RECLAIMING LOST NITROCF.N. 137 



of more practical value than the last. This also is dependent 

 upon bacteria, and upon a peculiar relation between bacteria 

 and the family of leguminous plants. As already noticed, ex- 

 perimental evidence indicates that ordinary plants are unable 

 to make use of atmospheric nitrogen. Long series of experi- 

 ments were conducted to test the matter and the more rigidly 

 the experiments were performed, the more evident did it be- 

 come that such an assimilation does not occur in ordinary 

 green plants. It \vas, however, shown in 1881, that this con- 

 clusion did not hold in regard to the great family of legumes. 

 It was then demonstrated very conclusively that peas and 

 beans, growing in a soil free from nitrogen and fed upon food 

 containing no nitrogen, did, in the course of a few weeks' 

 growth, increase the amount of nitrogenous material present in 

 the plant, and, inasmuch as the only possible source of this 

 nitrogen was the atmosphere, the conclusion was unhesitatingly 

 drawn that peas can assimilate atmospheric nitrogen. This con- 

 clusion, so contradictory to the belief accepted at the time, was 

 at first vigorously disputed ; but, upon being subjected to 

 further experimentation by many observers, was found to be 

 strictly correct. Many of the plants of the great family of 

 legumes certainly do have the power, under certain circum- 

 stances, of fixing atmospheric nitrogen and absorbing it into 

 their tissues. 



The question of the conditions under which such a fixing of 

 atmospheric nitrogen occurs, was recognized to be one of the 

 greatest importance. In the course of the next ten years there 

 was gradually unfolded by the experiments of botanists, chem- 

 ists and bacteriologists, a series of surprising facts which have 

 resulted in demonstrating that this fixation of atmospheric 

 nitrogen by the legume is dependent upon the growth of cer- 

 tain soil bacteria. The facts by which this conclusion was 

 finally demonstrated are briefly as follows : 



Root Tubercles. The first step was the observation that the 

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