78 Martin J. Prucha 



producing efficiency of the cultures is due to the dying-out of the organ- 

 isms. Whether the uijurious agent is some ingredient of the media, or 

 lack of a proper nutrient, or the accumulation of the metabolic products, 

 has not been determmed. The observation that a cop ous growth is 

 usually of reduced nodule-producing efficiency suggests the last as the 

 probable, or at least a partial, explanation. 



When one exammes leguminous plants growm under favorable conditions, 

 one finds that each plant has a lunited number of nodules and that the 

 number and the size of the nodules vary on the different plants. Hiltner 

 (1900), holding the relation between B. radidcola and the leguminous plants 

 to be of a pathological nature,' concluded from his experiments that the 

 variableness of the infecting power of the nodule-forming bacteria is the 

 limiting factor which determines the number and size of the nodules under 

 otherwise favorable conditions. Siichtmg (1904), in trying to account for 

 the limited number of nodules on each plant, emphasized the resistance 

 of the plants against the invasion of the bacteria. His explanation is 

 as follows: " In contrast to Hiltner' s immunity theory, I am of the opinion 

 that the nodule formation and their number are regulated by the relation 

 between the antibodies in the plant and the infecting substance of the 

 bacteria." "^ 



These explanations do not appear to the writer to explam the conditions. 

 No pathological explanation can account for a limited number of nodules. 

 The physical condition and the chemical composition of the soil, the 

 amount of moisture, and the microbial flora of the soil, are some of the 

 important factors that interfere, directly or indirectly, with the coming 

 together of B. radidcola and the roots of the plants. The stage m the 

 development of the plant, the rate of growth of the roots, the number 

 of root hairs, the character of the root tissue, and other factors, may also 

 play a part in limiting the development of nodules. 



ACKNOWLEDGMENTS 



It is with pleasure that the writer here acknowledges his indebtedness 

 to Professors Lewis Knudson and B. M. Duggar for much helpful advice 

 and criticism throughout the progress of this work. 



'Translation from the original German. 



