Cell Wall 7 



Cellulose is not coloured blue by means of iodine alone, 

 although it is so changed by the action of iodine and sul- 

 phuric acid, thus being distinguishable from starch, which has 

 the same percentage composition, but is turned blue by iodine, 

 Schultze's solution ' will also stain it. 



Cellulose is insoluble in water, both cold and boiling ; also 

 in alcohol, ether, and dilute acids. If it be treated with cold 

 concentrated sulphuric acid, it is converted first into dextrine 

 or British gum, which is the same composition as cellulose, 

 and then into grape sugar, CjHiaOg. It is found almost pure 

 in Cotton wool, but generally the cell wall contains mineral 

 ash and water in addition. 



During the growth of the cell the cellulose is often converted 

 into substances which are of an allied composition, but which 

 differ from it in various physical and chemical properties. 



The chief of these are — 



1. Suber, or Cuticularised Cellulose. — This is highly elastic 

 and almost impermeable to water ; when treated with iodine 

 or Schultze's solution it turns yellow ; when warmed (not 

 boiled) in concentrated solution of potash it is stained bright 

 yellow. It is met with in Cork, epidermis cells, and pollen 

 grains. 



2. Lig^nin, — In this case the cell wall is hard, inelastic, and 

 permeable to water. Treated with a solution of aniline sul- 

 phate acidulated with a few drops of sulphuric acid, it is stained 

 yellow. It is met with in wood cells. 



3. Mucilage. — "W'hen dry it is hard and horny, but it very 

 readily absorbs water, swelling and becoming gelatinous. The 

 reactions are the same as those of cellulose. Examples are to 

 be found in Linseed and Quince mucilage. 



The cell contents are very various ; we will note some of 

 the principal : — 



Protoplasm. — It is of this substance that the whole of the 



' Schultze's solution :— Dissolve zinc in hydrochloric acid ; permit the 

 solution to evaporate in contact with metallic zinc until it has attained a 

 syrupy consistence. Saturate the syrup with potassic iodide, and then add 

 enough iodine to make a dark sherry-coloured solution. The object to be 

 stained must lie placed in a little water, and then some of the above solu- 

 tion added. 



