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 vesre- 

 table (G9), and wliich is so readily detached from the succulent leaves 

 of such jjlants as the Stonecrop and the Live-for-ever (Seduip) 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 clilorophyll. 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, 

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



2)act layers of very thick-sided 

 cells (Fig. 222). It is generally 

 tliick, or hard and impermeable, 

 in the firm leaves of the Pitto- 

 sporum, Laurustinus, and other 

 plants, which wiU thrive, for this 

 very reason, where those of more delicate foliage are liable to per- 

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



222 



= ^=s= ==| ^ ^^S^ ^SSgSSSe^^aaoop oor- 



223 



FIG 222. Magnified section through a part only of the thickness of a loaf of the Oleander, 

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

 the two Tery compact layers of cylindrical cells standing endwise. 



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

 Schleiden ; exhibiting the epidermis (a) greatly thickened hy 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 crj-stals (9-1). 



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

 mata, and the deep intercellular space beneath it. 



