U INTRODUCTORY [CH. 



Thus Keeble & Armstrong were led to believe that they had made 

 artificial anthocyanin, and that preliminary reduction and subsequent 

 oxidation are the essential processes of anthocyanin formation. In 

 November of the same year, a note was published by Combes (234) 

 in the Comptes Rendus, giving a record of practically the same observa- 

 tion though in a much more complete form. By treating with sodium 

 amalgam the acid alcohol solution of a yellowish crystalline substance 

 which he had extracted from Ampelopsis leaves, he obtained a fine 

 purple pigment which crystallised in needles. This pigment, he claims, 

 on the basis of chemical and physical properties, to be identical with 

 crystals of natural anthocyanin from the same plant; but no analyses 

 are included. In addition, he found (235) that natural anthocyanin, 

 after treatment with hydrogen peroxide, gives a yellow product identical 

 with the natural yellow substance from which he started. 



Combes believes that these results entirely revolutionise our ideas on 

 anthocyanin formation. Thus he says: "La production experimentale 

 d'une antliocyane pent done etre consideree comme realisee. Ce resultat 

 permet d'entrevoir comme tres proche la solution du probleme de la 

 formation des pigments anthocyaniques pose depuis plus de 120 ans 

 et qui fut aborde par de nombreux physiologistes. On salt que dans 

 toutes les hypotheses relatives a cette question, formulees depuis 1825, 

 la pigmentation a toujours ete consideree comme un phenomene d'oxy- 

 dation; cette opinion ne peut plus etre soutenue, puisqu'il apparait 

 que 1'anthocyane des feuilles rouges prend naissance lorsque le compose 

 correspondant contenu dans les feuilles vertes est soumis a 1'hydrogene 

 naissant, c'est-a-dire dans un milieu qui est au contraire reducteur." 



The essential difference between Keeble & Armstrong's results and 

 those of Combes can be explained in the following way. The purple 

 pigment, as indeed the natural anthocyanin, will become colourless 

 on powerful reduction, though it regains its colour in air. Thus 

 Keeble & Armstrong's reduction was carried beyond the preliminary 

 formation of pigment to the colourless stage, the colour returning 

 again on exposure to air, whereas Combes's product was not reduced 

 so far. 



The production of a purple colour on reduction of flavone with 

 sodium amalgam had been known to chemists some time before the 

 work of Keeble & Armstrong. The almost universal distribution of 

 flavones naturally explains the appearance of the colour on reduction 

 of many plant extracts. The crystalline yellow product used by 

 Combes is undoubtedly a flavone also, as will be seen on referring to 



