THE PRODUCTS OF AEROBIC RESPIRATION 527 



two days 1 , so that the process may be even more active than normal" 

 respiration. It is possible that certain micro-organisms may even obtain 

 energy by the physiological combustion of hydrogen ' 2 . 



According to Winogradsky, nitro- and sulphur-bacteria produce little 

 or no carbon dioxide, but it is still doubtful whether any organic material 

 at all is oxidized by these organisms, or whether under certain circum- 

 stances organic substances may be consumed in respiration. 



The physiological combustion of carbon-compounds, either normally or 

 under special conditions, may frequently only proceed as far as the forma- 

 tion of organic acids, and certain vinegar-bacteria may evolve no carbon 

 dioxide when oxidizing an abundance of alcohol to acetic acid (Sect. 103). 

 Similarly the Crassulaceae exhale no carbon dioxide when first placed in 

 darkness, in spite of an active absorption of oxygen, and normal respiration 

 commences only when a large amount of malic or other acid has accumu- 

 lated. Hence if the acid were removed as fast as it was formed, the 

 excretion of carbon dioxide might be permanently suppressed in these 

 plants (Sects. 56, 86). In fungi, however, carbon dioxide is always exhaled 

 even when the production of oxalic, citric or other acids is greatest. 

 A decreased formation of oxalic or other organic acids is accompanied 

 by an increase in the ratio of carbon dioxide exhaled to oxygen absorbed, 

 and hence it is evident that the organic acids are direct products of physio- 

 logical combustion 3 . Organic acids, though imperfectly oxidized com- 

 pounds, are not produced because of any deficiency in the supply of oxygen 

 but because the metabolic activity assumes this character for the time being. 

 Similarly, nitrite-bacteria are permanently incapable of oxidizing the nitrous 

 acid they produce into nitric acid. Citromyces when deprived of sugar is, 

 however, able to oxidize citric acid still further into carbon dioxide and water, 

 and Bacterium aceti can do the same with acetic acid when alcohol is deficient, 

 so that the course of metabolism is here liable to regulatory modification 

 adapted to the prevailing conditions (Sect. 93). It does not however follow 

 that respiration always necessarily involves two or more such stages of 

 successive oxidation, for the combustion may proceed directly without the 

 intervention of any intermediate products such as oxalic acid, &c. Similarly 

 oil may either be directly consumed in respiration, or may merely partially 

 combine with oxygen ultimately producing sugar, as for example when oily 



1 \Yinogradsky, Bot. Zeitung, 1887, pp. 547, 590. Cf. Sect. 63, and Pfeffer, Energetik, 1892, 

 p. 207. There is no reason to give special names to these different varieties of respiration, as Detmer 

 proposes (JDetmer, Jahrb. f. wiss. Bot., 1879-81, Bd. xn, p. 245). On the structure of sulphur- 

 bacteria, cf. A. Fischer, Unters. iiber Cyanophyc. u. Bact., 1897, p. 88. 



2 According to Immendorf ^Landw. Jahrb., 1892, Bd. XXI, p. 323), certain micro-organisms are 

 able to cause the gradual combination of a mixture of hydrogen and oxygen. 



3 Cf. Sect. 86; also Wehmer, Bot. Zeitung, 1891, p. 535; Kunstmann, Uber das Verhiiltniss 

 zwischen Pilzernte u. verbrauchter Nahrung, 1895. 



