BACTERIA IN NATURAL PROCESSES. 109 



in the root. Now, by some entirely unknown 

 process, the legume and the bacteria growing to- 

 gether succeed in extracting the nitrogen from 

 the atmosphere which permeates the soil, and fix- 

 ing this nitrogen in the tubercles and the roots in 

 the form of nitrogen compounds. The result is 

 that, after a proper period of growth, the amount 

 of fixed nitrogen in the plant is found to have 

 very decidedly increased (Fig. 25 E.). 



This, of course, furnishes a starting point for 

 the reclaiming of the lost atmospheric nitrogen. 

 The legume continues to live its usual life, per- 

 haps increasing the store of nitrogen in its roots 

 and stems and leaves during the whole of its 

 normal growth. Subsequently, after having fin- 

 ished its ordinary life, the plant will die, and then 

 the roots and stems and leaves, falling upon the 

 ground and becoming buried, will be seized upon 

 by the decomposition bacteria already men- 

 tioned. The nitrogen which has thus become 

 fixed in their tissues will undergo the destructive 

 changes already described. This will result 

 eventually in the production of nitrates. Thus 

 some of the lost nitrogen is restored again to the 

 soil in the form of nitrates, and may now start 

 on its route once more around the cycle of food. 



It will be seen, then, that the food cycle is a 

 complete one. Beginning with the mineral in- 

 gredients in the soil, the food matter may start 

 on its circulation from the soil to the plant, from 

 the plant to the animal, from the animal to the 

 bacterium, and from the bacterium through a 

 series of other bacteria back again to the soil in 

 the condition in which it started. If, perchance, 

 in this progress around the circle some of the 

 nitrogen is thrown off at a tangent, this, too, 



