234 THE FUNGI 



256. The fixation of free nitrogen. One of the most impor- 

 tant relations of bacteria to agriculture and to plant life gener- 

 ally lies in the ability of some species to put the free nitrogen 

 of the air into chemical compounds that are available for absorp- 

 tion by green plants growing in barren soil. When crops are 

 taken off the land through a series of years the supply of nitrates 



FIG. 208. Tubercles on the roots of red clover 



I, section of ascending branches; 6, enlarged base of stem; t, root tubercles 

 containing bacteria 



is largely used up and the soil becomes impoverished or ex- 

 hausted. The nitrogen may be brought back to such soil by 

 fertilizers, but this is expensive. The restoration of nitrogen to 

 barren land has been one of the most serious problems of agri- 

 culture. There is one of the bacteria (Pseudomonas radicicola), 

 which lives on the roots of members of the legume, or pea fam- 

 ily, including such forms as the clover, alfalfa, and soy bean, and 

 develops swollen regions called root tubercles (Fig. 208). This 

 remarkable organism is able to take the free nitrogen from 



