62 Martin J. Prucha 



would be either very efficient or much reduced in infecting power, depend- 

 ing on which flowerpot was taken. 



It seems apparent, from the preceding discussion, that it is extremely 

 difficult to measure any variations in the infecting power of different 

 cultures. When the great danger of infection of plants grown in sterile 

 soil but not under sterile conditions is considered, and also the number 

 of other factors that may affect nodule development on plants, not many 

 clear-cut conclusions can be drawn from the second test. If the infecting 

 power is measured by the number of nodules, by their size, by the uni- 

 formity of their distribution, and by their location, the following con- 

 clusions seem reasonable: 



1. The cultures producing more than three nodules per plant are all 

 efficient. 



2. If one culture produced 3.7 nodules per plant and another culture 

 produced 15.8 nodules per plant, the belief that the latter culture possesses 

 greater infecting power than the former is not justified. 



3. Some cultures produced no nodules, or only a few nodules confined 

 to only one or two plants. Such cultures unquestionably lost their effi- 

 ciency, but this loss of efficiency was parallel with the condition of the 

 cultures. When no living organisms were found in a culture by using the 

 plate method, such a culture produced no inoculation; and when living 

 organisms were found, inoculation was produced. 



4. Propagating and keeping B. radicicola of Canada field pea for ten 

 weeks on media rich in nitrogenous matter, such as wheat bran, wheat 

 middlings, fresh cow feces, and potassium and calcium nitrates, did not 

 destroy the infecting power of the organism. If any injury to infecting 

 power was caused by these substances, it could not be detected by the 

 methods used in this experiment. 



Third test of infecting power (table 15) 



Cultures twenty weeks old were employed in this experiment. In 

 inoculating the plants a known number of organisms was introduced into 

 each flowerpot. By correlating the relation between the number of or- 

 ganisms used for the inoculation of each flowerpot and the number of 

 nodules on the plants, it was hoped to find a more nearly exact measure 

 of the infecting power of the organisms propagated in the different media. 

 The results are summarized in table 15: 



