120 AROMATIC COMPOUNDS [ch. 



Rosa gallica. It has been suggested that in such cases the pigment is 

 modified by other substances present in the cell-sap: thus it may be 

 present in one flower as a potassium salt, in another as an oxonium salt 

 of an organic acid, and in a third in the unaltered condition. But exactly 

 how these conditions are brought about is not clear. In one or two cases, 

 moreover, where there is a red or pink variety of a blue or purple flower, 

 the variety, when examined, has been found to contain a different pigment 

 and one less highly oxidized than that in the species itself The above 

 phenomena are exemplified in the Cornflower (Centaurea Gyanus). The 

 flowers of the blue type contain the potassium salt of cyanin, the purple 

 variety, cyanin itself, while those of the pink variety contain pelargonin. 

 The mode of origin of anthocyan pigments in the plant is as yet 

 obscure. It has been suggested ( Wheldale, 24) that they have an intimate 

 connexion with the flavone and flavonol pigments, which can be seen at 

 once by comparing the structural formula of quercetin with that suggested 

 for cyanidin: 



Wo 



^^ .O^ ^ .OH I ^ A , ,OH 



OH 



^" OH H 



Quercetin Cyanidin 



All the anthocyan pigments so far isolated, however, have been found 

 to contain the flavonol, and not the flavone, nucleus. 



Just as in the case of the flavone and flavonol pigments, some of the 

 anthocyan pigments are specific, while others, on the contrary, are common 

 to various genera and species. Also more than one anthocyan pigment 

 may be present in the same plant. 



It will be pointed out later that small amounts of a substance iden- 

 tical with cyanidin are said to be formed by reduction of quercetin with 

 nascent hydrogen, but this does not necessarily prove that the formation 

 of anthocyan pigments in the plant takes place on the same lines. If 

 we compare the formulae for a number of anthocyan with flavone and 

 flavonol pigments, it is seen that they may be respectively arranged in 

 a series, each member of which differs from the next by the addition of 

 an atom of oxygen : 



Luteolin, kaempferol and fisetin CigHioOa Pelargonidin C15H10O5 

 Quercetin C16H10O7 Cyanidin CisHjoOe 



Myricetin CieHioOs Delphinidin C15H10O7 



