RESPIRATORY PIGMENTS 145 



however, from his observations on the oxidative activities of 

 bacteria, found that fumaric acid could be activated as a 

 hydrogen acceptor, a state of affairs not provided for by the 

 Wieland theory. To quote the author's own words : The 

 inadequacy consisted in confining the activation only to hydro- 

 gen atoms. A more reasonable interpretation of the pheno- 

 mena was that activation of the substrate molecule as a whole 

 occurred, the activated molecule behaving as a hydrogen 

 donator or acceptor, according to the nature of the molecule 

 and the general circumstances under which the reactions 

 occurred. Further investigations with bacteria showed that 

 a variety of substances are activated as hydrogen acceptors, 

 e.g. nitrates and chlorates." 



RESPIRATORY PIGMENTS. 



Palladin * observed that a boiling water extract of many 

 plants contains a colourless chromogen which by the action of 

 a mixture of a peroxidase and hydrogen peroxide undergoes 

 a series of colour changes and ultimately produces a more or 

 less deeply coloured substance known as a respiratory pigment. 

 These pigments are concerned with the oxidation of hydrogen, 

 not with the oxidation of the carbon of organic compounds. 

 The role of respiratory pigments is therefore that of hydrogen 

 acceptors whereby they are reduced to the chromogen 



R + 2H = RH 2 . 



The chromogens are then oxidized by active oxygen arising 

 from the activity of an oxidase which results in the regenera- 

 tion of the pigment 



RH 2 + O = R + H 2 0. 



These chromogens, which occur in the plant in the form of 

 glucosides, are supposed to belong to the group of polyhydric 

 phenols and are therefore bodies of a tannin-like nature. 

 During regular respiration an equilibrium obtains between the 

 oxidation and reduction ; but if the oxygen absorption is 



* Palladin: "Per. deut. Bot. Ges.," 1908, 26a, 125, 378, 389; 1909, 

 27, no; see also " Jabrb. wiss. Bot. ( " 1910, 47, 431; " Zeit. Garungs. 

 Physiol.," 1912, 1, 91. 

 VOL II. — 10 



