278 FARM FRIENDS AND FARM FOES 



When we speak of two plants thus living together in mutual 

 helpfulness, we use the word symbiosis : so we say that the 

 nitrogen- gathering germs are Symbiotic Bacteria. 



These symbiotic bacteria are of great importance to prac- 

 tical agriculture. In most good systems of crop rotation, 

 clover or some other leguminous plant is grown every few 

 years in order that the supply of available nitrogen in the 

 soil may be increased. Clover is able to increase this supply 

 because of the help given by the germs living in the nod- 

 ules on the clover roots. One of the best ways to improve 

 poor soil is to grow a leguminous crop and plow it under 

 as green manure. The value of this process lies largely 

 in the presence of the nitrogen-gathering bacteria. When 

 an alfalfa field is well established, it may be left undisturbed 

 except for harvesting the crops, for many years. Presum- 

 ably it is able thus to yield its forage year after year be- 

 cause the bacteria in the nodules on the roots are gathering 

 free nitrogen from the air. 



A striking example of the effectiveness of these bacteria 

 in gathering nitrogen has been recorded by the Illinois Ex- 

 periment Station. Three sets of ten cowpea plants with 

 tubercles on the roots were analyzed to determine the per- 

 centage of nitrogen. Three other sets of ten cowpea plants 

 without tubercles were also analyzed for comparison. " The 

 infected plants contained nearly four times as much nitrogen 

 as the plants not infected, and about three fourths of the 

 total nitrogen in the infected plants was obtained from the 

 air. The roots and tubercles of the infected plants con- 

 tained six to seven times as much nitrogen as the roots of 

 the plants not infected." 



It Is an interesting fact that the. bacteria found in nodules 

 on the roots of red clover will not develop upon the roots 

 of cowpeas or several other related plants. Similarly, the 



