528 RESPIRATION AND FERMENTATION 



seeds germinate and assimilate large' quantities of free oxygen 1 . The 

 respiratory products may therefore vary qualitatively and quantitatively 

 according to the external conditions and the substances consumed. When, 

 however, a fungus fed with peptone produces an abundance of ammonia, the 

 latter probably arises not in the actual process of respiration but during 

 the preparatory changes which provide the material for combustion, and 

 energy may be obtained by the combustion of substances which take no 

 part whatever in constructive metabolism. 



Similarly the ratio of carbon dioxide exhaled to oxygen absorbed differs, 

 as might be expected, according to the respiratory activity and the material 

 consumed : thus it usually becomes smaller in fungi fed with carbon com- 

 pounds poor in oxygen, but may be greater than unity when the food contains 

 a high percentage of combined oxygen. Similarly any formation of organic 

 acid must also decrease the respiratory quotient, and as a matter of fact the 

 evolution of carbon dioxide by the Crassulaceae may almost or entirely cease 

 when the former process is active. In other cases the quotient is such as to 

 indicate that the substances consumed are completely oxidized into carhpn 

 dioxide and water, and in the case of carbohydrates the respiratory quotient 

 is then unity. This usually occurs in adult organs, whereas in growing plants, 

 owing to the production of organic acids and other substances, the quotient 

 is usually less than unity even when carbohydrates are consumed. Neither 

 the materials consumed nor the products afford any indication as to the 

 actual course of physiological combustion, and the constancy of the quotient 

 during changes of temperature simply indicates that the character of the 

 respiratory processes is not altered by their increased or diminished activity. 

 In members of the Crassulaceae and -in Aspcrgillns, however, a rise of 

 temperature may cause an increased combustion of organic acid and hence 

 a transitory increase in the amount of carbon dioxide evolved, until equili- 

 brium is again attained (Sects. 56, 86). 



In most plants and in all the higher plants carbon dioxide is the sole 

 gaseous product of respiration, if \ve neglect water-vapour-. Many lower organ- 

 isms may however produce other gases even when living aerobically, but it is 

 not quite certain whether this is due to a deficiency of oxygen (Sects. 98, 102) 

 or to other causes. Thus the ammonia evolved by fungi fed with peptone 

 does not seem to be a respiratory product, and similarly it is doubtful 

 whether the nitrogen liberated by certain aerobic nitro-bacteria, and a few 



1 Hellriegel, Journ. f. prakt. Chemie, 1855, Bd. LXIV, p. 102 ; Laskovsky, Versuchsst., 1874, 

 Bd. xvn, pp. 235, 240; Detmer, Unters. iiber die Keimung olhaltiger Samen, 1875, p. 30; God- 

 lewski, Jahrb. f. wiss. Bot., 1882, Bd. XIII, p. 507. 



2 Confirmed by the researches of Bonnier et Mangin, Ann. d. sci. nat., 1884, vi. sen, T. xvn, 

 p. 265; ibid., T. xvin, p. 314; T. xix, p. 228; Miintz, Ann. d. chim. et d. phys., 1876, v. ser., 

 T. vni, p. 67 ; Sachsse, Keimung von Pisum, 1872, p. 19. The older contradictory results obtained by 

 Fleury, Vogel, &c., were probably due to the lack of oxygen or to the presence of putrefactive bacteria. 



