148 



THE LEAVES. 



ferent is the organization of the two strata, that a leaf soon perishes 

 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 vege- 

 table (69), and which is so readily detached from the succulent leaves 

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

 the gardens. The epidermis 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 membrane. Its 

 cells 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. 221, and the Balsam, Fig. 220. It is composed of 

 two layers in cases where one might prove insufficient ; and in the 

 Oleander, besides the provision against too copious evaporation, 

 already described (263), the epidermis consists of three com- 

 pact layers of very thick-sided 

 cells (Fig. 222). It is generally 

 thick, or hard and impermeable, 

 in the firm leaves of the Pitto- 

 sporum, Laurustinus, and other 

 ' m plants, which will thrive, for this 



very reason, where tho=;e of more delicate foliage are liable to per- 

 ish, in the dry atmosphere of our rooms in winter. 



FIG 222. Magnified section through a part only of the thiekress of a leaf of the Oleander, 

 showing the epidermis of the upper surface, formed of three layers of thick-walled cells and 

 the two very compact layers of cylindrical cells standing endwise. 



FIG. 223. Magnified slice of the epidermis and superficial parenchyma of a Cactus, after 

 Sehleiden ; exhibiting the epidermis (a) greatly thickened by a stratified deposition in the cells : 

 and some cells of the parenchyma likewise nearly filled with an incrusting deposit The depo- 

 sition in such cases is always irregular, leaving canals or passages which nearly connect the 

 adjacent cells Several of the cells contain crystals (94) 



FIG 224 Similar section from another species of Cactus, passing through one of the sto- 

 mata, aud the deep intercellular space beneath, it. 



