HOW PLANTS OBTAIN FOOD. 93 



the root and down to near the centre of the root. As soon as 

 it begins to enter the cells of the root it stimulates the cells of 

 that portion to greater activity. So the root here develops a 

 large lateral nodule, or ' ' root tubercle. ' ' As this ' ' root 



Fig. 70, Fig. 71. 



Root-tubercle organism from vetch, old con- Root-tubercle organism from Medicago 



dition. denticulata. 



tubercle" increases in size, the fungus threads branch in all 

 directions, entering many cells. The threads are very irregular 

 in form, and from certain enlargements it appears that the rod- 

 like bodies are formed, or the thread later breaks into myriads 

 of these small " bacteroids. " 



165. The root organism assimilates free nitrogen for its 

 host. — This organism assimilates the free nitrogen from the air 

 in the soil, to make the proteid substance which is found stored 

 in the bacteroids in large quantities. Some of the bacteroids, 

 rich in proteids, are dissolved, and the proteid substance is 

 made use of by, the clover or pea, as the case may be. This is 

 why such plants can thrive in soil with a poor nitrogen content. 

 Later in the season some of the root tubercles die and decay. 

 In this way some' of the proteid substance is set free in the soil. 

 The soil thus becomes richer in nitrogenous plant food. 



The forms of the bacteroids vary. In some of the clovers 

 they are oval, in vetch they are rod-like or forked, and other 

 forms occur in some of the other genera. 



