50 THE PROPERTIES AND [CH. 



anthocyanin plus a yellow colour given by xanthogene. But in the 

 case of the flowers of Pelargonium, Papaver and Salvia the xanthogene is 

 absent and so the anthocyanin becomes blue or violet with alkalies. In the 

 later paper (132) he remarks on the resemblance of xanthogene to luteolin 

 (which we now know to be a flavone), but he was unable to establish 

 the identity. Thus Filhol's investigations brought him very near to 

 the truth. Similar views were advanced by Wiesner (135) in 1862. 

 From a series of reactions given by various flower pigments he concludes 

 that colourless sap contains, as a rule, a tannin giving a green reaction 

 with iron salts, and a yellow colour with alkalies. Like Filhol he 

 believes that anthocyanin itself gives a blue, never a green, reaction 

 with alkalies, and the green colour is due to admixture with yellow given 

 by the tannin. In plants free from tannins giving the yellow reaction, 

 anthocyanin turns blue with alkali. Wiesner's tannins are probably 

 for the most part flavones, since true tannins are rare in flowers. These 

 views on the reactions of anthocyanin gave rise to a certain amount 

 of controversy, for Wigand (136) and Nageli & Schwendener (138) 

 held the opinion that the green coloration is given by anthocyanin 

 itself, and may appear when the iron-greening tannins are absent. 

 Some of the arguments involved in the discussion are given by Wiesner 

 in a later paper (142). The alkali reaction of anthocyanin is also 

 mentioned by Overton (333), who considers the blue colour to be due 

 to the formation of an acid salt, the green colour to a neutral salt of 

 the pigment, anthocyanin itself being a dibasic acid. This view is 

 accepted by Grafe (197) as being in accordance with the reactions of 

 Althaea pigment which he prepared in a pure state. Anthocyanin 

 from Antirrhinum (Wheldale & Bassett, 254), purified from accompanying 

 substances, still gives the green alkali reaction. Willstatter (245), as 

 far as can be determined from his publications, considers the reactions 

 of anthocyanin to be as follows. The pure blue anthocyanin from the 

 Cornflower is not altered by alkali, i.e. pure anthocyanin gives a blue 

 colour with alkalies, but if a solution of the blue pigment has stood for 

 a time, the colour reaction with alkalies is green. This is due to the 

 fact that from the pigment a colourless isomer (see pp. 53, 72, 75, 78) 

 has been formed, and this gives a yellow colour with alkalies; hence 

 the blue plus yellow results in a green reaction. Crude extracts from 

 the flowers, he says, also give a green colour owing to the presence of 

 yellow pigments obviously flavones. (Die reine blaue Farbstoff- 

 losung zeigt auf Zusatz von wenig Soda zunachst keine Farbanderung, 

 eine gestandene Losung wird hingegen griinblau oder blaugriin weil 



