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 



