i] INTRODUCTORY 15 



his original communications. The question at issue is whether these 

 purple pigments are anthocyanins. 



The problem of the artificial formation of anthocyanin has been 

 attacked by Willstatter (257) also in 1914. By treating quercetin, 

 according to Combes's method, Willstatter in the same way obtains 

 a purple pigment, the bulk of which he finds is unstable in acid solution 

 on boiling, though a certain amount of colour remains after this process. 

 The product unstable to heat is, in Willstatter's opinion, not a true 

 anthocyanin, but he maintains that the small amount of stable product 

 left after heating is, when isolated, identical with the anthocyanin 

 (cyanidin) of the Cornflower. Thus Willstatter regards this reaction 

 as giving further confirmation to his views of the reduction hypothesis 

 of pigment formation, which we have seen to be based upon analyses 

 of the anthocyanins he has isolated, and to be practically a proof of 

 his suggested constitution of anthocyanin. To quote from his latest 

 paper: "Die Bildung von Cyanidin aus Quercetin hat zweifache 

 Bedeutung. Es ist dadurch eine Synthese von Cyanidin ausgefiihrt, 

 da das Quercetin selbst vor zehn Jahren von St. von Kostanecki... 

 synthetisch dargestellt worden ist. Ferner wird durch diese Umwand- 

 lung die Konstitutionsformel des Cyanidins bewiesen." 



The author has been so far unable to bring the results provided by 

 the case of Antirrhinum into line with Willstatter's views. In Antir- 

 rhinum the chromogen is, without doubt, apigenin ; when treated with 

 nascent hydrogen this flavone produces a purple pigment, yet the latter 

 does not resemble the natural anthocyanin of Antirrhinum, either in 

 composition or properties (255). 



Thus we have arrived at the last stage in the subject as far as inves- 

 tigation has gone. There is little doubt that anthocyauins are derived 

 from flavones, but further evidence is needed of the actual chemical 

 connection between these two classes of substances. Neither of Will- 

 statter's two lines of proof is beyond criticism. Actual comparison 

 of the percentage formula of any anthocyanin with any flavone, 

 though significant, does not prove their relationship in nature, by 

 reduction or otherwise, unless we know that they occur together. For 

 since the known flavones can be arranged in a series, each member 

 of which differs from the others by an atom of oxygen, any antho- 

 cyanin might be either an oxidation or a reduction product of a flavone. 

 What is really necessary is to know which flavone accompanies the 

 anthocyanin in a number of plants. At present Antirrhinum alone 

 supplies us with information, for as far as it is possible to judge, 



