270 TEXTBOOK OF PLANT PHYSIOLOGY 



ture is essentially the same as alcoholic fermentation caused by 

 yeast. Therefore, the view has been established that alcoholic 

 fermentation and respiration are closely related phenomena, the 

 former being but a preparatory stage of the latter. 



Formerly the connection between fermentation and respiration 

 was supposed to be of the following nature: The first products 

 formed from sugar are alcohol and carbon dioxide In an oxygen- 

 free medium the process stops at this point, but in the presence of 

 air the alcohol obtained is oxidized further to carbon dioxide and 

 water. This supposition has been given up, however, since during 

 respiration alcohol is oxidized less readily than sugar. The process 

 is now commonly supposed to take place in the following way. 

 Under the influence of zymase, which is always present in cells, 

 some intermediate products of alcoholic fermentation are formed 

 first. The subsequent fate of these products is varied. Alcohol and 

 carbon dioxide are formed of these substances when oxygen is 

 lacking. In the presence of oxygen they break down to carbon 

 dioxide and water before alcohol has been formed. 



Many of these chemical changes still remain obscure. More- 

 over, the fact that it is not known as yet which of the intermediate 

 compounds are oxidized in presence of air creates considerable 

 difficulties in the investigation. In the living plant, the oxidation 

 of sugar proceeds so readily and easily, that it is impossible to 

 determine any intermediate products. Only in certain cases, as in 

 fleshy plants, it may be observed that respiration is accom- 

 panied by an accumulation of organic acids, but these acids are 

 rather accessory than intermediate products. 



The readiness and ease with which physiological oxidation 

 takes place in the living cell, by no means indicates that this 

 process is a simple one. It may be due to a harmonious cooper- 

 ation of all the phases constituting it. In order to study the dif- 

 ferent phases and to destroy their connection the cells are usually 

 killed preferably by freezing, a method that has been noted, which 

 yields valuable results in the study of enzymatic activity. 



The killing of plants for the purpose of learning their respira- 

 tory mechanism has been applied especially by Palladin and his 

 students. He came to the conclusion, that respiration of plants 

 has to be considered as a summation of enzymatic activities. 

 Plants killed with sufficient rapidity and care will continue to 

 eliminate carbon dioxide and to absorb oxygen, but these processes 



