CHAPTER XXV 

 THE ROOT-NODULE BACTERIA OF LEGUMINOUS PLANTS 



EDWIN BROUN FRED 

 University of Wisconsin 



The growing of leguminous crops for soil improvement is probably one of the old- 

 est applications of bacteriology. More than two thousand years ago the Romans, and, 

 at an earlier date the Chinese, were aware of the fertilizing effect of leguminous plants. 

 Although the Roman farmers made use of these plants to increase the fertility of the 

 soil, they did not know how the beneficial effect was produced. It was not until the 

 early eighties of the last century that investigators demonstrated clearly that this 

 beneficial effect of leguminous plants was due to the fixation of nitrogen within the 

 root nodules. 



Outstanding among the early investigators were HellriegeP and his associate 

 Wilfarth in Germany, Atwater^ in America, and Lawes and Gilbert^ in England. 

 As a result of their work, intensive studies were carried out in various countries 

 and the conclusion reached that nodule formation was caused by the associated 

 growth of the leguminous plant and some lower organism, and also that nitrogen fixa- 

 tion takes place in the root nodules. For the first time in 1888, Beijerinck^ in Holland 

 obtained pure cultures of nodule bacteria and demonstrated their abihty to produce 

 root nodules. 



RELATION or BACTERIA TO HOST PLANT 



The life of the root-nodule organism outside of the plant and the agencies which 

 lead to its entry into the tissues are not well known. It has been suggested that the 

 bacteria are attracted to the roots by the secretion of a substance of unknown com- 

 position. At best, the movement of the bacteria in the soil is slow, perhaps i inch in 

 twenty-four hours.^ 



The bacteria get into the parenchyma of the root through the root hairs or 

 through other epidermal cells. These infected root hairs show a characteristic bend- 

 ing at or near the root tip. Once within the tissues the bacteria multiply rapidly, 

 forming threadlike filaments with many branches throughout the hair and into the 

 parenchyma of the root. Here in the innermost cells of the root cortex, just outside 

 of the endodermis, the bacteria bring about conditions favorable for a rapid multipli- 

 cation of the surrounding cells and thus the formation of the younj^; nodule begins. As 

 the cells multiply the young nodule soon pushes out the overlying cortical paren- 



' Hellriegel, H.: Lmidw. Vers. Stat., 33, 464. 1886. 

 ' Atwater, W. O.: Am. Chem. J., 8, 398. 1886. 

 3 Lawes, J., and Gilbert, J. : Proc. Roy. Soc, London, 47, 85. 1890. 

 ^Beijerinck, M. W.: Bot. Ztg., 46, 725. 1888. 



s Thornton, H. G., and Gangulee, N.: Proc. Roy. Soc, London, B, 99, 427. 1926. 



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