76 



GENERAL BIOLOGY 



reason for the improvement. It has been known for 

 a long time, too, that the rootlets of leguminous 

 plants (i.e. peas, beans, alfalfa, etc.) are beset 

 with little nodules or tubercles, which were supposed 



FIG. 26. Tubercles on roots of clover. (Osterhout.) 



to be pathological growths of the nature of galls. 

 Comparatively recently it has been demonstrated 

 that these tubercles harbor minute bacteria endowed 

 with the property of fixing atmospheric nitrogen, 

 and that this is the chief reason for the value of such 

 plants as fertilizers. We discover, therefore, that 



