FIXATION OF NITROGEN BY BACTERIA 193 



paratively few in number and attached by wide bases 

 to the tap root and stronger lateral roots. In the case 

 of many species the nodules are found in large numbers 

 on the smaller roots to which they are attached by 

 narrow necks. Some of them, such as those upon 

 Robima, Trifolium, Onobrychis, and Medicago, are elon- 

 gated and hot infrequently branched : while those of 



FIG. 34. a Root hair of pea showing infection-thread. 



b Infection-thread passing through cell of young nodule. 



Anthyllis, Serradella, and Phaseolus are usually small 

 round structures. 



The nodules are caused by the soil organism Pseudo- 

 monas radicicola (Beijk.), Moore (Bacillus radicicola, 

 Beijk.), which penetrates into the young roots chiefly 

 through the root hairs : in lupins and some other species 

 infection apparently takes place through the older parts 

 of the roots as well. After entry the organism multiplies 

 and forms a filamentous zoogloea, the infection- 

 thread which resembles a fungus hypha so closely that 

 it was at first confused with the latter. The infection- 

 thread consists of small rod-shaped bacteria imbedded in 

 a gelatinous substance from which they are difficult to 

 differentiate. It can be traced along the interior of the 

 root-hairs, and through the cells of the cortex. Where 



