144 



The extensive paper on nitrogen fixation which BEIJERINCK and 

 his collaborator VAN DELDEN published in 1902 l ) need not to be re- 

 viewed here in detail. The paper sets out extensive data regarding the 

 gain of nitrogen in cultures in which Azotobacter was growing to- 

 gether with other "oligonitrophilous" species. The conclusion was that 

 Azotobacter itself is unable to fix nitrogen and that its proliferation in 

 the enrichment cultures is exclusively due to its living in symbiosis 

 with other nitrogen fixing species. This view has now been definitely 

 refuted by the work of numerous other investigators. Nevertheless the 

 paper remains of interest on account of its detailed description of the 

 many other bacteria which regularly accompany Azotobacter in the 

 enrichment cultures. 



Six years later BEIJERINCK returned once more to the subject 2 ). 

 In this publication he revoked his opinion regarding the absence of 

 nitrogen fixing power in Azotobacter. This time, in collaboration with 

 his assistant MINKMAN, definite proof for nitrogen fixation in pure 

 cultures was given. In a final section of the paper a few observations 

 are recorded regarding the distribution of Azotobacter in soil. The 

 procedure applied, viz., the direct sowing of soil particles on elective 

 solid media, has later in the hands of WINOGRADSKY proved to be a 

 most valuable tool in soil microbiology 3 ). 



Finally, mention may here be made of a short paper - - published 

 only in the Dutch language - - which BEIJERINCK wrote in the last 

 year of his academic career 4 ) . Herein he gave his views on the signific- 

 ance to be attached to the more or less f requent occurrence of Azoto- 

 bacter in soils. BEIJERINCK seemed inclined to conclude that the num- 

 ber of Azotobacter cells detectable in soil would be an indicator of its 

 fertility. In contrast hereto he placed the observation that Granulo- 

 bacter Pastorianwn is equally frequent in fertile and infertile soils. 

 Although the data on which these conclusions are based are too 

 scanty to lend them more than a provisional character, the paper has 

 the merit of inciting further research in this direction. 



o. Investigations on urea-decomposing bacteria. 



As has already been observed in Chapter XX it was at the be- 

 ginning of this century that BEIJERINCK became fully aware of the 

 far-reaching importance of the principle of the enrichment culture. 

 His study on the group of the urea-decomposing bacteria which ap- 

 peared in 1901 and in which he for the first time made more general 

 remarks on the said principle, also afforded a splendid demonstration 

 of what can be attained by a well-designed application thereof 



1) Central!)!. I. Hakt. u. Parasitenk. II, 9, 3, 1902. 



2) Proc. Kon. Akad. v. \Yct. Amsterdam 11, 67, 1908. 

 J) Cf. Ann. de 1'Inst. Pasteur 40, 455, 1926. 



") Versl. Kon. Akad. v. Wet. Amsterdam 30, 431, 1921 

 5) Centralbl. f. Bakt. u. Parasitenk. II, 7, 33, 1901. 



