90 AGRICULTURE 



workshop inhabited by multitudes of germs, so small that 

 25,000 of them could be placed side by side on a line one 

 inch long. These germs are actively at work helping the 

 farmer. The tubercle in which they live serves as a house 

 for them. It is really a fertilizer factory, and the germs 

 are the workmen, busy making fertilizer that will be 

 used by the plant on the roots of which the tubercle 

 grows. The plant on which the tubercle forms is called 

 the host plant. It furnishes the germs in the tubercle 

 with starchy food made by the leaves. In exchange the 

 tubercles send up through the sap a fertilizer rich in 

 nitrogen. This fertilizer nitrogen is constantly being 

 made by the germs in the tubercle from the nitrogen gas 

 in the air. The farmer can help the germs to manu- 

 facture fertilizer nitrogen by plowing the land before sow- 

 ing legumes. Plowing or cultivation permits an abundance 

 of air, with the nitrogen gas which it contains, to pass 

 through the loose soil to the tubercle, where the tiny 

 workmen are ready to use it for the farmer's benefit. 



What the cowpea or clover plant does with nitrogen. 

 Let us consider what becomes of the nitrogen a tubercle 

 sends up in the sap current to the cowpea or clover plant 

 on which it is growing. A part of it is deposited in the 

 roots of the cowpea, another part in the stem, another 

 portion goes to make the leaves, and still another part 

 helps to make the seeds. All clovers and most other 

 legumes use their fertilizer nitrogen manufactured in the 

 tubercles just as the cowpea does, and they enrich the 

 soil in the same way. 



Even if the farmer mows and hauls away the vines for 



