these constituting one of the well-known types of plant 
diseases. Apparently these legume tubercles are produced in 
somewhat the same way, only instead of injuring the plant they 
benefit it. It appears too that the plant offers some resistance to 
the entrance of the bacteria into its root, and, when well nourished, 
is able to prevent their entrance. When there is plenty of nitrogen 
food in the soil, the plant grows vigorously, so that this resistance 
may be sufficient to prevent the formation of tubercles. When, 
however, the nitrogen food is scanty,’ the plant is weaker and 
cannot resist the 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 most readily upon 
plants that grow in soils somewhat deficient in nitrogen, and thus 
under exactly the conditions where nitrogen assimilation is 
needed. 
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, 
considering that some bacteria are known to possess this power of 
