1604 



A TEXTBOOK OF THEORETICAL BOTANY 



a continuous layer of cuticle of varying thickness. This is usually sharply 

 marked off from the individual cell walls below it. The latter are composed of 

 an inner cellulose layer, and between this and the cuticle there is interposed a 

 variable thickness of cutinized wall, consisting of a mixture of cutin and 

 cellulose. This layer may sometimes be absent, even in highlv thickened cells. 

 The thickness of the outer wall is not, therefore, a reliable index of its imper- 

 meability to water, since the intensity as well as the amount of cutinization 



Fig. 991. — Rhododendron ponticum. Transverse section of 

 leaf showing multiple palisade and double epidermis 

 \\ ith thick cuticle. A xeromorphic type of leaf. 



may vary considerably. Nevertheless it is generally true that more exposed 

 surfaces have a thicker cutinization than those less exposed, and it follows 

 that the upper surfaces of dorsiventral leaves have usuallv a thicker coating 

 than the lower surfaces, sometimes twice as thick. Compared with the amount 

 of water vapour which escapes through the stomata, the loss of water through 

 the cuticle is extremely small. Haberlandt showed by experiment that the 

 loss of water attributable to evaporation through the cuticle itself varied 

 between 0-23 (Aesculus) and o-oo6 {Hedera) of the evaporation from an 

 equivalent free water surface. 



