SYMBIOTIC BACTERIA AND LEGUMINOUS PLANTS. 99 
normal roots even without being planted in the soil. By dipping the 
tip of a needle into cultures of the microorganisms and then pricking 
the rootlets of young legumes at various points, the development of 
tubercles will almost inevitably follow such slight wounds. In 
favorable experiments the tubercles appear in six days after the 
inoculation and always at the point of inoculation. These facts 
proved that the cultures are concerned in the development of the 
tubercles. 
The study of the organisms themselves and of their relation to the 
legume tubercle has proved somewhat puzzling. The organisms 
isolated are ordinary bacteria, B. radicicola, and in laboratory 
FIG. 26. Showing the bacterioids found in root tubercles. 
culture media they resemble other bacteria, occasionally producing 
the peculiar bacterioid forms. Usually there is nothing in their 
growth in the laboratory culture media to suggest that they may pro- 
duce the peculiar bodies found in the tubercles. When such cultures 
are inoculated into the roots of legumes the results are not always 
successful, sometimes no tubercles following. But, as a rule, the 
inoculation is followed by the growth of the tubercle, the develop- 
ment of the curious tube-like filaments growing among the cells, and 
there is the subsequent appearance of the bacterioids in the fila- 
ment. The appearance of the pouch-like threads and the bacteri- 
oids has been a puzzle that has not yet been wholly explained. 
It is evident that the tubercle bacteria must exist in the soil. 
But in spite of careful search no bacteria have yet been isolated 
from the soil which have the power of producing tubercles when 
inoculated into legumes. This, together with the irregularity of 
