220 THE CARBOHYDRATES 



was suggested by Wiesner that the substance was actually 

 phloroglucinol, but the presence of this substance in wood has 

 not been established, and it is more likely to be substance 

 of a tannin-like nature which, after all, is closely related 

 to phloroglucinol, which is responsible for the reaction. 



The opinion generally held is that the colour reactions 

 for Hgnified tissue are not due to lignin itself — which forms 

 about 50 per cent by weight of wood — but to small quantities 

 of substances of an aldehydic nature which may have been 

 adsorbed upon the surface of the lignin from the cambial sap, 

 but their exact nature is still largely a matter of surmise. 



That the colour reactions are not due to hgnin itself but to 

 small quantities of substances accompanying lignin * is sup- 

 ported by two facts, firstly, as shown by Wichelhaus and 

 Lange,t pine or firwood when distilled with superheated steam 

 at 180-200° yields a distillate which gives all the characteristic 

 colour reactions of the original wood, and secondly, that 

 lignin once isolated from its association with cellulose in wood 

 generally no longer gives the colour reaction. 



In attempting to find an explanation of the nature of this 

 substance, suspicion at first fell upon vanillin and coniferyl 

 aldehyde, since both these substances give the reaction with 

 phloroglucinol and hydrochloric acid, but no proof has been 

 furnished for the universal occurrence of these substances in 

 hgnified tissues. It is true that coniferin, a glucoside giving 

 rise to coniferyl alcohol, occurs in the cambial sap of most 

 conifers and that it is easily oxidized to vanillin. 

 OH OH 



^OCHs /^\oCH3 



^^CH = CH— CH2OH -^^Q 



Coniferyl alcohol Vanillin 



Also vanilhn itself is widely distributed, having been reported 

 in many resins, in dahlia tubers, potato peel, asparagus shoots, 



* vSee Cross and Doree : "Researches on Cellulose, IV.," London, 

 1922, p. 153. 



t Wichelhaus and Lange : " Ber. deut. chem. Gesells.," 1916, 49, 

 2001 ; 1917, 50, 1683. Wichelhaus : " Chem. Zeit.," 1923, 47, 865. 



