NITROGEN ASSIMILATION; THE NITROGEN CYCLE 95 



ship between soil microorganisms and the roots of the plant. 

 These results have been confirmed many times, and in that same 

 year Beijerinck isolated the bacteria concerned and grew them 

 in pure cultures. 



These bacteria (Bacterium, Rhizobium, or Pseudomonas radi- 

 cicola) develop only on unsterilized soil and are of many different 

 strains or races, each of which grows best in association with a 

 particular legume. Thus nodules will form on pea roots better if 

 soil is taken from a field where peas have been grown previously 

 than when the peas are inoculated with soil from a clover field. 

 There is even a difference between the race which grows on alfalfa 

 and the one on clover. Formerly the inoculation was made by 

 importing soil from a place where the crop was previously grown. 

 Now the experiment stations keep on hand the various strains in 

 pure cultures, and these are furnished to the farmers throughout 

 the State who wish to inoculate a particular crop growing in soil 

 deficient in the necessary strain. 



The bacteria enter the root through the root hairs which they 

 surround and finally penetrate, passing into the cells of the cortex 

 as a bacterial filament. Having well entered the cortex, the fila- 

 ment branches rapidly, while the parenchyma cortex cells in its 

 neighborhood divide actively. The result is a swelling (the tubercle) 

 on the side of the root. The central part of the nodule contains 

 the bacterial cells, which are now surrounded by the root paren- 

 chyma. The whole tubercle is covered with a corky layer and is 

 connected by vascular strands to the root. The sheath which 

 surrounds the bacterial filament later disintegrates, and the bac- 

 teria are thus freed inside the cells, where they enlarge and form 

 peculiar Y-, V-, and T- shaped structures known as bacteroids. This 

 bacteroid tissue becomes depleted of its contents, which are then 

 used by the plant, after which the cells form cysts or colonies of 

 spores. These then become disseminated in the soil after the 

 destruction of the tubercle and are now capable of infecting other 

 roots. 



The bacteria have the power of fixing nitrogen in the form of 

 organic nitrogen compounds from the free nitrogen in the air in 

 the soil. The host plant is then able to absorb and use this organic 

 nitrogen or some similar compound resulting from its partial 

 decomposition. In just what manner the legume takes the nitrogen 

 from the bacteria is not known, but the relation between the two 



