252 PIGMENTS 



glucosides or may be decomposition products of proteins. 

 These pro-chromogens, by the action of enzymes, give origin 

 to chromogens. 



Chromogens are widely distributed in the vegetable king- 

 dom, in fact are universally present in those parts of plants 

 which are respiring ; they, however, vary in amount at differ- 

 ent seasons of the year and according to the physiological 

 condition of the plant. For instance, in the spring they occur 

 in abundance in the young leaves, and in the autumn the old 

 and dead leaves also contain much owing to the lack of co- 

 ordination of enzymic activity. 



At other times the amount of chromogens is not very 

 great, but may be increased by suitable treatment. Thus 

 Palladin found that leaves kept for a week in a strong solu- 

 tion, 20-30 per cent, of cane sugar showed a great increase, 

 whereas leaves kept in distilled water and also untreated leaves 

 of the plant showed no such increase. A bright illumination 

 also increases the amount of chromogens. 



The chromogens are acted upon by oxidases in the 

 presence of oxygen and yield pigments which may be reduced 

 by reducing enzymes or reductases. Carotin and Xanthophyll 

 provide convenient examples. Carotin, C^gH^g, is acted upon 

 by an oxidase and converted into Xanthophyll, Cj^Hj^^-Og, which 

 in turn is acted upon by a reductase yielding carotin. This 

 action is comparable to that of the haemoglobin in the blood, 

 and in fact Palladin has termed all such respiratory pigments 

 of plants, no matter what their composition may be, phyto- 

 haematins. 



It does not follow that all definite coloured compounds 

 are formed during respiration ; it all depends on the relative 

 activities of the oxidases and the reductases. A pigment will 

 make its appearance provided the oxidases are the more 

 potent, but if the reductases are the more active no pigment 

 will appear. 



The method of indicating the presence of a chromogen is 

 obvious ; the material to be examined is extracted and heated 

 to a degree of temperature sufficient to destroy any enzymes 

 present. To this preparation is added peroxidase and hydrogen 

 peroxide ; if a chromogen were present originally, then a 

 coloration will result, usually brown, red or purple. 



