104 PHYSIOLOGICAL ACTIVITIES 



that this property is due to bacteria. These plants, and 

 several other kinds as well, have on their roots larger or 

 smaller nodules (Fig. 69), spoken of as ''root tubercles," 

 which are at certain stages filled with bacteria. When 

 conditions are favorable, these bacteria live in symbiotic 

 relationship with the plant tissues, receiving carbonaceous 

 and other food material from them and in return furnishing 

 nitrogenous compounds to the plant. This nitrogenous 

 material is built up from free nitrogen absorbed from the 

 air by the bacteria. The utilization of this peculiar prop- 

 erty 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 in so far as the nitrogen is 

 concerned. The technical name of these bacteria is Rhizo- 

 biimi leguiiiinosarum. 



Fig. 70.— Free-living nitrogen absorbing bacteria " Azotobacter." Note 

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



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 

 under natural conditions. One of the most important of 

 these types is the aerobic "Azotobacter" (Fig. 70), while 

 another is the anaerobic Clostridium pasteurianuvi. The 

 nitrogen which is absorbed is built up into the protein 

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

 ability 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 



