2 26 PHYSIOLOGY OF NUTRITION 



extract of the plant tissue is prepared with boiling water and filtered. The 

 addition of peroxidase and hydrogen peroxide to the nitrate thus obtained 

 produces a red (rarely lilac or violet) color, due to the respiration pigment formed 

 by oxidation of the chromogen, and this rapidly changes with further oxidation, 

 to a dark violet or black. 



Respiration chromogens appear to exist in plant tissues mainly in the form 

 of pro-chromogens, which may be glucosides. To obtain the pro-chromogen of 

 wheat embryos, the material is first extracted with alcohol and the pro-chromo- 

 gen is precipitated from the extract, by acetone. It is soluble in water and is 

 decomposed by emulsin, with the production of the chromogen. The latter is 

 oxidized by peroxidase, without hydrogen peroxide, into the red respiration 

 pigment. The experiments of Combes 1 showed that the transformation of the 

 chromogen into the pigment is accompanied by increased respiratory activity. 

 An alkaline solution of chromogen absorbs oxygen very actively. 2 Some of 

 the natural plant dyes are obtained by the complete oxidation of chromogens. 3 



The chromogens appear to belong in the same class with ortho-dioxy- 

 benzene. 4 Urushiol, the chromogen of Japanese lacquer (from Rhus vernicifera, 

 etc.), has the formula C 2 oH 3 o0 2 and its structure is that of o-dioxy-benzene with 

 a large, unsaturated side-chain. 



By forming water, the respiration chromogens remove the hydrogen pro- 

 duced by the respiration process. If the pigment be represented by the letter 

 R, this reaction is shown by the equation: R (pigment) + H 2 (hydrogen) = 

 R-H 2 (chromogen). By the action of oxidase, the chromogen, as it is pro- 

 duced, absorbs oxygen from the air and forms water and the pigment, as accord- 

 ing to the equation: R-H 2 (chromogen) + O (oxygen) = H 2 (water) + 

 R (pigment). Thus the respiration pigments may be regarded as acceptors of 

 hydrogen (see page 210.) 



§10. Respiratory Enzymes. 5 — Recent studies agree in indicating that plant 

 respiration is the summation of a number of fermentation or enzymatic proc- 

 esses. If plants are killed without destroying their enzymes, the production 

 of carbon dioxide and the absorption of oxygen still continue, but in such cases 

 only the primary, anaerobic phase of the process (corresponding to alcoholic 

 fermentation) is present. In some kinds of plants thus killed, in spite of the 

 fact that they are plentifully supplied with peroxidase, the secondary, direct- 



1 Combes, Raoul, Les ^changes gazeux des feuilles pendant la formation et la destruction des pigments 

 anthocyaniques. Rev. gen. bot. 22: 177-212. 1910. 



2 Rupe, Hans, Die Chemie der naturlichen Farbstoffe, Braunschweig, 1900, 1909. 2 v. [This state- 

 ment and citation are omitted in the 7th Russian Edition. — Ed.] 



3 Palladin, V. I. [W.], and Tolstaia, Z. N., Sur l'absorption de l'oxygene par les chromogenes respira- 

 toires des plantes. [Russian.] Bull. Acad. Imp. Sci. St.-Petersbourg VI, 7 : 93-108. 1913. [Also reported 

 in the following reference.] Palladin, W., and Tolstaja, Z., Ueber die Sauerstoffabsorption durch die- 

 Atmungschromogene der Pflanzen. Biochem. Zeitsch. 49: 381-397. 1913- 



4 Majima, R., and S. Cho, Ueber einen Hauptbestandteil des japanischen Lackes. (Vorlaufige Mitteil- 

 lung.) Ber. Deutsch. Chem. Ges. 40 IV : 4390-4393- 1907. Majima, Riko, Ueber den Hauptbestandteil 

 des Japanlacks. (I. Mitteilung.) Ueber Urushiol und Urushiol-dimethylather. Ibid. 42 7 : 1418-1423. 

 1909. Idem, Ueber den Hauptbestandteil des Japanlacks. (II. Mitteilung.) Die Oxydation des Uru- 

 shioldimethylathers mit Ozon. (I. Mitteilung.) Ibid. 42 /7/ : 3664-3673. 1909. Idem, Ueber den Haupt- 

 bestandteil des Japanlacks. (III. Mitteilung.) Die katalytische Reduktion von Urushiol. Ibid. 4S 7/ : 

 2727-2730. 1912. 



6 Palladin, 1909. [See note 3, p. 207.] 



