228 PHYSIOLOGY OF NUTRITION 



by free oxygen (compare page 224). When oxygen is first admitted to tissues 

 in which the hydrogen acceptors have already been satisfied, the renewed 

 evolution of carbon dioxide is very vigorous. 



As has been stated (pages 203, 211), in the presence of methylene blue 

 and a catalyzer, formic acid decomposes into carbon dioxide and hydrogen, so 

 long as absence of oxygen prevents the regeneration of the methylene blue from 

 the leuco-compound formed from the dye by union with hydrogen; HCOOH + 

 M = C0 2 + M-H 2 , where M represents methylene blue and M-H 2 represents 

 the leuco-compound. Access of oxygen allows the removal of the extra hydro- 

 gen from the leuco-compound and regenerates the methylene blue (M-H 2 + O 

 = M + H 2 0), thus rendering it again able to absorb hydrogen. Consequently, 

 the evolution of carbon dioxide begins anew and it appears, superficially, as 

 though this were the result of the oxidation of the carbon of the formic acid by 

 atmospheric oxygen. Here the methylene blue behaves as an acceptor of hydro- 

 gen, as such acceptors are supposed to act in plant respiration. 



Bach and Battelli/and also Palladin, 2 regard all the carbon dioxide eliminated 

 in respiration as the product of anaerobic fermentation. Palladin thinks water 

 enters into this decomposition reaction; thus, C 6 Hi 2 6 (glucose) + 6 H 2 = 

 6 C0 2 + 12 H 2 . Since much hydrogen should result from this sort of reaction 

 and since hydrogen is never actually given off by higher plants, it follows that 

 the differing capacities of different kinds of plants for the anaerobic evolution 

 of carbon dioxide depend upon the various powers of the plants to carry out 

 reductions that result in alcohol, and upon the differing amounts of hydrogen 

 acceptors present. 



In living plants, the hydrogen produced by anaerobic decompositions is 

 taken up by the respiration pigments, forming the corresponding chromogens. 

 From these it is subsequently removed and oxidized to form water, through the 

 action of oxidase. The reactions are shown by the two following equations, 

 where R represents the pigment and R-H 2 the chromogen. (1) 12 H 2 + 

 12 R = 12 R-H 2 . (2) 12 R-H 2 + 60 2 = 12 HoO + 12 R. It thus appears 

 that the respiration enzymes are water-producing enzymes, carrying out the 

 same reactions in the living plant as they do in vitro. Thus oxidase (or peroxi- 

 dase together with hydrogen peroxide) oxidizes colorless hydroquinone (chromo- 

 gen) to form red quinone (pigment) and water, according to the equation: 



Hydroquinone Quinone 



C 6 H 6 2 + O = C 6 H 4 2 + H 2 0. 

 In plants that have been killed without destroying their enzymes the con- 

 trols that govern the various activities during life are greatly disturbed, and the 

 respiration pigments in such tissues remove not only the hydrogen that they 

 normally take up (this being then oxidized to form water), but also the hydrogen 

 simultaneously being produced by, and taking part in, the anaerobic processes. 

 Consequently, such killed tissues that are rich in chromogens give off more 

 carbon dioxide in the air when they have been previously kept for a time in 



' Bach, A., and Battelli, F., Degradation des hydrates de carbone dans l'organisme animal. Compt. 

 rend. Paris 136: I35i-i353- 1903- 



■ Palladin, 1912 (1, 2). [See note 3. P- 210.] 



