204 PHYSIOLOGY OF NUTRITION 



oxidation phase is entirely absent; in other kinds of plants direct oxidation 

 occurs, but quite difierently from its occurrence in the living organism. 



In an atmosphere free from oxygen the anaerobic phase of respiration, in 

 plants killed without injury to the enzymes, is accompanied by the production 

 of both carbon dioxide and ethyl alcohol, but in most plants the production of 

 alcohol is the less vigorous of the two processes. Under such conditions the 

 formation of carbon dioxide ceases after a time, and if oxygen is then admitted 

 to the tissues this formation may begin again or not, according to the kind of 

 plant employed. In some forms (e.g., wheat embryos) no further production 

 of carbon dioxide takes place. In other forms the killed plants (with their 

 enzymes still intact) give off carbon dioxide even more vigorously in the presence 

 of oxygen than they did in its absence. , In this latter case, however, the carbon 

 dioxide produced after the admission of oxygen is not to be considered as the 

 product of direct oxidation. For example, Palladin and Kostychev* found that 

 germinating peas, killed without injury to the enzymes, developed considerably 

 larger amounts of carbon dioxide when air was admitted than they did in the 

 absence of oxygen. Alcohol formation was Hkewise increased, however, so 

 that the acceleration of carbon dioxide formation cannot be regarded as the 

 direct result of oxidation. In such cases Ivanov^ supposes that the oxygen is 

 supplied by the activity of the enzyme zymase. 



Another example may be presented. Etiolated bean leaves that had been 

 killed by freezing were deprived of oxygen until carbon dioxide ceased to be 

 given off, after which air was admitted, when the ehmination of carbon dioxide 

 was resumed and the leaves became black, as a result of the oxidation of the 

 chromogen. Although the renewed production of carbon dioxide was not here 

 accompanied by alcohol formation, still, we must refrain from supposing that it 

 was the direct result of oxidation, as will become clear from the following 

 considerations. 



Anaerobic decomposition is accompanied not only by the evolution of carbon 

 dioxide but also by the production of hydrogen. In some plants this hydrogen 

 disappears in the reduction (to form alcohol) of intermediate products of the 

 decomposition, but in most plants little alcohol is formed. In the latter case 

 the hydrogen must unite either with the respiration pigment or with some o&er 

 hydrogen acceptor, and when all acceptors of hydrogen become satisfied (having 

 taken up all the hydrogen they can) the further decomposition (and, conse- 

 quently, the evolution of carbon dioxide) should stop. When the cells are ex- 

 posed to the air the acceptors of hydrogen oxidize their hydrogen to water, 

 however, and thus become able to absorb still more hydrogen. Therefore, 

 exposure to air results first in the regeneration of the hydrogen acceptors, which 

 is accompanied by a renewal of hydrogen absorption and a consequent renewal 

 of anaerobic decomposition, the latter being, of course, accompanied by the 

 giving-off of carbon dioxide, just as occurred at first. It is thus seen how this 



' Palladin and Kostytschew, 1906. [See note 37 p. 195.] 



2 IwanoSf, Leonid, Ueber die Sogenannte Atmung der zerriebenen Samen. Ber. Deutsch. Bot. Ges. 

 29: S63-570. 191 1. 



