114 Prof. Keeble, Dr. Armstrong, and Mr. Jones. 



It is to be remarked that this hypothesis postulates a larger number of 

 glucose residues in the molecule of the yellow pigment than is the case in the 

 varieties which have been studied. Iu none of these are more than two 

 sugar residues present. Moreover, even in such cases, the sugar residues are 

 not attached to different hydroxyl groups, but are united to one another to 

 form a disaccharide. Further, the experience gained with amygdalin* 

 suggests that such complex glucosides are only broken down by enzymes in 

 one way, and hence, if this be the case, it is not probable that different 

 enzymes act on the glucosides in such a manner ,as to set free different 

 groups. 



The yellow pigments are regarded by Miss Wheldale as belonging to the 

 flavone or xanthone classes, and in this connection it may be pointed out that 

 the constitution of the hydroxyflavone glucosides offers great possibilities of 

 variation. Thus the sugar residue, which may be either glucose or rhamnose r 

 is joined by an hydroxyl group to the flavone. The hydroxyl group may be 

 attached to carbon in the oxygen ring or substituted in either of the phenol 

 groups. 



O OH 



Such differences of constitution will correspond with differences in the 

 properties of the several glucosides, and these different properties will be 

 manifested particularly when the glucosides are acted on by enzymes or 

 hydrolysed by acids. 



A further possibility of the derivation of varied products from these 

 glucosides follows from A. G. Perkin's observations (vide infra) to the effect 

 that the glucosides of this class occur commonly in plants not singly but in 

 association. In a valuable series of memoirs dating from 1895, A. G. Perkin 

 has described a number of natural hydroxyflavone glucosides, and at the 

 present time the structure of at least 12 of these bodies is known. The 

 researches of Perkin have been supplemented by Kostanecki's synthetical 

 work on the hydroxyflavone group. 



The outstanding fact revealed by Perkin's investigations is that, of the 

 yellow pigments derived from many and diverse varieties of plants, all, 

 without exception, are hydroxyflavone derivatives ; indeed, with this evidence 

 before us, it is not too much to assume that all the soluble yellow pigments 

 of plants belong to this class. The observations described in this section 



* Compare Armstrong, ' The Simple Carbohydrates,' 1912. 



