FUNGI 137 
root-tubercle bacteria have been cultivated artificially at 
the United States Department of Agriculture in such a way 
Fria. 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- 
