THEIR ANATOMY. 155 



expose the least possible surface, obviously serve to protect the 

 loose parenchyma beneath from the too powerful action of direct 

 sunshine. This provision is the more complete in the case of 

 plants indigenous to arid regions, where the soil is usually so 

 parched during the dry season, that, for a long period, it affords 

 only the scantiest supply of moisture to the roots. Compare, in 

 this respect, the leaf of the Lily (Fig. 179), where the upper stra- 

 tum contains but a single layer of barely oblong cells, with that of 

 the Oleander (which is obliged to stand a season of drought), the 

 upper stratum of which consists of two layers of long and narrow 

 vertical cells as closely compacted as possible (Fig. 184). So 

 different is the organization of the two strata, that a leaf soon per- 

 ishes if reversed so as to expose the lower surface to direct sunshine. 



264. A further and more effectual provision for restraining the 

 perspiration of leaves within due limits is found in the epidermis, 

 or skin, that invests the leaf, as it does the whole surface of the veg- 

 etable, and which is so readily detached from the succulent leaves 

 of such plants as the Stone-crop and the Live-for-ever (Sedum) 

 of the gardens. The Epidermis (69) is composed of small cells 

 belonging to the outermost layer of cellular tissue, with the pretty 

 thick-sided walls very strongly coherent, so as to form a firm mem- 

 brane. Its cells usually contain no chlorophyll. In ordinary 

 herbs that allow of ready evaporation, this membrane is made up 

 of a single layer of cells ; as in the Lily, Fig. 179, and the Balsam, 

 Fig. 178. It is composed of two layers in cases where one might 

 prove insufficient ; and in the Oleander, besides the provision al- 

 ready described, the epidermis consists of three layers of very 

 thick-sided cells (Fig. 184). It is generally thick, or hard and' 

 impermeable, in the firm leaves of the Pittosporum, Laurustinus, 

 &c., which will thrive, for this very reason, where other plants are 

 liable to perish, in the dry atmosphere of our rooms in winter. 



265. In such firm leaves, especially, the walls of the epidermal 

 cells are soon thickened by secondary deposition (39), especially 

 on the superficial side. This is well seen in the epidermis of the 

 Aloe, and in other fleshy plants, which bear severe drought with 

 impunity : in Fig. 185, it is shown, at a, in the rind of a Cactus, 

 where the green layer of the whole stem answers the purpose of 

 the leaves. Sometimes an exterior layer of this superficial de- 

 posit in the epidermis, or a secretion from it, may be detached 

 in the form of a continuous, apparently structureless membrane, 



