SULPHUR AND NITROGEN BACTERIA 231 



Bibliography to Lecture XVIII. 



BAUMANN. 1888. Landw. Versuchsstationen, 35, 217. 



BEIJERINCK. 1895. Centrbl. Bakt. II, i, i. 



BEIJERINCK. 1900. Ibid. II, 6, 193. 



BEIJERINCK. 1901. Ibid. II, 7, 33. 



[DELDEN. 1903. Centrbl. Bakt. II, 9, 8 1.] 



ENGELMANN, TH. W. 1888. PFLUGER'S Archiv. 42, 183. 



FRANKLAND, G. and P. 1889. Zeit. f. Hygiene, 6, 373. 



FRANKLAND, G. and P. 1890. Phil. Trans. Roy. Soc. B, 181, 107. 



GODLEWSKI. 1895. Anzeiger d. Akad. Krakau. 



HERAEUS. 1886. Zeit. f. Hygiene, i, 193. 



HOPPE-SEYLER. 1 886. Zeit. f. phys. Chem. 10, 201. 



HUPPE. 1887. Biolog. Centrbl. 7, 701. 



[MiQUEL. 1904. In LAFAR'S Handb. d. techn. Mykologie, 3, 82.] 



MOLISCH. 1892. Die Pflanze und ihre Beziehungen zum Eisen. Jena. 



NATHANSOHN. 1902. Mitt. d. zoolog. Station Neapel, 15, 655. 



OMELIANSKI. 1899. Centrbl. Bakt. II, 5, 473. 



OMELIANSKI. 1902. Ibid. II, 9, 63. 



[OMELIANSKI. 1904. LAFAR'S Handb. d. techn. Mykologie, 3, 214 and 234.] 



[RULLMANN. 1904. In LAFAR'S Handb. d. techn. Mykologie, 3, 193.] 



SCHLOSSING and MUNTZ. 1877-79. Compt. rend. Paris, 84, 301 ; 85, 1018 ; 86, 



892 ; 89, 891, 1074. 



WARINGTON. 1888. Centrbl. Bakt. 6, 498. 



WINOGRADSKY. 1887. Ueber Schwefelbakterien, Bot. Ztg. 54, 493. 

 WINOGRADSKY. 1 888 a. Bot. Ztg. 46, 261. 

 WINOGRADSKY. 1888 b. Beitr. z. Morphologic und Physiologic der Bakterien, I; 



Schwefelbakterien. Leipzig. 

 WINOGRADSKY. 1890-91. Recherches sur les organismes de la nitrification. An- 



nales Institut Pasteur, 1890: Mitt. I : 4, 213-231 ; Mitt. II : 4, 257-275 ; 



Mitt. Ill : 4, 760-771 ; 1891 : Mitt. IV : 5, 92-100 ; Mitt. V : 5, 577-616. 

 WINOGRADSKY. 1892. Contrib. a la morphologic des organismes de la nitrification 



(Archives d. Sc. biolog. St. Pe'tersbourg i). 



[WINOGRADSKY. 1904. LAFAR'S Handb. d. techn. Mykologie, 3, 132.] 

 WIXOGRADSKY and OMELIANSKI. 1899. Centrbl. Bakt. II, 5, 329. 



LECTURE XIX 



DENITRIFICATION AND NITROGEN FIXATION. SYMBIOSIS AND 

 METABIOSIS. CIRCULATION OF CARBON AND NITROGEN. 



WE have already drawn attention on several occasions to the problems 

 connected with the circulation of nitrogen in the organic world. In Lecture XI 

 we established the fact that the green plant supported itself in the first instance 

 on nitric acid, and constructed proteid out of that substance ; in Lecture XVII 

 we found that on the death of the green plant the proteid was broken down 

 by micro-organisms in such a way that the nitrogen was finally transformed 

 for the most part into ammonia. When, owing to the combined activity of 

 nitrite- and nitrate-bacteria, the ammonia is changed again into nitric acid, 

 the circle is completed, and the nitrogen once more appears in a form which 

 green organisms can appropriate. The cycle is not, however, quite so simple 

 as this as regards all the nitrogen ; a complication appears when in certain 

 processes gaseous nitrogen is formed, and when in other cases free nitrogen may 

 be seen to undergo transformation into a combined form. Reference has been 

 already made to these processes, but some of them require further explanation. 



We have already seen (p. 219) that free nitrogen as well as ammonia may 

 be produced during the decomposition of proteid. The conditions under 

 which this takes place have not as yet been fully determined, but the process 



