342 PIGMENTS 



arctic plants, for instance, which live under conditions un- 

 favourable for metabolic activities. 



Combes holds views similar to those of Palladin, that 

 anthocyanin is closely connected with respiration. If the sugar 

 content increases, the rate of respiration is accelerated, and this 

 leads to the formation of the pigment. 



Although it is not proposed to enter into a detailed con- 

 sideration of the phenomena of respiration here, brief men- 

 tion may be made of Palladin's * conceptions on the subject 

 on account of the role he ascribes to colouring matters and 

 allied substances in respiratory activity. 



Occurring in plants are pro-chromogens which may be 

 glucosides ; these pro-chromogens, by the action of enzymes, 

 give origin to chromogens. Their presence is indicated by the 

 appearance of a reddish colour on the addition of peroxidase 

 and hydrogen peroxide to a hot-water extract of the tissue. 



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 to 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 increased 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. 



A chromogen which satisfies the requirements of Palladin's 

 respiratory pigments has been found to occur in Mercurialis 



* Palladin : " Ber. deut. bot. Gesells.." 1908, 26a, 125, 378. 389 ; 

 1909, 27, no. 



