PROTOTROPHIC BACTERIA 



soil, and the importance of nitrification in Nature's food 

 cycle cannot be overestimated. 



Nitrogen-fixing Bacteria. Nitrogen-fixing bacteria, or 

 legume bacteria, occur in the nodules which are found on the 

 roots of various legumi- 

 nous plants. Growing 

 in these nodules these 

 bacteria are able to fix 

 the nitrogen of the air 

 into a form which is 

 available for plant use, 

 and while they live in 

 the plant they are not 

 to be regarded as para- 

 sites, or even as meta- 

 trophic bacteria, but 

 they belong to the 

 group under discussion in this chapter; namely, the pro to- 

 trophic bacteria. They are bacilli so far as their form is con- 

 cerned, and there are many different kinds. Whether these 

 different kinds are separate species or merely varieties, is a 

 question that is not fully settled. These germs are not strictly 

 prototrophic, but may be cultivated upon artificial media in 

 which they make use of organic food material. There are 

 different forms associated with the different plants. One 

 peculiarity, however, of all forms is the tendency, at a 

 certain stage in their life history, to assume irregular or 



FIG. 44. Bacteroids from legumes, (i) From 

 Melilotus alba. (2, 3, 5) From Medicago 

 sativa. (4) From Vicia villoso. After Har- 

 rison and Barlow. (Lipman.) 



