96 PLANT PHYSIOLOGY 



organisms is apparently one of mutualism. The bacteria get their 

 carbon foods, minerals, and water from the legume and after their 

 death give to it in return the manufactured nitrogen compounds. 

 The bacteria certainly profit from the relationship, and there can 

 be little doubt of a similar advantage gained by the legume from 

 the presence of the bacteria. When the plants are cut and the 

 roots left in the soil, the crops which come after are able to use 

 the excess bound nitrogen which has accumulated in the soil thus 

 enriched by the leguminous plants. 



For a long time it was believed that the legumes were the only 

 plants associated with tubercle-forming, nitrogen-fixing bacteria, 

 but recent workers have found many other plants which derive 

 some benefit from similar associations. Miss Spratt (1919) re- 

 ports nodules in the families of the Eleagnacese, Myricaceae, 

 Cycadaceae, Podocarpaceae, and in the genera, Alnus and Ceano- 

 thus. In these plants the bacteria are associated with the roots as 

 in the legumes; but in the leaves of certain tropical Rubiaceae have 

 been found tubercles containing bacteria which are able to fix nitro- 

 gen from the air in the same manner as those which inhabit roots. 

 The natives use the leaves of these plants as green manure. 

 Leaves of Dioscorea macroura also have been found to contain 

 nitrogen-fixing bacteria. 



Nitrogen Fixation in the Soil. — Not all nitrogen-fixing bacteria 

 are mutualistic and associated symbiotically with higher plants. 

 Berthelot (1885) showed that the nitrogen of the soil was in- 

 creased by means of bacteria living free in the soil, but the deter- 

 mination and isolation of these forms were left for later workers. 

 Two forms in particular have been much studied, — Clostridium 

 and Azotobacter. Winogradski (1893), working with the former 

 genus, has shown that it is an anaerobic form and that small 

 quantities of ammonium salts aid in the fixation process. Beijer- 

 inck (1901) later isolated the other nitrogen-fixing bacterium 

 (Azotobacter) and found it capable of fixing nitrogen in the presence 

 of oxygen. 



Other bacteria and fungi have been reported as capable of fixing 

 free nitrogen in the soil. Among these are Phoma betae, sl fungus 

 which grows parasitically upon sugar beets, and several species of 

 Penecillium and Aspergillus. Wann (1921) reported several species 

 of green algae as possessing this power; and Lipman and Taylor 

 (1924) announced that wheat and barley can fix free nitrogen. 



