232 PRINCIPLES OF SOIL MICROBIOLOGY 



1889 that algae are able to fix atmospheric nitrogen; his results were 

 substantiated by other workers, who used impure cultures of algae, 

 Kossowitch, 45 however, who was the first to use pure cultures of algae, 

 namely a species of Cystococcus (Nageli) and Chlorella vulgaris Beij., 

 demonstrated that algae do not fix any atmospheric nitrogen, and that 

 some of the bacteria are the only organisms capable of doing that. In 

 general, whenever pure cultures of algae were employed, nitrogen fixa- 

 tion was found to be negative. 43 ' 46 



But even if algae do not fix any nitrogen by themselves, they were 

 found to exert a favorable effect on the process of nitrogen-fixation by 

 non-symbiotic bacteria. Kossowitsch, therefore, suggested that the 

 algae work in a manner symbiotically with nitrogen-fixing bacteria, 

 furnishing them, through photosynthetic activity, with carbohydrates. 

 These results were confirmed by other investigators, 47 especially in the 

 case of Azotobacter and algae. 48-51 



Beijerinck considered those organisms that can live on media to 

 which no nitrogen has been added (without being, however, free from 

 combined nitrogen), to be able to fix atmospheric nitrogen; he con- 

 cluded, therefore, that various Cyanophyceae are able to fix atmospheric 

 nitrogen, since they grew in media almost free from combined nitrogen. 

 These media were not free from nitrogen-fixing bacteria, however, and 

 no analytical data were presented. In a series of carefully controlled 

 experiments with pure cultures of several grass-green algae, Schramm 52 



46 Kossowitsch, P. Untersuchungen iiber die Frage, ob die Algen freien Stick- 

 stoff fixieren. Bot. Ztg. 52: 97-116. 1894. 



46 Charpentier, P. G. Alimentation azot£e d'une algue, le Cystococcus humi- 

 cola. Ann. Inst. Past. 17: 321-334, 369-420. 1903. 



47 Kriiger, W., and Schneidewind, W. Sind niedere chlorophyllgrune Algen 

 imstande den freien Stickstoff der Atmosphaere zu assimilieren und den Boden 

 an Stickstoff zu bereichern? Landw. Jahrb. 29: 776-804. 1900. 



48 Reinke, J. Symbiose von Volvox and Azotobacter. Ber. deut. bot. Gcsell. 

 21: 481. 1903. Fischer, H. Uber Symbiose von Azotobacter mit Oscillarien. 

 Centrbl. Bakt. II, 12: 267-268. 1904. 



49 Heinze, B. Einige Beitrage zur mikrobiologischen Bodenkunde. Centrbl. 

 Bakt. II, 16: 640-663, 703-711. 1906; Uber die Stickstoffassimilation durch nie- 

 dere Organismen. Landw. Jahrb. 35: 889-910. 1906. 



60 Nakano, H. Untersuchungen fiber die Entwicklungs- und Erniihrungs- 

 physiologie einiger Chlorophyceen. Jour. Coll. Sci., Tokyo Univ. 40: 66. 1917. 



" Pringsheim, 1913 (p. 220). 



62 Schramm, J. R. A contribution to our knowledge of the relation of certain 

 species of grass-green algae to elementary nitrogen. Ann. Mo. Bot. Gard. 1: 

 157-184. 1914. 



