124 AGRICULTURAL BACTERIOLOGY 



formation and consequently growth. If the teachings were 

 heated, no effect was noted. It was thus evident that the 

 causal factor was a biological one. Soon after the discovery 

 of the relation of the nodule to the nitrogen needs of the 

 plant, the nodule-forming bacteria were isolated by the 

 Dutch bacteriologist,, Beyjerinck. 



When leguminous plants are spoken of, the cultivated 

 legumes come to mind, such as the clovers, alfalfa, the peas 

 and beans, the vetches, lupines, and serradella. The native 

 flora of all soils and of all parts of the world is made up 

 in large part of leguminous plants. It has been determined 

 that 20 per cent, of the flora of the Western prairies con- 

 sists of wild or native legumes, while still higher figures 

 have been obtained in the Eastern States. In size they 

 vary from the smallest clovers to full sized trees, such as 

 the honey locust. All, as far as is known, have the same 

 relation to the nodule-forming bacteria, and possess the 

 ability to use the free nitrogen of the air. Leguminous 

 plants are found growing on every type of soil. Some are 

 adapted to acid soils, while others grow best on alkaline 

 soils. Many are adapted to high land, and a few are 

 water plants. 



It is certain that the leguminous plants and the associated 

 bacteria have been the chief factors in the gradual storing 

 of nitrogen in the soil. Under their influence, the elemen- 

 tal nitrogen of the air is brought into combination in the 

 form of proteins, which, as they have undergone decompo- 

 sition in the soil, have left a residue of humus. 



The legumes are differentiated from the grasses and 

 grains by their peculiar relation to a group of bacteria and 

 by their composition. Both the seed and vegetative parts 

 contain much more nitrogen than is the case with the iion- 

 leguminous plants, as is shown in the following table ; 



