THE OXIDASES IN RELATION TO PIGMENTATION 289 



the pigments of this class studied by Willstatter. They 

 are accordingly regarded as oxidation and condensation 

 products. 



(6) The production of dark-coloured substances in injured 

 plant-tissues or in expressed sap is undoubtedly due to the 

 action of an enzyme-system upon more or less colourless chro- 

 mogens. Though this is so, there appears to be no direct 

 proof that oxidases are concerned in the production of the 

 anthocyan pigments, inasmuch as a precursor of antho- 

 cyanin has not as yet been converted into this pigment by 

 the action of added enzyme. Of indirect evidence there 

 is, on the other hand, a considerable amount. Thus, the 

 very general parallel between intensity of oxidase action 

 and depth of pigmentation may be considered to be a 

 weighty argument for this view, even though the distribu- 

 tion of chromogen is the main factor limiting the occur- 

 rence of anthocyanin. To this parallel there are exceptions, 

 notably in the genus Iris, so it cannot be taken as con- 

 clusive evidence. 



The presence of areas free from anthocyanin in tissues, in 

 which it is elsewhere abundant, has often been described. 

 The localization of an inhibitor of oxidase action in such 

 areas furnishes to the writer's mind the most convincing 

 of the proofs as yet available that oxidases are concerned 

 in the processes leading to the formation of this pigment. 



(c) The physiological and biochemical investigations 

 upon the occurrence of anthocyanin and large quantities 

 of sugar in the same tissues point to the importance of 

 the latter in the synthesis of the former. This may be 

 partly explained by the fact that the pigment is a gluco- 

 side, but the quantities of sugar required appear to be far 

 in excess of the proportion necessary to unite with the 

 modified flavone to produce the amount of anthocyanin 

 actually found. 



