S68 



ECOLOGY 



on the upper leaf surfaces in mesophytes and in xerophytes, there are 

 no breaks whatever in the epidermis, which thus contrast strikingly with 

 the chlorenchyma. In most dicotyls the cells are nearly isodiametric 

 (figs. 8ii, 812, 911), while in monocotyls they usually are elongated in 

 the same direction as is the leaf (figs. 796, 804). Commonly the epi- 

 dermal cells of air leaves differ from the mesophyll cells in the absence of 

 chlorophyll (except in the guard cells) and in the presence of a cutinized 

 outer layer, the cuticle; such an epidermis soon ceases to have any 

 r61e other than that of protection. In submersed plants, however, the 

 epidermis contains chlorophyll and remains uncutinized, thus taking part 

 in absorption and in synthesis. The epidermal cells of hydrophytes and 



(j of mesophytes usually are 



larger than are like cells in 

 xerophytes, and the growth 

 of mesophytes in xerophytic 

 conditions commonly results 

 in a decreased cell size. 



Fig. 808. — a cross section through the leaf epi- y^g gj^^g;. epidermal walls; 



dermis of the century plant {Agave americana), . . . _i 



showing the ceUulose layer (c"), the cuticular layer CUtmtZaUon. — i he outer 



(c'). the cuticle (c), and a superficial layer of wax wall of the epidermis, origi- 



grains (A) which constitute a glaucous bloom; highly n^lly thin, and also perme- 



magnified. 1.1 u a c 



able because composed of 

 cellulose, in the adult leaf commonly is thickened through the deposi- 

 tion of citlifi, a fatty substance highly impermeable to water. Usually 

 the cutinized portion forms a continuous yellowish coat, the cuticle 

 (figs. 807, 810), below which is the slightly modified cellulose portion of 

 the outer wall. In some xerophytes a cuticular layer is interposed be- 

 tween the cellulose and the cuticle, the wall thickening progressively 

 inward before it becomes cutinized (fig. 808). In Pinus the encroach- 

 ing wall finally fills the entire lumen (fig. 1039). In the grasses and in 

 Equisetum, in addition to cutin, silica is deposited in the cell walls. 

 Highly cutinized walls are characteristic of xerophytes, and particularly 

 of evergreen xerophytes, such as conifers, ericads, and many broad- 

 leaved trees of warm temperate regions, for example, the live oak and the 

 olive. Heavy cutinization characterizes many alpine and arctic plants, 

 and also plants of peat bogs (as Ledum, Andromeda, Chamaedaphne) 

 and of tropical salt marshes (as the mangroves) ; even mesophytic ever- 

 greens, as the yew and the hemlock, may have a prominent cuticle. 

 Many succulent xerophjrtes, such as Sedum and Salsola, have a very 



