ANTHOCYANINS 343 



and is known as hermidin.* This chromogen, on oxidation 

 by atmospheric oxygen, is converted into a blue compound, 

 cyanohermidin, which in turn may be further oxidized to a 

 yellow compound, chrysohermidin. Fresh chrysohermidin is 

 reduced to cyanohermidin, and cyanohermidin is readily re- 

 duced to hermidin by the plant, and in vitro by such reducing 

 agents as nascent hydrogen or sodium hydrosulphite. These 

 changes, therefore, are easily reversible, a lability indispensable 

 in a respiratory chromogen. 



It will be obvious that the amount of pigment produced 

 depends on the relative activities of the oxidizing and reducing 

 agents ; if the former be the more potent, a coloured substance 

 will be formed, but if the latter be dominant, no pigment will 

 appear. In the living healthy plant, if the thesis is held to be 

 true, the balance will be justly maintained, the mechanism 

 being one for the transference not the holding of oxygen ; 

 wherefore no colour change will be visible. The opinion of 

 Oparin that chlorogenic acid is also a respiratory pigment has 

 been alluded to on page 306. 



In addition there is another respiratory pigment first 

 described by MacMunn and subsequently reinvestigated by 

 Keilin f ; it is a modified haemochromogen, known as cyto- 

 chrome, and occurs in the tissues of many animals, in aerobic 

 bacteria, yeast, and in the non-green parts of higher plants 

 such as the bulbs of shallot and allied plants, potato tubers 

 and so on. 



Cytochrome is an intracellular respiratory catalyst, the 

 spectrum of which in the reduced state shows four character- 

 istic absorption bands ; in the oxidized form, these bands 

 disappear, a faint shading only being visible. Oxidation 

 easily is effected by the air, and reduction is brought about 

 by the normal activity of cells or by a simple chemical agent 

 such as sodium hydrosulphite. These spectral changes are 

 recognizable only under the microspectroscope ; no colour 

 change is visible to the unaided eye. 



* Haas and Hill : " Biochem. Journ.," 1925, 19, 233, 236 ; " Ann. 

 Bot.," 1925, 39, 861 ; 1926, 40, 709. 



t Keilin : " Proc. Roy. Soc," B., 1925, 98, 312 ; 1926, 100, 129. 



