RECLAIMING LOST NITROGEN. 



139 



power of legumes was demonstrated, it became evident that 

 the fixation of nitrogen was associated with the formation of 

 tubercles. Only such plants as developed tubercles were able 

 to increase the amount of nitrogen in their tissues, and the 

 amount of nitrogen fixation was roughly proportional to the 

 development of tubercles. Plants without tubercles showed 

 no increase ; those with a moderate number, a slight increase ; 



FIG. 20. 



Aleguminous plant (vetch) without tubercles, showing less vigorous growth. 



and those with abundant tubercles, a larger increase in nitro- 

 gen. ( Figs. 19 and 20.) 



This, of course, raised the question as to the nature of these 

 tubercles and led to a series of experiments in regard to their 

 formation. It appeared that the tubercles would not form 

 upon the roots of legumes if the plants were grown in steril- 

 ized soil. Under these circumstances the plants developed 

 no tubercles, fixed no nitrogen and, unless fed with ni- 

 trogenous food, made very little growth, being stunted and 



