LIGNIN 229 



72 per cent sulphuric acid to remove lignin, still retains its 

 original organised structure, shows that lignin is merely mixed 

 with or incorporated with cellulose ; the argument, however, 

 is not convincing since cotton cellulose can be nitrated without 

 destroying the structure of the fibres. The view of Wislicenus * 

 on the mode of origin of lignified cell walls is that the original 

 cellulose wall is a colloidal hydrogel which adsorbs from the 

 sap other colloidal materials that produce lignin ; the union 

 between the lignin and the cellulose is accordingly only one 

 of physical adsorption, possibly reinforced by supplementary 

 valencies of oxygen atoms. Robinson f also favours the view 

 of physical mixture from observations on the microscopical 

 features of mechanical strains in timber which he explains 

 as being due to displacement of films of lignin overlaying the 

 ground cellulose of the tracheids. On the other hand, 

 Schmidt and others J have published some theoretical specu- 

 lations on the relation of cellulose to the incrustation and 

 conclude that they are united in ester-like combination, while 

 Mehta § considers that lignin is combined with cellulose as 

 an aromatic glucoside. 



MICROCHEMICAL REACTIONS. 



Lignified tissues give the following reactions :— 



1. A brownish-yellow colour is given with iodine. 



2. A brown colour is obtained with the use of chlorzinc 

 iodide. 



3. Calcium chloride iodine solution turns lignin yellow to 

 yellow-brown. 



4. Insoluble in cuprammonia. 



5. Aniline sulphate or aniline chloride in aqueous solution 

 and acidified with the corresponding acid turns lignified walls a 

 bright yellow. 



6. If the sections be soaked for about a minute in an 

 alcoholic solution of phloroglucin and then mounted in a drop 



* Wislicenus : " Kolloid Zeit.," 1910, 6, 17 and 87 ; " Cellulosechemie," 



1925. 6, 45. 



t Robinson : " Trans. Roy. Soc," B., 1920, 210, 49. 



J Schmidt, Haag, and Sperling: " Ber. deut. chem. Gesells.," 1925, 



58, 139. 



§ Mehta : " Biochem. Journ.," 1925, 19, 958. 



