vii THE HISTOLOGY OF THE CELL 77 



showing it at all. The external layer of the epidermal cells is 

 called cuticle, which is almost impermeable to water. 



The walls of corky cells have the same properties as cuti- 

 cularised cell-walls, and they give the same reaction with iodine. 

 The corky walls consist of a substance called suberin. Both 

 cutin and suberin contain about 74 per cent of Carbon. 



2. The cellulose of the cell-wall may, owing to the deposition 

 of lignin in the wall, become lignified. A lignified wall gives 

 a blue colour when treated with aniline chloride and hydro- 

 chloric acid. Lignification, while it makes the cell-wall harder 

 and more elastic, does not prevent water being able to readily 

 traverse it. Lignification takes place most largely in woody 

 tissue, and to a less extent in other parts of the plant. 



3. In some cases the cell-wall may become more or less 

 mucilaginous. This change is caused by the conversion of 

 cellulose into mucilage, which may be either a form of cellulose 

 or a form of gum. 



EXPT. 70. Obtain a small quantity of Spirogyra, which is found in 

 ditches and ponds during summer. Mount some of it in water, and 

 examine it under a low power of the microscope. . Note 



(i) That the filament is surrounded by a cell-wall. 



(ii) That each cell contains protoplasm. 



(iii) That a nucleus is present in the protoplasm. 



Place a small quantity of Spirogyra in a watch glass and cover it 

 with iodine solution. Mount it in water and examine it first with a low 

 power, then with a high power. Note 



(i) The cell-wall is but slightly stained yellow. 



(ii) The protoplasm is coloured a deeper yellow or brown. 



(iii) The nucleus is still more deeply stained than the protoplasm. 



EXPT. 71. Take a Date stone and scrape away the brown coat. 

 Cut sections from the reserve material (which is cellulose) stored up in 

 the seed. This can be done by using either the heel of the razor or a 

 strong knife. Mount a thin section in glycerine, and examine first 

 under a low power and then under a high power. Note 



(i) The thick cell-walls with a number of thin places called pits in 

 them ; the membrane which closes those pits is called the closing 

 membrane, and is, in reality, the primary cell- wall. For this reason 

 the closing membrane is sometimes called the middle lamella (Fig. 102). 



(ii) The granular protoplasm. 



(iii) Soak a section for a few minutes in iodine, and mount in 

 glycerine. The cell- wall is stained slightly yellow. 



(iv) Mount another section which has been soaked in iodine and a 

 drop of strong sulphuric acid. Examine it under a low power only. 

 Observe how the cell-walls swell, lose their sharp outline, and assume 



