NODULE BACTERIA OF LEGUMINOUS PLANTS 



By F. Lohnis, Soil Biologist, Bureau of Plant Industry, United States Department of 

 Agriculture, and Roy Hansen, Professor of Soils, University of Saskatchewan, 

 Saskatoon, Sask. 1 



INTRODUCTION 



Despite the fact that the nodule bacteria of the leguminous plants 

 have been made the subject of numerous publications, it is not to be 

 disputed that their true morphological and physiological character, as 

 well as their correct systematic position, are by no means sufficiently 

 known. This is especially clearly demonstrated by the fact that they 

 are still proclaimed by several writers to be the representatives of a 

 special genus Rhizobium, once established by A. B. Frank as the result 

 of rather inadequate studies upon this subject. In the new classification 

 of bacteria, adopted by the Society of American Bacteriologists, the 

 nodule bacteria again are widely separated from closely related species, 

 and the error concerning the so-called genus Rhizobium has been re- 

 vived once more. 



Comparative investigations upon the symbiotic and the nonsymbiotic 

 nitrogen-fixing bacteria of the soil, published in 1905 by the senior author, 

 have proved conclusively that the nodule bacteria are not representatives 

 of a special genus Rhizobium, but that they are closely related to Bacillus 

 radiobacter Beijerinck and further to B. lactis viscosum Adametz, B. 

 pneumoniae Friedlander, and B. aerogenes Escherich. The last three 

 organisms are immotile, while the first one is motile; but here again the 

 very close relationship between the immotile B. aerogenes and the motile 

 B. coli has to be kept in mind. In fact, there can be easily isolated from 

 every soil numerous varieties of B. radiobacter, which lead gradually up 

 to B. coli, acquiring the power of fermentation and that type of growth 

 on solid substrates which is characteristic of the last-named species. 

 It has been pointed out in detail that all species mentioned above differ 

 only gradually, not principally, in their physiological and morphological 

 qualities, and especially that those branched or otherwise changed cell 

 forms which are frequent in the root nodules are equally common with 

 all members of this group of capsule bacteria, if these are tested ade- 

 quately. 2 The ability to fix the atmospheric nitrogen was shown to be 

 common in this group of organisms. 



1 Most of the experiments discussed in this paper were made in the summer of 1919, at the University of 

 Illinois, where at that time the junior author held the position of Associate in Soil Biology. The photo- 

 graphs accompanying the paper were made by Mr. F. L. Goll, of the Bureau of Plant Industry, United 

 States Department of Agriculture. 



2 It is not superfluous to emphasize once more that persistence in calling these forms " bacteroids " is by 

 no means to be recommended. They are true bacteria, not foreign bodies looking like bacteria, as Frank's 

 pupil Brunchorst erroneously believed. To speak of a "bacteroid" growth of bacteria is no less absurd 

 than it would be to speak of a "fungoid" growth of fungi. 



Journal of Agricultural Research, Vol. XX, No. 7 



Washington, D. C Jan. 3, 1921 



wg Key No. G-215 



17777°— 21 3 (543) 



