230 THE CARBOHYDRATES 



of strong hydrochloric acid, the lignified walls are turned a 

 bright red. 



7. A concentrated solution of thallin sulphate in 50 per 

 cent alcohol gives a yellow to orange-yellow coloration. 



The sections should be treated first with alcohol, and the 

 thalhn sulphate solution should be freshly prepared. 



8. If lignified tissues be treated with chlorine water and 

 then with sodium sulphite, a deep magenta colour is produced. 



9. Lignocelluloses induce the formation of Prussian blue 

 in the greenish-red solution produced by mixing ferric chloride 

 with potassium ferricyanide. 



CUTINIZED MEMBRANES. 



The surface of the subaerial parts of the majority of vascu- 

 lar plants is covered by a secretion of the epidermis. This 

 secretion is known as cutin and forms a continuous trans- 

 parent layer, the cuticle, which may be so well developed as 

 to give a shining surface to the plant member, the upper 

 surface of a holly leaf, for example. The cuticle may be 

 quite distinct from the underlying cellulose membrane of the 

 epidermal cells, to which it is closely applied, as in the leaf 

 of the hellebore ; in other cases the distinction between the 

 cutinized and non-cutinized parts is not sharply defined, as 

 in Selaginella, the one merging gradually into the other. 

 When thick, the cuticle not infrequently shows stratification, 

 and wax-Hke substances may be present. The thickness of the 

 cuticle varies much in different plants and with the conditions 

 of growth. Its greatest development is found on leaves and 

 shoots which are exposed to arid conditions such as high 

 insolation, the prevalence of dry winds, growth in soils poor 

 in available water, together with other factors. Its chief 

 physical property is its high degree of impermeability to water 

 vapour and gases, its presence, therefore, impedes the evapora- 

 tion of water from the surface of the plant. 



Lee and Priestley * conclude that cuticle is formed by the 

 migration of fatty substances liberated at the surface of the 



* Lee and Priestley : " Ann. Bot.." 1924, 38, 525. Priestley : " New 

 Phyt. " 1921, 20, 17. Lee : " Ann. Bot.." 1925, 39, 755. 



