60 Martin J. Prucha 



of the organisms in the soil. Moreover, if the infecting power of the 

 organism were easily affected by the medium, it would be easily affected 

 by the soil into which the organisms were introduced at the time of in- 

 oculation. The result would be that in course of time the infecting power 

 of the different organisms, although different at the beginning, would 

 become the same for all the organisms. If this is true, the difficulty in 

 measuring the infecting power of a given culture is very evident. 



4. Resistance of the 'plant against the invasion of the organism. — Nothing 

 is known of this in connection with the leguminous plants and the nodule- 

 forming organism. If such a character does exist in the plants, it probably 

 varies in the different individuals and would influence the number of 

 nodules formed. 



5. Infection from other sources than the inoculating material. 



6. Infecting power of the organisms. 



It is evident from the above discussion that the number of nodules on 

 plants may be the result of several factors operating simultaneously, and 

 that it would.be a difficult matter to determine their influence singly. 

 The number of nodules, therefore, is not an accurate measure of the in- 

 fecting power of the different cultures. Unfortunately, however, no other 

 measure is available. In interpreting the above data, therefore, the 

 limitations in the accuracy of the method must be borne in mind. 



Comparing the data in table 13 with those in table 10, a close relation 

 may be observed between the condition of the cultures when ten weeks old 

 and the number of nodules on the plants inoculated with these cultures. 

 The following cultures had no living organisms: 



Medium 334 + 2 per cent Witte's peptone 



Medium 334 + 5 per cent Witte's peptone 



Medium 334 + 0.5 per cent resorcin 



Medium 334 + 1 per cent salicin 



Medium 334 + 0.5 per cent potassium oxalate 



Medium 334 + 0.5 per cent potassium citrate 



Medium 334 + 0.2 per cent phloroglucin (few organisms) 



Soy bean hay 



Soy bean roots 



Canada field pea hay 



Corn meal 



Reference to table 13 shows that the plants inoculated with these cultures 

 produced three nodules or less per plant. Whether the few nodules found 



