57° 



ECOLOGY 



Fig. 809. — A leaf of a 

 broad-leaved sclerophyll, the 

 American holly {Ilex opaca)^ 

 constituting a representative 

 coriaceous leaf of xerophytic 

 aspect, its stiffness being due 

 largely to cutin. 



The stiffness of most evergreen leaves is due to the cuticle, which thus 

 is of mechanical value, giving protection from vnnds and storms and 

 also from fungi, insects, and grazing animals (fig. 8og). Doubtless the 

 thick epidermis of such leaves as those of Nerium and Ficiis is of 

 mechanical value in addition to its importance in checking evaporation; 

 in the bromelias there is a thin outer layer 

 of cutinized epidermal cells that checks 

 transpiration and a thick but non-cutinized 

 inner layer, whose role is chiefly mechanical. 

 In some leaves stiffness is increased further 

 by bast fibers, collenchyma, cells, and sclereids 

 (p. 639). The advantage of heavy cutiniza- 

 tion to mesophytes (as Taxus, Tsuga, and 

 Ficus) is more difl&cult to discern. Possibly 

 such plants have limited root systems and 

 relatively slight absorption or imperfect conduction (as in some coni- 

 fers), the cuticle thus being advantageous in reducing transpiration. 

 Possibly the cutin has no role of importance in such plants, repre- 

 senting a surviving structure once of use in some former xerophytic hab- 

 itat, or, perhaps, a structure which was never of particular advantage. 



Surface peculiarities; wax and resin deposits. — Many leaves, espe- 

 cially those that appear " glaucous," have a bluish gray surface film of 

 wax, sometimes known as bloom, which is 

 readily removed by rubbing (fig. 810). Some- 

 times these wax deposits are thick, forming a 

 brittle crust, as in Sempervivum and in the 

 wax palms, or layers of vertical rods, as in 

 the sugar cane. Wax coats are best developed 

 in xerophytes (e.g. Agave, Crassulaceae), and 

 like cutin, they appear to be increased by 

 excessive transpiration. In common with 

 other xerophytic features, glaucous leaves are 

 abundant in peat bogs and in maritime situa- 

 tions; in solutions of increasing concentration 

 the layer of bloom increases in thickness, 

 coats effectively impede transpiration, the mere rubbing of a glaucous 

 leaf sometimes inducing an increase of a third in the transpired 

 water. Wax coats also retard the heating of leaves. As with 

 hairs, but not with cutin, wax coats are best developed on the 



Fig. 810. — A cross section 

 through the leaf epidermis of 

 a xerophytic individual of the 

 glaucous willow (Salix glauco- 

 phylla), showing just outside 

 the cuticle (c) a thick layer of 

 wax grains (6), constituting a 

 glaucous bloom; highly mag- 

 nified. 



Thin as they are, wax 



