FUNGI 



137 



root-tubercle bacteria have been cultivated artificially at 

 the United States Department of Agriculture in such a way 



r 



FIG. 123. Root-tubercles on sweet clover. 



that they can be shipped anywhere at small cost and used 

 to inoculate soils that are deficient in tubercle-forming bac- 

 teria. This deficiency may be discovered either by the ab- 



