138 AGRICULTURAL BACTERIOLOGY. 



fixation of nitrogen by legumes is associated with the develop- 

 ment upon the roots of little nodules known as tubercles. 

 (Fig. 19.) These tubercles are little swellings on the roots, 

 sometimes very numerous, and varying from the size of a 

 pinhead to the size of a pea. They have been known for a 

 long time, since they can easily be found on nearly any legume 

 growing luxuriantly in the soil, if the roots are carefully pulled 

 up from the soil in such a way as to prevent these nodules from 



FIG. 19. 



A leguminous plant (vetch) showing root tubercles. 



being destroyed, and if the soil is carefully washed away. Their 

 nature was unknown and they were in general regarded as 

 galls upon the roots, similar to those that appear upon the 

 leaves and branches of trees, and, therefore, were looked upon 

 as a type of disease. It was, however, evident that if they 

 were of the nature of a disease, they did the plants no injury, 

 for the plants developing these tubercles were as luxurious as 

 those without them. Indeed, as soon as the nitrogen-fixing 



