BACTERIA FIXING ATMOSPHERIC NITROGEN 125 



The causative organism was isolated in 1888, in pure culture, by Bei- 

 jerinck, 108 who named it Bacillus radicicola. Beijerinck described three 

 stages in the development of the organism. 



1 . The organism is present in the soil in the form of small rods which 

 can penetrate the root hairs of the leguminous plants and from there it 

 is transferred to the "infectious tissue." 



2. The organism changes into a motile bacillus. 



3. It changes into the bacteroid form which functions as the sym- 

 biotic organism. 



The organism was soon grown, on artificial culture media, by a 

 number of investigators. 109 The mechanism of root infection by pure cul- 

 tures of bacteria was worked out by Prazmowski in 1889. 109a Schloesing 

 and Laurent 110 demonstrated that the nitrogen is actually obtained by 

 the bacteria in the form of nitrogen gas from the atmosphere. Legumi- 

 nous plants were grown in sterile glass cylinders containing sterile 

 sand and watered with sterile water. When the composition of the gas 

 in the cylinder was determined, it was found that, while the uninoculated 

 plants showed a gain of only 0.6 mgm. of nitrogen and no nodule forma- 

 tion, inoculated plants showed a gain of 34.1 and 40.6 mgm. of nitrogen 

 and abundant nodule formation. Nobbe and Hiltner 111 concluded that 

 the fixation of nitrogen by leguminous plants is closely related to the 

 formation of bacteroids in the nodules. 



Nomenclalure. The causative organism of the nodules on the roots of 

 leguminous plants is referred to by different names, depending on the 

 particular system of classification. As pointed out above, the discoverer 



108 Beijerinck, 1888 (p. 103). See also Prazmowski. Das Wesen und die 

 biologische Bedeutung der Wurzelknollchen der Erbse. Bot. Centrbl. 39: 356- 

 362. 1889; Landw. Vers. Sta. 37: 161-238. 1890. 



109 For a review of earlier literature see Voorhees and Lipman, 1907 (p. 491). 

 Hiltner, 1904 (p. 128). Lohnis, 1910. Burrill and Hansen, 1917 (p. 126); Miiller, 

 A., and Stapp, C. Beitriige zur Biologie der Leguminosenknollchenbakterien 

 mit besonderer Beriicksichtigung ihrer Artverschiedenheit. Arb. Biol. Reich- 

 sanst. L. u. Forstm. 14: 455-554. 1925. 



loss Prazmowski, A. Die Wurzelknollchen der Erbse. Landw. Vera. Sta. 37: 

 161; 38: 5. 1890. 



110 Schloesing, Th., and Laurent, E. Recherches sur la fixation de l'azote 

 libre par les plantes. Compt. Rend. Acad. Sci. Ill : 750-754. 1890; 113 : 776-778, 

 1095-1060. 1891; 115: 1017. 1892; Ann. Inst. Past. 6: 65-115, 824-940. 1892. 



111 Nobbe, F., and Hiltner, L. Wodurch werden die knollchenbesitzenden 

 Leguminosen befahigt, den freien atmospharischen Stickstoff fur sich zu ver- 

 werten. Landw. Vers. Sta. 42: 459-478. 1893. 



