RESPIRATION. 259 



With regard to the physiological significance of these pig- 

 ments in the Algae, the work of Gaidukov * on complementary 

 chromatic adaptation may be consulted. 



RESPIRATION. 



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'sf 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 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 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 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^H^, is acted upon 

 by an oxidase and converted into Xanthophyll, C 40 H 56 O 2 , 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 



*See Blackman : "New Phytologist," 1904, 3, 237. 



t Palladin : " Ber. deut. hot. Gesells.," 1908, 26a, 125, 378, 389 ; 1909, 27, no. 



