548 Journal of Agricultural Research vol. xx, No. 7 



The results of our investigations leave no doubt that the earlier find- 

 ings of the junior author were correct so far as the polar flagellation and 

 the peculiar morphological and cultural features of the cowpea-soybean 

 organisms are concerned. On the other hand, it could now be ascer- 

 tained with equal certainty that the bacteria producing nodules on 

 clover, alfalfa, vetch, and other plants originally cultivated in Europe 

 are all peritrichic and exhibit all the characteristics of Bacillus radici- 

 cola, as described by Beijerinck and other authors. 



Those findings which were obtained most frequently and which may 

 be considered as being typical for the two groups of nodule bacteria are 

 compiled in Table I, together with analogous data pertaining to Bacillus 

 radiobacter. Photographs of the most characteristic details are repro- 

 duced on Plates 68 and 69. 



When grown from the root nodule on Harrison and Barlow's ash agar, 

 mannite agar, or similar agar of low nitrogen content, the two groups of 

 nodule bacteria are best characterized and differentiated by the very 

 slow growth of colonies in the cowpea-soybean group and the com- 

 paratively quick growth of those of Bacillus radicicola (6, pi. 11, fig. 1-11). 

 Frequently, but not always, the development of B. radiobacter is still 

 somewhat more rapid than that of B. radicicola; in the macroscopical 

 as well as in the microscopical aspects, however, the colonies of these 

 two species on such media are so very much alike that it is almost im- 

 possible to distinguish them with certainty. Both, when developing on 

 the surface, are perfectly round, drop-like, soft, watery or slimy, glisten- 

 ing, transparent. Often a whitish center or whitish streaks become 

 visible within the more transparent mass, especially if the surface colony 

 is the outgrowth of an imbedded colony. Sometimes it appears as if 

 this whitish center were regularly to be seen only with certain strains of 

 Radicicola and Radiobacter. This is not the case, however. Its pres- 

 ence or absence is erratic and can not be used for differentiation. The 

 imbedded colonies are always small, white, opaque, mostly lentiform, 

 less frequently circular. Under the microscope the surface colonies 

 present themselves as sharp-edged disks, pure white at the outside with 

 yellowish-grayish granulation in the center. In a few cases a radiate 

 structure becomes visible. The colonies of the cowpea-soybean group 

 appear macroscopically, as well as microscopically like young colonies 

 of the Radicicola type. The presence of Radiobacter colonies on the 

 plate stimulates their growth markedly. 



In cell morphology there is again a more pronounced relationship be- 

 tween Radiobacter and Radicicola than between the nodule bacteria of 

 the clover-vetch group on the one side and of the cowpea-soybean group 

 on the other. This holds true with the regular rod forms as well as with 

 the very pleomorphic, curved, swollen, branched, or otherwise changed 

 types of growth characteristic of these groups. The photographs on 

 Plate 68, D-L, represent the pictures we have seen most frequently, but 

 they do not pretend to give a complete illustration of the wide pleomor- 



