276 



MICROBIOLOGY OF SOIL. 



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 compounds. It was, therefore, made 

 clear by these investigations, and by 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 



FIG. 69.- Sections through root tubercles, i, Cell from tubercle of Pisum sativum, 

 showing bacterial filament; 2 and 3, cells with bacterial filaments from tubercle of 

 Trifolium pannonicum. (After Stefan from Lipman.) 



their soil so as to prevent the survival and subsequent entrance of B. 

 radicicola, and deprived the leguminous plants of the ability to utilize 

 atmospheric nitrogen. 



MODES OF 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 hollow 

 thread or tube that seems to consist of a gelatinous material (Fig. 69). 

 The tubes branch out as they pass from cell to cell and carry the invading 

 organisms with them. The bacteria which may be readily detected 

 within the tubes and cells are the involution forms of Ps. radicicola and 

 assume various irregular shapes. They are designated as bacteroids. 



