58 THE ELEMENTS OF VEGETABLE HISTOLOGY 

 CHEMICAL PROPERTIES OF CELL WALLS 



The walls of primitive cells are usually composed 

 of cellulose; but in the building up of more complex 

 structures the cell walls undergo more or less change 

 in chemical composition as well as in appearance. 

 The substance most frequently occurring in the cell 

 walls are cellulose, lignin, cutin, suberin and gums, 

 including pectinous substances. Many of the aniline 

 stains give characteristic colorations with these sub- 

 stances, and these reactions will be considered in the 

 section on Staining. 



Cellulose. — This is a carbohydrate to which the 

 empirical formula (C6HioOs)i has been assigned. The 

 following reactions may be used in the identification 

 of cellulose : 



1. A blue coloration with iodine and sulphuric acid. Treat 



the material with iodine-potassium iodide solution, then 

 add a few drops of sulphuric acid (70 per cent) . 



2. A violet coloration with zinc chloriodide solution. 



3. Solubility in cuprammonia solution. 



Lignified Walls. — Lignification of cell walls, or 

 deposition of woody materials upon the cellulose 

 membrane, occurs in those tissues of the plant which 

 are used for support. The existence of a definite sub- 

 stance termed lignin is open to question, and it is 

 possible that the reactions given by lignified walls 

 may be due to one or more compounds present in 

 woody materials. The following reactions may be 

 used in the identification of lignified tissues : 



1. A yellow or brown coloration with iodine and sulphuric acid. 



2. A yellow or brown coloration with zinc chloriodide solution. 



3. A red or violet coloration with phloroglucin solution and 



concentrated hydrochloric acid. 



