THEIK ANATOMICAL STRUCTURE. 149 



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

 cells are soon thickened by internal deposition (44), 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. 223, it is shown, at a, in the rind of a Cactus, 

 in which the green layer of the whole stem answers the purpose 

 of leaves. Sometimes an exterior layer of this superficial deposit 

 in the epidermis may be detached in the form of a continuous, ap- 

 parently structureless membrane, which Brongniart and succeeding 

 authors have called the Cuticle. That it may shed water readily, 

 the surface of leaves is commonly protected by a very thin varnish 

 of wax, or else with a bloom of the same substance in the form of a 

 whitish powder, which easUy rubs off (85), as is famiharly seen in a 

 Cabbage-leaf. 



266. A thickening deposit sometimes takes place in the cells of 

 parenchyma immediately underneath the epidermis, especially in 

 the Cactus Family, where the once thin and delicate walls of the 

 cells become excessively and irregularly thickened (Fig. 223, 224), 

 so as doubtless to arrest or greatly obstruct exhalation through the 

 rind. Something like this choking of the cells must commonly 

 occur with age in most leaves, particularly those that live for more 

 than one season (311). 



267. But the multiplication of these safeguards against exhalation 

 miglat be liable to defeat the very objects for which leaves are prin- 

 cipally destined. Evaporation from the parenchyma of the leaves 

 is essential to the plant, as it is the only method by which its exces- 

 sively dilute food can be concentrated. Some arrangement is requi- 

 site that shall allow of sufficient exhalation from the leaves while 

 the plant is freely supplied with moisture by the roots, but restrain 

 it when the supply is deficient. It is clear that the greatest demand 

 is made upon the leaves at the very period when the supply through 

 the roots is most hkely to fail ; for the summer's sun, which acts so 

 powerfully on the leaves, at the same time parches the soil upon 

 which the leaves ( through the rootlets) depend for the moisture they 

 exhale. So long as their demands are promptly answered, all goes 

 well. The greater the force of the sun's rays, the greater the speed 

 at which the vegetable machinery is driven. But whenever the 

 supply at the root fails, the foliage begins to flag and droop, as is so 

 often seen under a sultry meridian sun ; and if the exhaustion pro- 

 ceeds beyond a certain point, the leaves inevitably wither and perish. 



13* 



