NUTRITION 83 



this function. Pseudomonas radicicola, lives, as its name 

 implies, in roots, and chiefly in those of leguminous plants, 

 such as clover, lupine, locust, peas, beans, alfalfa, and 

 their near relatives. The presence of the bacterium 

 causes little swellings or nodules on the roots, commonly 

 known as leguminous tubercles (Figs. 61 and 228). 



Nitrification, or the oxidation of ammonium compounds 

 into nitrates, was at first thought to be simply a chemical 

 process, but early in the nineteenth century it was found 

 to depend upon living organisms. Numerous bacteria, 

 molds and other fungi (Mucor, Penicillium, Botrytis) , and 

 the yeast Torula are active in the formation of ammonium 

 salts by the disintegration of organic compounds. Follow- 

 ing this process nitrification results. Two important 

 nitrifying bacteria live in the soil; one (Nitrosomonas) 

 forms nitrites from ammonia, and the other (Nitrobacter) 

 forms nitrates from nitrites (Fig. 229). 



Root-nodules, caused by nitrogen-fixing organisms, oc- 

 cur also on roots of certain non-leguminous plants, includ- 

 ing Elaeagnaceae (Oleaster family), Myricaceae (Bayberry 

 family), Podocarpineae, the genus Alnus, of the Betulacese 

 (Birch family), and Cycadaceae (Cycas family). The 

 roots of Cycas contain two kinds of nitrogen-fixing organ- 

 isms, Pseudomonas radicicola and Azotobacter (Fig. 241). 



82. Value of Leguminous Crops. Because of the 

 presence in their roots of organisms that can use the 

 free nitrogen of the air to form compounds of nitrogen, 

 leguminous crops are of inestimable value to agriculture. 

 In fact, they are absolutely necessary in order to maintain 

 the fertility of the soil. When any leguminous crops 

 are harvested, the roots are left in the soil with their 

 tubercles rich in compounds of nitrogen, and the com- 



