430 SECTIONAL TRANSACTIONS.— D. 



These factors are all dependent upon structural changes in the antho- 

 cyanin molecule, that is, the differences are internal. However, conditions 

 external to the molecule may also affect the colour of the anthocyanins. 

 The most important of these is a phenomenon known as copigmentation, 

 which will be referred to in connection with the anthoxanthins. Secondly, 

 modification of the flower colour can be brought about by variation in the 

 pH of the cell sap, the colour becoming bluer as the pH is increased. 



The Anthoxanthins. — The substances included under this heading are 

 closely related chemically to the anthocyanins, but differ in colour, ranging 

 from pale ivory to deep yellow. Like the anthocyanins, they are sap soluble 

 and usually occur as glycosides. Structural variation is greater thaii in the 

 case of the anthocyanidins, but the majority are analogous and fall into two 

 classes, the flavones and flavonols, which differ in that the flavones have no 

 hydroxyl group at position 3. 



HO- 



^ Q . . 



>-0H "O-MVCD 



,0H 

 OH 



-OH 



w 



^ OH OH 



Apigenin - a flavone. Querceh'n - a flovonol. 



Increase in the number of hydroxyl groups present in an anthocyanidin 

 molecule results in increased blueness. A similar effect is manifest in the 

 flavones and flavonols, which become more yellow. 



There are four ways in which the anthoxanthins are concerned in flower 

 colour : 



(a) In flowers which have no anthocyanin they may be directly responsible 

 for some or all of the colour. 



(b) If a yellow anthoxanthin occurs together with an anthocyanin, the 

 resultant colour is a blend of the two. 



(c) In the presence of anthocyanins, ivory anthoxanthins, as would be 

 expected, do not contribute directly to the colour ; nevertheless they may 

 be of great importance on account of their ' copigmenting ' action. When 

 present in the same solution as an anthocyanin they combine loosely, in 

 some way as yet unknown, with the anthocyanin, to give a much bluer 

 colour. The effect is very marked, and flowers with a copigmented cyanidin 

 derivative may appear bluer than those containing an uncopigmented 

 delphinidin glycoside. It is probably not an exaggeration to say that 

 upwards of 70 % of garden flowers are copigmented, to some extent 

 at least. The degree of copigmentation varies with the nature of the 

 anthocyanin, delphinidin being most readily copigmented and pelargonidin 

 derivatives least. It also varies with the nature of the flavone or flavonol. 

 There is no exact information on this point, but observations show that 

 yellow anthoxanthins do not generally behave as copigments. 



(d) It has been pointed out that there is a close relationship between 

 anthocyanins and anthoxanthins, as is shown by inspection of their respec- 

 tive formulae. Therefore their syntheses in the plant might be expected 

 to follow similar lines. Evidence in favour of this suggests that the 

 anthocyanins and anthoxanthins are formed from the same starting material, 

 which may be limited in quantity. This results in competition between 

 the two, and if most of the source is utilised in the synthesis of one pigment, 

 then of necessity less of the other is produced. For example, the presence 

 of much anthoxanthin may lead to almost complete suppression of antho- 

 cyanin, resulting in delicately flushed flowers. 



