336 PIGMENTS 



such circumstances was first observed in the case of the simpler 



substance pyrone — 



o 



/ \ 



CH CH 



11 II 



CH CH 



\ / 

 CO 



Pyrone 



which dissolves in hydrochloric acid, forming an additive com- 

 pound of the formula — 



H CI 



O 



/ \ 

 CH CH 



CH CH 



\ / 

 CO 



the oxygen becoming tetravalent. Such additive compounds 

 of anthoxanthins with acids are easily dissociated and do not 

 occur in plants, though it will be seen on page 345 that in the 

 case of the anthocyanins analogous compounds do actually 

 occur naturally. 



FURTHER LITERATURE. 



Kostanecki : " Bull. Soc. chim. Paris," 1913, [3], 29, i-xxxvii. 



Perkin, A. G., and others : " J. Chem. Soc. Lond.," 1895, 67 ; 1896, 

 69 ; 189?' 71 ; 1898, 73 ; 1899, 75, etc. 



Wheldale : " Proc. Camb. Phil. Soc," 1909, 15, 137 ; " Biochem. 

 Journ.," 1914, 8, 204, etc. 



Perkin and Everest : " The Natural Organic Colouring Matters," 

 London, 191 8. 



ANTHOCYANINS. 



OCCURRENCE, CONDITIONS OF FORMATION, AND 

 PHYSIOLOGICAL SIGNIFICANCE. 



The occurrence of a red, blue, or purple pigment, either 

 dissolved in cell sap — the exact colour depending on the acid, 

 alkaline, or neutral reaction of the cell sap — or in an amorphous 

 or crystalline state as in Delphinium spp., Passiflora spp., 

 Ruhus spp., and many other plants,* is a common phenomenon, 



* See Gertz : " Studier ofver Anthocyan," Lund, 1906. 



