106 PHYSIOLOGICAL ACTIVITIES 



the plant tissues, receiving carbonaceous and other food 

 material from them and in return furnishing nitrogenous 

 material to the plant. This nitrogenous material is built up 

 from free nitrogen absorbed from the air by the bacteria. 

 The utilization of this peculiar property through the proper 

 cultivation of clover, alfalfa, soy beans and other legumes 

 is one of the best ways of building up and maintaining soil 

 fertility insofar as the nitrogen is concerned. The technical 

 name of these bacteria is Pseudomonas radicicola. 



There are also types of "free-living," as distinguished 

 from these symbiotic, bacteria which absorb the free nitro- 

 gen of the air and aid materially in keeping up this supply 



Fig. 75. — Free-living nitrogen absorbing bacteria "Azotobaoter." Note 

 their large size as compared with other bacteria shown in this book. 



under natural conditions. One of the most important of 

 these types is the aerobic "Azotobacter" (Fig. 75), while 

 another is the anaerobic Bacillus (Clostridium) pastorianus. 

 The nitrogen which is absorbed is built up into the protein 

 material of the cell body and this latter must in all proba- 

 bility be "worked over" by various types of decomposition 

 bacteria and by the nitrous and nitric organisms and be 

 converted into utilizable nitrates just as other protein 

 material is, as has been discussed in Chapter X. At any 

 rate there is as yet no definite knowledge of any other 

 method of transformation. Up to the present no intentional 

 practical utilization of this valuable property of these free- 

 living forms has been made. 



