NITROGEN FIXATION BY BACTERIA 423 



decomposable carbohydrates needed by the denitri- 

 fying organisms decreases or precludes their activity. 



287. Nitrogen fixation through symbiosis with 

 higher plants. It has long been recognized by farmers 

 that certain crops like clover, alfalfa, peas, beans, etc., 

 improve the soil, making it possible to grow larger 

 crops of cereals after these crops have been upon the 

 land. The benefit was, within the past century, traced 

 to an increase in the nitrogen content of the soil, and 

 the specific plants so affecting the soil were found to be, 

 with perhaps a few exceptions, those belonging to the 

 family of legumes. It has furthermore been demon- 

 strated that these plants utilize, under certain con- 

 ditions, the uncombined nitrogen of the atmosphere, 

 and that they contain, both in the aerial portions 

 and in the roots, a very high percentage of nitrogen. 

 In consequence, the decomposition of even the roots 

 of the plants in the soil leaves a large amount of 

 nitrogenous matter. 



288. Relation of bacteria to nodules on roots. 

 It has also been shown that the utilization of atmos- 

 pheric nitrogen is accomplished through the aid of 

 certain bacteria that live in nodules (tubercles) on 

 the roots of the plants. These bacteria acquire the 

 free nitrogen from the air in the soil, and the host plant 

 secures it in some form from the bacteria or their 

 products. The presence of a certain species of bacteria 

 is necessary for the formation of tubercles. Legumi- 

 nous plants grown in cultures or in soil not containing 

 the necessary bacteria do not form nodules, and do 

 not utilize atmospheric nitrogen, the result being that 



