40 



BRITISH PLANTS 



II. Internal Xerophytic Tendencies. 



1. Thickening of the Cuticle. The cuticle is a layer or 

 membrane formed by the external walls of the epidermal 

 cells (Pig. 8). In xerophytes this becomes thickened and 

 strongly cuticularized. The latter condition is brought 

 about by the deposition in the walls of a waxy substance 

 called cutin, a body closely allied to cork, which renders 

 the membrane impermeable to water. 



FIG. 8. SECTION OF PART OF A PINE-LEAF, WITH DEEPLY-SUNK 

 STOMA (a). (HIGHLY MAGNIFIED.) 



b, guard-cells ; c, stomatal pit ; d, air-cavity ; e, cuticle ; /, epidermis ; 

 g, thick-walled hypodermis (sclerenchyma) ; h, chlorophyll-tissue. 



2. Diminution in the Volume of the Intercellular Air- 

 Spaces. The cells of the leaf are packed closely together, 

 thereby impeding transpiration. 



3. The Stomata are reduced in Number and placed in 

 sheltered positions in pits and grooves, the entrance to 

 which is often closed by hairs e.g., heaths (Fig. 9), 

 marram-grass (Fig. 11). The stomata thus come to be 

 in moist chambers. In most plants, however, each 

 stoma is sunk in its own pit e.g., pine (Fig. 8). 



