FIXATION OF ATMOSPHERIC NITROGEN 413 



course of their studies on the cause of nitrogen accumulation by 

 legumes. They found that when leguminous plants were grown in 

 sterile sand, nodules were formed on the roots only after the addition 

 of a small portion of aqueous extract of fertile soil, or an extract of 

 crushed nodules, or in some cases (lupines and seradella) by soil itself 

 from a field on which these crops had been grown. The first successful 

 artificial production of nodules by the aid of pure cultures was made 



FIG. 137. Roots of plant B with nodules (Fig. 135). 



in 1889 by Prazmowski in the course of studies on the method of 

 entrance of the organism to the root hairs of the host plant. 



The first inoculation experiments in a large way were those made in 

 1887 at the Moor Soil Experiment Station, Bremen, Germany, where 

 earth taken from fields that had borne luxuriant crops of various 

 legumes was scattered over reclaimed heath or swamp soils upon which 

 legumes had not previously grown, with the result that in every instance 

 the yield on the inoculated portions of land was greater than on the 



