32 Martin J. Prucha 



GENERAL DISCUSSION OF RESULTS OF EXPERIMENTS 



The nine experiments reported in Part II, on the factors affecting the 

 development of nodules on Canada field peas, are not extensive enough 

 to allow any broad and general deductions, but the results point to several 

 conclusions. Nodules develop readily on Canada field peas in nutrient 

 solutions, provided the proper nutrient salts are added. Varying the 

 concentration appears to have a marked influence on nodule development. 

 Aeration has no appreciable effect. Nodules developed on the long roots at 

 as great a depth -as 30 centimeters below the surface of the nutrient solu- 

 tion. If air is essential for the development of nodules, enough of it was 

 dissolved in the nutrient solution under the conditions in the experiments. 

 The presence of nitrates in the nutrient solution or in the soil tends to 

 inhibit the development of nodules; the reason for this is not known. 

 If the plants are grown in the presence of nitrates for about a week and 

 then inoculated, a few nodules will develop, but no further development 

 of nodules takes place; in water cultures without nitrates and inoculated, 

 a continuous nodule development takes place as long as the roots 

 grow. 



The chemical composition of the various soils used for agricultural 

 purposes differs. What influence this has on the various groups of the 

 nodule-forming organism and on nodule development has never been 

 extensively investigated. The results of the foregoing experiments em- 

 phasize its importance. The limited distribution of the different groups 

 of the nodule-forming organism in some soils, the failures in inoculations, 

 and the difficulty in growing certain legumes, may be explained in certain 

 cases as being due to the composition of the soil. It is known that the 

 addition of lime to certain soils has a beneficial effect on nodule develop- 

 ment and on the growth of some legumes. It is highly probable that 

 the addition of other substances to the soil may be beneficial to other 

 legumes. 



PART III. INFLUENCE OF VARIOUS MEDIA ON THE INFECTING POWER 

 AND THE VITALITY OF BACILLUS RADICICOLA 



As indicated previously, and again further developed in the discussion 

 of this subject, the view has been maintained that the infecting power, 

 or " virulence," of the nodule-forming organism may be impaired by 

 cultivating it on certain media. In order to determine whether or not 



