76 Martin J. Prucha 



The fact that a pure culture is obtained from a nodule and resembles in 

 its cultural characteristics the true nodule-forming organism is not a suffi- 

 cient proof that such an organism is the nodule-forming organism. De' 

 Rossi (1907), in his investigation, emphasizes this point, and unques- 

 tionably the information on the general subject of nitrogen fixation by 

 leguminous plants has been colored by results from experiments in which 

 some organism other than the nodule-forming orgarism was employed. 



As stated on page 59, the number, size, and location of the nodules 

 on the roots are probably influenced by a number of factors. Since the 

 number of nodules produced on the plants in a given time was used 

 as the measure of the mfecting power of the culture with which the 

 plants were inoculated, it seemed advisable to study the mfiuence of 

 several factors and thus to determine whether Canada field peas readily 

 form nodules under the conditions that it was planned to use in Part III. 

 The results of these experiments in Part II tend to point to the conclusion 

 that, in general, the conditions favormg the normal development of plants 

 favor also the development of nodules. An exception to this is found in 

 the fact that the presence of nitrates tends to inhibit the development 

 of nodules, and at the same time favor the normal development of the 

 plants. No satisfactory explanation for this phenomenon has as yet been 

 given. The plants appear not to be injured by the presence of nitrates, 

 and neither do the nodule-forming organisms seem to be injured when 

 propagated on a medium m which nitrate is present (see experiment 13). 

 The explanation that the plants are made more vigorous when supplied 

 with nitrogen, and can more readily resist the invasion of the microorgan- 

 isms, cannot be taken seriously. It might be noted here that the root 

 system of Canada field peas grown in Pfeffer's nutrient solution appears 

 normal, tending to become slightly brownish; but when the nitrate in the 

 same solution is replaced by the chloride of the same metal, the root 

 system becomes larger, the roots being more numerous and longer. Con- 

 sequently, the rate of gro\\i;h of the root tissue is accelerated by the ab- 

 sence of nitrates. Whether this somewhat rapid growth of the root 

 tissue has any relation to nodule formation is not known. It is highly 

 probable that a biological factor also influences the development of nodules. 

 The microbial flora of the soil or of the solution in which the plants are 

 grown is undoubtedly influenced by the composition of that soil or solution. 

 The microorganisms that thrive best in a highly nitrogenous soil or solu- 



