INTERMEDIATE ORGANISMS 193 



galls ( ), commonly known as tubercles (Fig. 90), 



on the roots of clover and other leguminous ( ) 



plants. These serve a very important purpose in that they derive nitro- 

 gen directly from the air and supply it to the clover. This makes it 

 possible for clover to grow in soil very poor in nitrogen, while the over- 

 production of nitrogen leaves the soil richer than it was before. 



The galls themselves are filled with rather large x- and y-shaped 

 bacteria, easily seen under the microscope. These bacteria die, and 

 the nitrogen which they contain is added to the surrounding soil, some- 

 times directly, and sometimes through the intermediate plant to which 

 it is attached. 



