82 



THE VEGETATIVE FUNCTIONS OF PLANTS 



potassium, calcium, magnesium, ammonium, and others, 

 forming potassium nitrate (KN0 3 ), calcium nitrate 

 (Ca(NOs) 2 ), ammonium nitrate (NH^NOs, and so forth. 

 Nitrates and ammonia may result from the disintegration 

 of plant and animal bodies, but the supply must con- 

 tinually be renewed by drawing upon the free nitrogen 



FIG. 61. Root- tubercles on yellow sweet clover (Melilotus officinalis), 

 caused by and containing Pseudomonas radicicola. 



of the air. The ability to "fix" atmospheric nitrogen, 

 that is, to form nitrogen compounds with the free nitrogen 

 of the air, is possessed by several kinds of bacteria, includ- 

 ing species of Azotobacter, Clostridium, and Pseudomonas. 

 There is also evidence that certain lower fungi possess 



