SECTIONAL TRANSACTIONS.— D. 429 



Mr. W. J. C. Lawrence and Mr. J. R. Price. — Flower pigments. 



The Chemical Basis of Flower Colour. 



The substances responsible for flower colour may be divided into two 

 classes, sap-soluble and non-sap-soluble. The sap-soluble pigments 

 comprise the anthocyanins, anthoxanthins and certain nitrogenous 

 substances . 



The Anthocyanins are the most important flower colouring matters, and 

 are responsible for scarlet, red and blue colours. They occur as glycosides, 

 that is, they are compound molecules formed by the union of the true 

 colouring matter with one or more molecules of a sugar. 



The colour-producing part of the anthocyanin molecule, known as the 

 anthocyanidin, may be derived from one of three main structures — pelar- 

 gonidin, cyanidin and delphinidin — which differ only in the number of 

 hydroxyl groups ( — OH) in the phenyl ring : 



CI. CI CI 



As the formulae show, cyanidin has one and delphinidin two more oxygen 

 atoms in the molecule than pelargonidin. These differences represent 

 one of the principal factors upon which variation in flower colour depends, 

 since an increase in the number of oxygen atoms (in the form of hydroxyl 

 groups) results in a marked increase in blueness of tone. This is illustrated 

 in the first part of the exhibit. 



As mentioned above, the anthocyanins occur as compounds involving one 

 or more molecules of a sugar . Of these sugar molecules one is always attached 

 at the 3 position ; if there is a second sugar molecule it may be attached 

 either directly to the first one or it may unite with the anthocyanidin in 

 a different position, at 5. Hence there are two classes of glycosides : 

 {a) those with one or two sugar molecules attached at position 3, and 

 {b) those with sugar molecules at both 3 and 5. These two classes are 

 visibly different in colour and constitute another important factor in flower 

 colour variation, the 3 : 5-diglycosides being bluer than the corresponding 



3-type- . , , . . 



A third variable involving structural difference m the anthocyanms is 

 the existence or otherwise of methylated hydroxyl groups, where the 

 hydrogen atom of a hydroxyl group has been replaced by a methyl (CH3) 

 radicle. This results in an increase in redness. As a rule the only hydroxyl 

 groups methylated are those at positions 3' and 5' ; that at 4' is never 

 methylated. Thus there is one methyl ether of cyanidin and there are two 

 of delphinidin (3 '-mono- and 3' : s'-di-). 



So far three factors influencing the colour of anthocyanins have been 

 dealt with, namely : 



(a) The number of hydroxyl groups in the molecule. 

 \b) The position of attachment of the sugar molecules, 

 (c) The methylation of hydroxyl groups. 



Combinations of these three give rise to twelve anthocyanins, differing 

 slightly in colour and covering a wide range from scarlet to purple. 



