F. Keeble and E. F. Armstrong 279 



particularly by Palladin and Miss Wheldale. The latter investigator 

 (1910 loc. cit.) has formulated in a clear and schematic manner the 

 following hypothesis as to the course of events which leads to the 

 formation of pigment in plants. The colourless chromogen by the 

 oxidation of which pigment is produced occurs in the plant as the 

 constituent of a glucoside. In this combined form it resists oxidation. 

 Enzymes of the emulsin type hydrolyse the glucoside and liberate 

 chromogen which is then oxidised by atmospheric oxygen made active 

 by the oxydase. Thus an anthocyan or soluble sap pigment is formed. 

 Such anthocyan pigments, which are common in the flowers and other 

 parts of plants, are of very different chemical constitution and origin 

 from the plastid pigments which are also of common occurrence. We 

 are concerned here only with the former. The anthocyan pigments 

 are generally red, violet or blue. 



The chemical nature of the anthocyan pigments is obscure. They 

 are regarded by Miss Wheldale as flavone derivatives which are known 

 to be widely distributed in plants in the form of glucosides. 



An illuminating paper suggesting a mode of formation of organic 

 pigment has been published recently by Chodat (1912). He shows 

 that when tyrosinase acts on ^-cresol in the presence of one or other 

 of the products of protein hydrolysis a series of diversely coloured 

 pigments resembling the natural pigments is produced. Chodat believes 

 that a pigment of a given kind is produced by the action (1) of an 

 oxydase on (2) a phenolic compound in the presence of (3) an amino 

 compound, and suggests that, as the compositions of the phenolic 

 compound and the amino compound vary, so the composition and hence 

 the colour of the pigment varies. 



Gortner (1910 and 1911) working independently has already pub- 

 lished conclusions which in some respects are similar to those of Chodat. 

 In a series of valuable papers Gortner has produced evidence in support 

 of the view, put forward previously by Miss Durham (1904) and other 

 observers, that the black or brown melanin pigments of animals are 

 formed by the action of tyrosinase on a product of prot,ein hydrolysis, 

 namely tyrosine. He holds further, as described in the text, p. 299, that 

 inhibition of pigment formation may be brought about by the con- 

 version of part of the tyrosine into a closely allied substance which is 

 not only itself resistant to tyrosinase but also checks the action of 

 that substance on tyrosine. Gortner applies this hypothesis to the 

 elucidation of the suppression of pigment which is known on genet ical 

 grounds to occur in certain white forms of plants and animals. 



