THE CHEMICAL PROCESSES IN PLANT RESPIRATION I45 



one-third of all the CO2 may be traced to oxygen respiration 

 when air is admitted; hence the amount of sugar used in respira- 

 tion is one-ninth of that used in fermentation, and the energy 

 derived from the oxygen respiration is about two and a half 

 times as great as that furnished by alcohoHc fermentation when 

 oxygen is excluded. The previously unexplained meaning of 

 oxygen for the propagation of yeast has now become easily 

 understood. When oxygen is excluded the energy production 

 of the yeast is so low that the multipHcation of cells proceeds 

 but slowly. 



So with free access to oxygen, alcohoHc fermentation is a 

 wholly unessential process for yeast. For that reason it con- 

 tinues only because the yeast cells contain a large quantity of 

 the enzymes of fermentation which are essential to the anaerobic 

 life of the yeast. But in the presence of oxygen a complete 

 combustion of such enormous quantities of sugar would be 

 wholly superfluous. So with good aeration the unoxidised, 

 intermediate products of fermentation are changed into alcohol 

 and CO2, since the atmospheric oxygen does not attack these 

 intermediate products. Kostychev and Eliasberg have shown 

 by means of direct determinations of sugar, alcohol and COo, 

 that some mucors such as M. racemosMs, M. mucedo and Rhi- 

 zopus nigricans behave like yeast and in the presence of oxygen 

 maintain oxygen respiration and alcoholic fermentation simul- 

 taneously. The explanation given for yeast is strengthened in 

 many respects by studies of the Mucoraceae. For example, the 

 alcoholic fermentation of Rhizopus nigricans is so weak in the 

 presence of oxygen that it often does not suflice for the require- 

 ments of the life of the fungus. Yet it continues during 

 unrestricted aeration because the ratio of the oxidising enzymes 

 to those of fermentation prevents a complete combustion of 

 sugar. 



Second Case. — The quantities of oxidising and fermentation 



enzymes are so coordinated in the plant cells that the velocity 



of the primary sugar cleavage is always less than the velocity 



of the subsequent oxidation. Consequently alcohol is never 



formed when air is admitted, since all the intermediate products 



pass over into the end-products of oxygen respiration. This 

 10 



