[Chap. XLIII BACTERIA OF THE SOIL 535 



into compounds usable by all plants. The chain of processes in this 

 transfonnation is teniied nitrogen-fixation. 



It has been known for more than 20 centuries that certain legumes, 

 such as clover, beans, and peas, enrich the soil. For this reason legumes 

 have long been used in crop rotations because farmers have known that 

 other crop plants grow better and yield more following the plowing 

 under of legumes. 



It was not definitely known that nitrogen compounds accumulate in 

 legumes through the agency of nitrogen-fixing bacteria until 1886-1888.- 

 Since that time the life cycles of these organisms have been thoroughly 

 studied. These bacteria occur as microscopic motile rods free in the 

 soil. When legumes are planted and roots form, the bacteria invade the 

 cells of the roots by wav of the root hairs. The root becomes infected 

 and enlarges locally in the form of nodules ( Fig. 231 ) . These nodules 



Fig. 231. Nodules containing nitrogen-fixing bacteria on the roots of soybean. 

 Photo from Agricultural Extension Department, Ohio State University. 



- First shown by Hellriegel and Wilfarth. Pure cultures of these bacteria were first iso- 

 lated by Beijerinck in 1888. 



