Anaerobic Bacteria 193 



on the basis of their abilit\ to iitili/.c the available sources of energy 

 in a nonsymbiotic manner. Other organisms are able to obtain the 

 carbon for their energy and for cell synthesis from the growing plant 

 with which they live symbiotically. These organisms are not obli- 

 gate, so far as the nitrogen is concerned, since they are also able to 

 obtain their nitrogen from organic or inorganic compounds. 



I. Nonsymbiotic nitrogen-fixing bacteria. 



1. Anaerobic organisms. 



a. Clostridium pastcttriantim, comprising the group of non-starch-fer- 

 menting type of Clostridia. 



b. Bacillus saccharobuitjricits, comprising the starch-fermenting Clostridia 

 and occasional plectridia. 



c. Plcctridium group, including the starch-fermenting Tlcctridium, 

 which differ from the plectridia of the previous group by forming 

 long, slender, often curved rods, with thick oval spores as their ex- 

 treme ends, and by being more proteolytic and less fermentati\e in 

 nature. 



d. But>l-alcohol-forming group, morphologically related to the second 

 group of starch-fermenting Clostridia. 



2. Aerobic organisms. 



a. Azotohacier, comprising fi\e distinct species: Az. cJiroococcum, Az. 

 beijerinckii, Az. vinelandii , Az. agilis, and Az. indicum. 



b. Diplococcus pneumoniae, Aerobacter aerogenes, and other non-spore- 

 forming bacteria. 



c. Bacillus astcrosporus and other spore-forming bacteria. 

 II. Symbiotic nitrogen-fixing bacteria. 



1. Bacteria living in the roots of leguminous plants. 



2. Bacteria living on and in the roots of nonleguminous plants. 



3. Bacteria li\ing in the leaves of certain plants. 

 III. Nitrogen fixation by blue-green algae. 



Anaerobic Bacteria 



The nitrogen-fixing capacity is well distributed among the an- 

 aerobic butyric acid bacteria but to a varying degree. 



The number of nitrogen-fixing Clostridia in the soil has been found 

 to be greater than 100,000 per gram. They are much more abundant 

 than the members of the Azotohacier group. This led various inves- 

 tigators to conclude that the genus Clostridium rather than Azoto- 

 hacter is the most important group of nonsymbiotic nitrogen-fixing 

 bacteria. Diiggeli reported 100-1,000,000 anaerobic bacteria and 

 0-100,000 aerobic nitrogen-fixing bacteria per gram of soil. Plots 

 receiving sodium nitrate as a source of nitrogen contained 10,600- 

 12,000 cells of Clostridium and 4,900-6,300 Azotohacier. Plots re- 

 ceiving no nitrogen, but potassium and phosphorus fertilizers, con- 



