32 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



"nitrification." This genus, together with the preceding 

 genus, are of considerable importance in determination of 

 soil fertility. 



Azotobacter.^ — Organisms belonging to this genus are 

 relatively large rods, sometimes spherical, and occasionally 

 almost yeastlike in appearance. They are typically soil 

 forms securing their growth energy by the oxidation of 

 carbonaceous material, and of unusual importance because 

 of their ability to fix atmospheric nitrogen for their own 



\» 



ITiG. 10. — AzoTOBACTEE. FiG. 11. — EhizObium. A. Species mo- 

 tile by peritrichie flagella. 1. 

 Young motile cells. 2. Involu- 

 tion forms. B. Species motile . 

 by means of polar fla'^ella. 1. 

 Young cells. 2. Involution 

 forms. 



use, thus increasing the nitrogen content of soils. They 

 do not live upon the roots of leguminous plants, but free 

 in the soil. In the laboratory their ability to use atmos- 

 pheric nitrogen may be shown by growing them in solutions 

 containing carbohydrates but deficient in nitrogen com- 

 pounds. 



Rhizobium. — These organisms are minute rods, motile 

 when young, by means of flagella which may be either polar 

 or peritriehous. The cells may occur free in the soil, but 

 usually are present in the characteristic nodules on the 



1 Other names sometimes used for Azotoiacter are Parachromatium, 

 and Azotomonas. 



