AEROBIC AND ANAEROBIC RESPIRATION 549 



subsequent secondary reactions (Sects. 77 and 98). Moreover in certain 

 cases the action of free oxygen may be mainly confined to inhibiting 

 these secondary reactions, and thus a marked apparent result may be 

 produced, although the actual anaerobic respiration (or fermentation) 

 continues unchecked. The pronounced general reducing action is probably 

 for the most part, if not entirely, due to certain products of anaerobic 

 metabolism, and is not directly caused by the induction of those affinities, 

 which in aerobic life cause free oxygen to enter into molecular combination 

 with the protoplasm. Most plants, including many ferment-organisms, 

 have no such reducing power, and no such action is exercised upon reagents 

 imbibed by the plasma even when all free oxygen is removed. It is, 

 however, possible that the protoplast in certain cases withdraws oxygen 

 from substances with which the latter is loosely combined, so that certain 

 anaerobes may obtain oxygen from reducible substances 1 . 



The reduction of solutions of indigo, litmus, and other substances which do 

 not penetrate the protoplast is undoubtedly due to the action of special metabolic 

 products, for in coloured gelatine the reduction may occur at a certain distance 

 from the bacterial colonies 2 . The sulphuretted hydrogen which is frequently 

 evolved suffices to produce this result, which, however, in other cases is due to 

 a different product, while in others again, no reducing action is exhibited. 

 Saccharomyces is able to evolve a slight amount of this gas under certain con- 

 ditions, and hence to exercise a corresponding power of reduction 3 . 



Nitrates may in part be reduced to nitrites by the action of excretory products, 

 but this does not occur in all cases where indigo is reduced 4 . Nascent hydrogen 

 may exercise very marked reducing powers within the cell and may perhaps be 

 even capable of producing sulphuretted hydrogen from sulphates. This, however, 

 must occur under special conditions only, for usually no reduction of sulphates is 

 induced in spite of the formation of hydrogen, nor does the latter always cause 

 nitrates to be reduced to nitrites 5 . 



1 The growth of certain anaerobic bacteria is favoured by the presence of reducible substances 

 (cf. Kitasato u. Weyl, Zeitschr. f. Hygiene, 1890, Bd. vin, p. 41, and Bd. ix, p. 96). 



2 Cf. Fliigge, Mikroorganismen, 1896, Bd. I, p. 169; Hiippe, Methode d. Bakterienforschung, 

 1891, 5. Aufl., p. 257 ; Pfeffer, Oxydationsvorgange, 1889, p. 510, and the literature here quoted; 

 also Smith, Centralbl. f. Bact., 1896, Bd. xix, p. 187. 



3 Rubner, Archiv f. Hygiene, Bd. xix, p. 174; Beyerinck, Centralbl. f. Bact., 1895, Abth. ii, 

 Bd. I, p. 5 ; Nastukoff, Compt. rend., 1895, T. cxxi, p. 535, and Ann. d. 1'Inst. Pasteur, 1895, 

 T. ix, p. 766. 



* The literature on reducing bacteria is given by Burri u. Stutzer, Centralbl. f. Bact., 1895, Abth. ii, 

 Bd. I, p. 259. Cf. also Rubner, Koch's Jahresb. d. Gahrungsorganismen, 1893, p. 93; Beyerinck, 

 Centralbl. f. Bact., 1895, Bd. I, p. 58 (Methods). According to Laurent (Ann. d. 1'Inst. Pasteur, 

 1890, T. iv, p. 742), a few mould- fungi exercise a reducing action, which power Beyerinck states is 

 absent from yeast (Butylalkoholgahrung, 1893, p. 48). No reduction of nitrates to nitrites occurs in 

 higher plants (Molisch, Sitzungsb. d. Wien. Akad., 1887, Bd. xcv, Abth. i, p. 242). 



5 Outside of the cell nascent hydrogen reduces nitrates, but not sulphates (Fitz, Ber. d. Chem. 

 Ges., 1876, p. 1349; l8 79> P- 4 S ; Rubner, I.e., 1893, p. 94). 



