222 THE CARBOHYDRATES 



Whilst it may be generally accepted that the commonly 

 employed colour reactions for lignified tissue are given not by 

 lignin itself but by a substance, most probably coniferyl 

 aldehyde, which accompanies lignin, there are nevertheless 

 two colour reactions which may be regarded as being pro- 

 duced by the lignin complex itself ; these are known as 

 the lignone chloride reaction of Cross and Bevan * and of 

 Maiiie's reagent. The former depends on the formation of a 

 yellow colour when lignified material is exposed to moist 

 chlorine gas or bromine, and which on addition of sodium sul- 

 phite changes to red ; Maiiie's reaction also consists in the pro- 

 duction of a red colour when wood is treated successively with 

 potassium permanganate, hydrochloric acid, and ammonia. 

 Probably this is a modified form of the Cross and Bevan re- 

 action, since permanganate followed by hydrochloric acid 

 evolves chlorine. The colour obtained is, however, not uni- 

 form and tends, in the case of wood of deciduous trees, to be 

 brown instead of red. 



The Isolation and Constitution of Lignin. 



As stated above, the process of lignification consists in the 

 incorporation into the cell wall of a substance known as lignin ; 

 but opinions are divided as to whether such hgnin is chemically 

 combined with the cellulose or only physically adsorbed. 

 The facts that lignified cellulose, or lignocellulose as it is called, 

 is not soluble in cuprammonia solution and is also incapable 

 of entering into such chemical reactions as can cellulose with 

 carbon disulphide and caustic soda, for example, suggest that 

 there is some kind of chemical union between lignin and cellu- 

 lose. The very great technical importance of cellulose free 

 from lignin necessitated the provision of methods for separating 

 these two substances, the two best known and most widely 

 employed being those of heating the wood with caustic soda 

 or with calcium disulphite ; both these methods are somewhat 

 drastic, and it is reasonable to suppose that the lignin so 

 isolated will differ somewhat from the form in which it existed 

 in the original material ; in the case of the bisulphite method 



* Cross and Bevan : " J. Chem. Soc," 1889, 55, 199. 



