4o8 



MICROBIOLOGY OF SOIL 



Hellriegel and Wilfarth's studies were soon confirmed by the inves- 

 tigations of others. Wigand showed in 1887 that the tubercles con- 

 tained within them true bacteria. In the following year Beyerinck 

 reported the successful isolation of these bacteria on artificial media, 

 and named the organism B. radicicola (Fig. 133). Prazmowski also 

 isolated pure cultures of Ps. radicicola, and followed the entrance of 

 the organisms into the root hairs of young plants, their passage through 

 the cell- walls, and their transformation into bacteroids. These facts 

 were all confirmed by other investigators, and it was further shown by 

 Schloesing and Laurent that properly inoculated legumes not only can 

 grow in soils devoid of combined nitrogen, but that when growing in 

 such soils in a confined atmosphere they decrease the quantity of 

 nitrogen gas surrounding them by transforming it into nitrogen com- 

 pounds. It was, therefore, made clear by these investigations, and by 



FIG. 134. Sections through root tubercles, i, Cell from tubercle of Pisum 

 sativum, snowing bacterial filament; 2 and 3, cells with bacterial filaments from 

 tubercle of Trifolium pannonicum. (After Stefan from Lipman.) 



others not mentioned here for lack of space, that the belief of practical 

 farmers in the soil enriching qualities of legumes was amply justified. 

 It was shown, further, that the later experiments of Boussingault, as 

 well as those of Lawes, Gilbert and Pugh failed to solve the problem 

 because these investigators treated their soil so as to prevent the 

 survival and subsequent entrance of Ps. radicicola, and deprived the 

 leguminous plants of the ability to utilize atmospheric nitrogen. 



MODES or ENTRANCE AND DEVELOPMENT. Tubercle bacteria con- 

 sisting of small motile rods usually enter the legumes by way of the root- 

 hairs. For this reason young tubercles, with but few exceptions, are 

 found on young roots. The organisms multiply at the point of infection 

 and penetrate into adjacent plant-tissue by means of a hypha-like 



