LEGUMINOUS PLANTS AND SYMBIOTIC FIXATION I07 
bacterial infusion from these cultures: the usual result has been 
the growth of abundant tubercles and the fixation of nitrogen. 
Some striking experiments have been made with germinating 
peas. Such peas, if kept moist and warm, will grow for several 
days, sending out their normal roots even without being planted 
in the soil. By dipping the tip of a needle into cultures of the 
microédrganisms and then pricking the rootlets of young legumes 
at varlous points, the development of tubercles will almost 
inevitably follow such slight wounds. In favorable experiments 
MC 
1 Le 
Fic. 25.—Showing the bacterioids found in root tubercles. 
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 culture media they resemble other bacteria, occasion- 
ally producing the peculiar bacterioid forms. Usually there is 
nothing in their growth in the laboratory culture media to suggest 
that they may produce 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 follow- 
ing. But, as a rule, the inoculation is followed by the growth of 
the tubercle, the development of the curious tube-like filaments 
growing among the cells, and there is the subsequent appearance 
