398 



General Botany 



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F. Lohnis, U.S. D. A. 



Fig. 245. A, cross-section of root nodule of a legume; B, a single root cell showing nitro- 

 gen-fixing bacteria within it ; C, branched bacteria from a nodule. 



corn. The practice of using legumes in crop rotations was fol- 

 lowed long before the real cause of the increase in soil nitrogen 

 was discovered, and even before it was understood how the dif- 

 ferent elements in the soil contribute to its fertility. By expe- 

 rience it was learned that other plants flourish on land after 

 leguminous plants have been grown on it, and for this reason 

 the farmer included legumes in his scheme of crop rotation. 



It is now clearly understood that nitrogen compounds accumu- 

 late in leguminous plants only because of the presence of certain 

 nitrogen-fixing bacteria. These bacteria occur in many soils, 

 and when the legume is planted and develops roots, they invade 

 the cells of the root. This causes the infected parts of the root 

 to enlarge, forming nodules. If a nodule from a clover or alfalfa 

 root is crushed and examined under a microscope, it will be found 

 to be filled with bacteria. These bacteria are parasites and take 

 their food from the legume. A part of it they use in building 

 their tissues ; the remainder is oxidized and the energy used in 

 changing nitrogen from the soil air into nitrogen compounds, 

 just as the other nitrogen-fixing bacteria mentioned in the pre- 



