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 



