102 PLANT PHYSIOLOGY 



The'development of thought on the chemical nature of respira- 

 tion proceeded for a very long time in two directions, which 

 seemed to be irreconcilable. One theory was based on the 

 activation of hydrogen, and the other, on the activation of 

 oxygen. 



The theory of activation of hydrogen was developed chiefly 

 by Wieland. According to Wieland's theory, during the proc- 

 esses of oxidation that take place in complex systems containing 

 the atoms of carbon, hydrogen, and oxygen in different com- 

 binations, one phase of the system of oxidized substances loses 

 hydrogen, while the other gains it. The substances binding 

 hydrogen are called '' hydrogen acceptors," and the first phase 

 of oxidation may be expressed by the following equation: 



XH2 + acceptor — ^ x + acceptor H2. 



In the following and final stage of oxidation, the hydrogen, 

 united to the acceptor, connects with oxygen. The acceptor, 

 being freed, may again take on hydrogen from the substance 

 oxidized. 



Wieland extended his theory of dehydrogenation to processes 

 of oxidation-reduction that proceed with the introduction of the 

 elements of water. Thus for instance, according to his opinion, 

 in oxidation of aldehydes a hydrate is first formed through 

 addition of a molecule of water. 



CH3COH + H2O -> CH3CH(OH)2. 



Then 2 atoms of hydrogen are transferred to the acceptor. 



CH3CH(OH)2 + acceptor -^ CH3COOH -f acceptor H2. 



The role of the acceptor is frequently carried out by a second 

 molecule of aldehyde, which transforms into a molecule of alcohol. 

 Thus one of the most important reactions of oxidation-reduction, 

 taking place readily in water solutions of aldehydes and leading 

 to a simultaneous formation of alcohol and acid, is the Cannizzaro 

 reaction. It is usually represented by the following simplified 

 equation : 



CH3COH H2 -^ CH3CH2OH 



+ I 

 CH3COH O -^CHaCOOH 



The Cannizzaro reaction was very often used to explain the 

 processes taking place during fermentation (Neuberg, Kosty- 



