100 RECLAIMING LOST NITROGEN. 
results obtained in trying to use the cultures of B. radicicola, has led 
to suspicions that the actual bacterium that produces the tubercle 
may not be the B. radicicola which has been isolated, but some 
other, which has escaped observation and which is frequently at- 
tached to B. radicicola. Indeed, DeRossi recently claims to have 
found the B. radicicola associated with what he thinks is a second 
bacterium that has quite different properties. The latter is much 
more like the bacteria forms that appear in the young tubercle, and 
shows a tendency to form bacterioids in culture media. According 
to DeRossi, it always produces the tubercles when inoculated into the 
root tissue of legumes. These bacteria do not grow well in culture 
media, not becoming visible for about two weeks, and have been 
overlooked in previous experiments since they are hidden by the 
vigorously growing B. radicicola with which they are closely as- 
sociated. DeRossi thinks this a new organism and the cause of the 
tubercle rather than the species ordinarily accepted as the cause. 
It is doubtful whether this is anything different from B. radicicola. 
But whichever result is reached, it remains equally true that the 
tubercles are the result of the action of bacteria that enter the root 
tissues, and stimulate the root cells to excessive growth, although, 
perhaps, B. radicicola is not the real exciting cause. This conclu- 
sion of DeRossi, if true, would in a measure explain the irregularity of 
results obtained by the use of what were previously supposed to be 
pure cultures of the tubercle organism (see page 107). 
The Production of Tubercles by the Bacteria. Just how the 
bacteria produce the tubercle is not known. Tubercles, 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 pre- 
vent 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 
