308 PLANT PHYSIOLOGY 



Hexose 



through zymase action 



I 



Labile Intermediate Substances 



/ \ 



oxygen absent oxygen present 



/ \ 



Carbon Dioxide and Alcohol Carbon Dioxide and Water. 



Since pyruvic acid and acetaldehyde have been found in both 

 aerobic and anaerobic respiration, they are doubtless some of the 

 " labile intermediate substances" that are ultimately converted 

 into carbon dioxide and alcohol or water, depending upon the 

 absence or presence of oxygen. 



Source of the Energy. — Precisely how the energy is derived 

 in such respiration is not known. It will be seen that even in so- 

 called aerobic respiration, the oxygen does not unite with the 

 food but with the hydrogen produced by its hydrolysis. In this 

 sense, even normal respiration is anaerobic; so that it is not sur- 

 prising to find that energy is derived from the splitting of the 

 molecules or in other ways which do not require an access of free 

 oxygen. Many plants grow where oxygen is not abundant, as in 

 marshes, flooded soils, standing water, etc. It may be advan- 

 tageous to them to be able to live without free oxygen, when the 

 power to respire anaerobically would consequently have high sur- 

 vival value in the struggle for existence. 



Fermentation. — The term fermentation was originally used 

 to mean the decomposition of organic substances. The breaking 

 down of sugars into alcohol and carbon dioxide, the spoiling of 

 fruit, the souring of milk, the formation of vinegar, the rotting of 

 vegetables, and the development of butyric acid in butter, were 

 all called fermentation. Pasteur was the first to realize that these 

 operations were the result of the growth of microorganisms under 

 conditions (in general) when little oxygen was present, and he 

 therefore regarded fermentation as "life without oxygen." The 

 word came later to mean the decomposition of materials by or- 

 ganisms and was thought to be a property peculiar to life itself, 

 a so-called vital process. It is now known that these fermentation 

 processes are all brought about by enzymes, which may be isolated 

 from the organisms and made to do their work in vitro. The term 



