92 CUTICLE AND CUTICULARISATION 



ridges which appear as a faint striation in surface sections. 

 Cuticularisation may sometimes extend to the lateral walls, 

 which in these cases are generally thickened, so that in section 

 they appear as pegs projecting inwards. 



The cuticle is insoluble in cuprammonia and concentrated 

 sulphuric acid, being the only part of the epidermis that does 

 not dissolve in the latter reagent. With iodine and sulphuric 

 acid it yields a brown colouration, and it is easily stained with 



B 



FIG. 43. Transverse sections through the leaves of the Holly (A) and 

 Ranunculus auricomus (B). In the former only half of the vertical 

 extent of the leaf is shown. Cu., cuticle ; Ep., epidermis ; Hy. t 

 hypodermis ; Nu., nucleus ; Pal., palisade layers ; Sp., spongy 

 parenchyma. The chloroplasts are shown black. 



Scharlach red, probably as a result of the presence of fatty 

 substances. 



Owing to its toughness, the cuticle, especially when strongly 

 developed, renders the epidermis more efficient as a protection 

 against mechanical injury; but its main function is certainly 

 the restriction of transpiration to the stomata. In correspond- 

 ence with this its thickness depends mainly on the nature of 

 the habitat ; thus the cuticle is most strongly developed in 

 plants of dry situations (e.g. Gorse, Fig. 81, B, Psamma, Fig. 85, A, 

 etc.), whilst it is extremely thin in submerged aquatics (Fig. 89), 



