HOW NODULE BACTERIA WORK 



179 



per acre. The amount gathered by the bacteria may vary from 

 forty to two hundred pounds to the acre per year, depending upon 

 the amount of crop growth and soil conditions. This does not mean 

 that the nodule bacteria actually add to the soil this amount 

 of nitrogen, but rather it is the amount the bacteria furnish to 

 the legume. 



How Nodule Bacteria Work. Nearly everyone is interested 

 in knowing how the minute nodule bacteria aid legumes in getting 



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FIG. 120. The inside of a lupine nodule, magnified. (U. S. D. A.) (See Fig. 121.) 



their nitrogen. These organisms naturally live in the soil, and when 

 they come in contact with the legume root they enter it and grow 

 rapidly, causing an abnormal growth known as tubercles or nodules. 

 As the plant grows, the bacteria multiply and the nodules increase 

 in size and in number (Figs. 120 and 121). Each nodule contains 

 millions of these bacteria. They feed upon the plant juice and, in 

 return, furnish the plant with nitrogen which they take from the 

 air in the soil and combine it in a form suitable for the plant. 

 About the time seeds form on the legume the nodules cease grow- 

 ing, lose their plump appearance, begin to shrink, and finally die, 



