Bacteria and Fungi 



259 



Bureau of Agriculture, P. I. 

 Fig. 151. A field of cowpeas. Like other legumes, cowpeas accumulate nitrogen 

 compounds, and when plowed under they add nitrogen and humus to the soil. 



crop rotations existed long before the real cause of the increase 

 in soil nitrogen was understood, or even before it was under- 

 stood how different elements in the soil contribute to its 

 fertility. By experience it was learned that other plants 

 flourish in land after leguminous plants have been grown in 

 it, and for this reason the farmer included legumes in his 

 scheme of crop rotation. 



It is now clearly understood that nitrogen compounds ac- 

 cumulate 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 



