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Prof. Keeble, Dr. Armstrong, arid Mr. Jones. [Feb. 3, 



the fronds of the Royal fern (Osmunda rcgalis). Loss and recovery of colour 

 are therefore phenomena of very general occurrence, and may be regarded 

 as characteristic of many, if not of all, kinds of anthocyan pigments. 



The wallflower is of special interest in this connection, in that the brown 

 varieties with which we have worked contain representatives of the two 

 types of pigment — anthocyanic and plastid-derived pigments— to either or 

 both of which the colour of flowers may be due. The brown colour of the 

 wallflower is produced by a purple anthocyan pigment and a yellow plast 

 pigment. "When acted on by alcohol a brown petal becomes decolorised ; 

 but it recovers to a purple colour when treated with water. The recovery 

 to purple instead of brown is due to the fact that the yellow plastid-pigment 

 which contributes to the original brown colour is soluble in alcohol and is 

 therefore extracted from the tissues by this reagent. Thus only the colour- 

 less antecedent of the purple anthocyan pigment is left in the cells. Treated 

 with water that antecedent gives rise to a purple pigment which, since it is 

 no longer mixed with yellow, produces its proper optical effect. The yellow 

 pigment may be obtained free from the anthocyan pigment by evaporating 

 the alcoholic solution and washing the residue with water, in which the 

 plast pigment is insoluble. The power of recovering to the original colour 

 serves as a means of distinguishing the pigments of the anthocyan class from 

 those which are derived from the plastids. 



The reproduction of the original colour in the petals of stocks and other 

 plants is open to two alternative interpretations. On the one hand, it may 

 be regarded as a phenomenon of like nature to that exhibited by indicators ; 

 on the other hand it may be attributed to the oxidation of a chromogen. 



Immediate choice between the two interpretations is rendered difficult by 

 reason of the fact that acids and alkalis exercise marked and definite effects 

 on the colours of the anthocyan pigments contained in the flowers. Thus, 

 in the presence of alkalis the pigment in the petals of stocks assumes 

 a green-blue colour and in the presence of acids it becomes pink. Moreover 

 the chromogen extracted by means of 50-per-cent. alcohol from the petals of 

 stocks behaves as a very sensitive indicator. Dilution of the alcoholic 

 extract with ordinary distilled water — which contains carbon dioxide — 

 suffices to produce a pale pink colour. With mineral acids the colour 

 becomes intense and with alkalis it passes through blue and blue-green to 

 green. 



Nevertheless, and in spite of the complication introduced by these 

 indicator effects, the evidence of the experiments now to be described points 

 very definitely to the conclusion that the indicator hypothesis must be 

 discarded in favour of its alternative. 



