143 



fixation. One might thus expect that he would not have expressed his 

 firm belief in the great importance of Azotobacter chroococcum without 

 having convinced himself that the cultivation of this species indeed 

 leads to a noticeable gain in nitrogen of the medium. However, in 

 BEIJERINCK'S first publication there is no indication that he even 

 attempted to do so. In this connection WINOGRADSKY rightly re- 

 marked : 



"Obgleich wir nun, Dank BEIJERINCK, die genaue Charakteristik 

 dieser Arten jetzt besitzen, bleibt doch immer der wichtigste Punkt 

 noch unaufgeklart, namlich ob dieselben atmospharischen Stickstoff 

 assimilieren konnen oder nicht. Die blosse Thatsache ihres Vorkom- 

 mens in stickstoffarmen Nahrlosungen beweist naturlich nichts." 



In view of all this there remains only one explanation for BEIJE- 

 RINCK'S discovery, namely, intuition or even better, genius! And if 

 WINOGRADSKY in 1893 failed to deal adequately with the situation, 

 the reason can only be that at that time his genius had been too much 

 captivated by his great discovery of Clostridium Pastorianum. 



After this circumstantial historical introduction to the Azotobacter 

 discovery only a few more remarks will be made on BEIJERINCK'S 

 further contributions to the problem of microbial nitrogen fixation. 

 In the first place it should be emphasized that the way in which he 

 described the various stages of development of Azotobacter chroococcum 

 is exemplary. It is noteworthy that he succeeded in completely 

 avoiding the pit-falls of which several later investigators have been 

 become the victims. 



Then it is characteristic for BEIJERINCK'S universality and thor- 

 oughness that already in his first publication he described a second, 

 clearly distinct species of his new genus, viz., Azotobacter agilis. 

 BEIJERINCK found that this second species, with its much larger cells, 

 usually predominates in the enrichment cultures, if canal water, in- 

 stead of soil, is used for the inoculation. In a fairly recent paper, 

 published 32 years after the discovery of Azotobacter agilis, it was 

 concluded that this organism had until that time not been isolated 

 except from Dutch canal waters l ).lt seems probable that this second 

 Azotobacter species which also exhibits a good nitrogen fixing power, is 

 of material significance for the economy of fresh-water communities, 

 at least, in those regions in which the water is not free from pollution. 



Of the later publications of BEIJERINCK on "oligonitrophilous 

 microbes" we pass over those dealing with the photo-synthetically 

 active microbes, because they have been considered in Chapter XVI. 



i) A. J. KLUYVER und W. J. VAN REENEN, Archiv f. Mikrobiol. 4, 280, 1933; cf. also 

 A. J. KLUYVER und M. T. VAN DEN BOUT, Ibid. 7, 261, 1936. 



It is interesting to add that since the appearance of the first paper, HUGH NICOL, at 

 Rothamsted, isolated a strain of A . agilis from a drainage ditch at Oby Mill, Norfolk. 

 England (Private communication; cf. E. J. RUSSELL, Soil Conditions and Plant 

 Growth, 7th Ed., 1937, p. 384). More recently WINOGRADSKY has also isolated typical 

 strains of A . agilis from surface waters in France. Cf. S. WINOGRADSKY, Ann. de 1' Inst. 

 Pasteur 60, 351, 1938. 



