112 REACTIONS INVOLVED IN [CH. 



there is to hand, it seems most likely that the leaf is the organ where 

 such a synthesis takes place. 



The flavones. moreover, are usually present as glucosides in the plant, 

 one or more hydroxyl groups being replaced by sugar ; hence the crude 

 alcohol or water extracts of many plants are only pale yellow (the 

 auxochrome groups being replaced). On hydrolysis with dilute acids, 

 the sugar is split off, and the colour deepens ; at the same time a deposit 

 of flavone is formed, as the free pigment is less soluble than the glucoside. 

 According to Witt, the capacity for dyeing of the flavones depends 

 on the presence of free hydroxyl groups. Thus, it comes about that 

 aqueous or alcoholic extracts of most plants dye but slightly when 

 boiled with mordanted cloth, but after hydrolysis of the glucoside with 

 dilute acid, and neutralisation, the same extract dyes more deeply. 



It has been suggested by the author (226) that the flavones may, 

 in many cases, be the chromogens from which anthocyanins are derived. 

 The reactions involved would then be expressed in very general terms 

 as follows: 



glucoside + water "*~^ chromogen (flavone) + sugar 

 x (chromogen) + oxygen -* anthocyanin 



It is suggested that the first reaction is controlled by a glucoside- 

 splitting enzyme or enzymes, and the second reaction by an oxidising 

 enzyme. Also that several of the hydroxyl groups of the flavones, as 

 they actually exist in the cell-sap, are replaced by sugar. After hydro- 

 lysis of one or more, but not necessarily all, of these hydroxyl groups, 

 oxidation of the flavone molecule, accompanied by condensation of 

 either two flavone molecules, or of a flavone with other aromatic 

 substances, may take place at these points. Hence the final product 

 anthocyanin would be itself a glucoside, and the reacting substances 

 would at all times be in the glucosidal state. 



Exception has been taken by Everest (248) to the above hypothesis, 

 which will in future be referred to as the glucoside hypothesis, and 

 it is advisable to consider his criticisms at this point, since they do not 

 affect the general evidence for the hypothesis to be considered later. 



Everest makes the following statements which are to some extent 

 the outcome of his investigations : 



1 . No known glucoside of a flavone has more than two hydroxyl groups 

 replaced by sugar ; most of the glucosides contain only one sugar molecule. 



2. All anthocyanin pigments present in the natural state in plants 

 are glucosides. No free anthocyanidins (non-glucosidal) have been 

 detected (Willstatter & Everest, 245). 



