252 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



roots. The family Leguminosce is a relatively large one. 

 Some forms are trees, some are shrubs, and many are 

 herbaceous. They are found in abundance from the tropics 

 to the arctic region, and from the low plain to the limits of 

 vegetation upon the mountain. Some species develop best 

 in very moist soil, others under relatively arid conditions. 

 Some species prefer soils which are distinctly acid. Most 

 species prefer a neutral to a somewhat alkaline soil. It is 

 evident, therefore, that the Rhizobium leguminosarum is 

 practically world-wide in its distribution. Apparently the 

 only conditions under which it does not grow are those 

 under which legumes will not grow. 



Rhizobium leguminosarum may be readily cultivated in 

 the laboratory from the nodules of legumes. It grows best 

 on neutral media containing suitable sugars and phosphate. 

 Peptone is not necessary. The colonies which develop upon 

 sugar agar are usually somewhat slimy, sometimes almost 

 clear and colorless. In sugar broth the medium becomes 

 clouded and with some strains considerable amounts of gum 

 are produced. This may be precipitated by the addition of 

 alcohol. The bacteria grow readily upon a nitrogen-free 

 medium and are able to fix appreciable quantities of atmos- 

 pheric nitrogen. In determining the relative nitrogen- 

 fixing power of various types of these bacteria, it is cus- 

 tomary to compare the number of milligrams of atmospheric 

 nitrogen fixed per gram of sugar (usually glucose) used. 

 Usually Rhizobium leguminosarum is able to fix from one 

 to five milligrams of nitrogen per gram of glucose utilized. 

 The organism is aerobic, completely oxidizing sugar with 

 the development of carbon dioxide and water. There is, 

 therefore, an abundant evolution of gas, but as the oxidation 

 can take place only near the surface of the medium and in 

 the presence of an abundance of oxygen, gas bubbles are not 

 observed and no gas is formed in the closed arm of the 

 fermentation tube. 



