HOW TREES AND PLANTS IMPROVE THE LAND 89 

 is an indication that the plants are legumes and that they 



FIG. 57. TUBERCLES ON THE ROOTS OF A YOUNG VETCH PLANT 



are at work making the soil rich. These knots are root 

 tubercles or root nodules (Figs. 56, 57). 



The farmer's tiny helpers. Each tubercle is a busy 



