The Production of Anthocyanins and Anthocyanidins. 327 



— from yellow glucosides of the flavonol series by reduction in acid solution 

 as described by the author (loc. cit), is of vital interest to those studying the 

 production of these pigments in plants. 



Willstatter bases his criticism of the author's conclusions upon the 

 following observations: — (1) The red pigments obtained by reduction are 

 unstable, becoming decolorised even in acid solution. (2) The decolorised 

 solutions of these pigments do not recover their colour on acidification, even 

 with considerable excess of acid. The anthocyans do not behave in this 

 manner, they are stable in acid solution, and, in neutral solution — with a few 

 •exceptions — become decolorised by isomerisation, but the decolorised solution 

 •on acidification regains its colour. 



The author has carefully examined the red pigments obtained by him, and 

 •compared them with cyanin. The glucosides and the one non-glucoside 

 •examined gave results in every way similar to those given by cyanin and 

 •cyanidin respectively, and support the conclusions reached in the author's 

 previous paper {loc. cit.), viz. : that these pigments are true anthocyanins and 

 anthocyanidins. 



It is perhaps necessary to point out at this stage that, for the comparisons 

 made by Willstatter and now to be discussed, the anthocyanins — glucosides — 

 give far more reliable results than the anthocyanidins — non-glucosides — 

 which latter compounds are more unstable in solution, even when faintly 

 .acid, than the corresponding anthocyanins. This fact was first observed 

 when working with cyanin ; moreover, the decolorisation by isomerisation and 

 return of colour by the action of acids is far more difficult to carry out 

 satisfactorily with the anthocyanidins, particularly if they are somewhat 

 impure, than with the anthocyanins. With the anthocyanins this series of 

 changes is most characteristic and very easily carried out, even when the 

 pigment is in quite a crude and impure condition. 



As Willstatter appears to have used the non-glucosides only in makino- 

 his comparisons, the foregoing considerations may perhaps explain the 

 -discrepancy between his results and those obtained by the present writer. 



In every case the red pigments — glucosides — obtained by the author are 

 stable in aqueous or alcoholic acid solution ; indeed many of them have 

 remained in open test-tubes for many days without losing their colour. In 

 no case has decolorisation under such conditions been observed. These 

 pigments become decolorised, it is true, even in acid solution, if reducing or 

 ■oxidising agents are present, but this is equally true of anthocyanins. 



The non-glucosides when in neutral, or very faintly acid, solution are 

 -somewhat less stable than the glucosides, but in this they resemble the 

 -anthocyanidins. 



