156 



THE LEAVES. 



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 easily rubs 

 off (86), as familiarly 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, so as doubtless 

 greatly to obstruct or arrest all exhalation through the rind. Some- 

 thing like this choking of the cells must commonly occur with age 

 in most leaves, particularly those that live for more than one season. 



267. But the multiplication of these safeguards against exhala- 

 tion might be liable to defeat the very objects for which leaves are 

 principally destined. Evaporation from the parenchyma of the 

 leaves is essential to the plant, as it is the only method by which 

 its excessively dilute food can be concentrated. Some arrange- 

 ment is requisite 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 likely to fail : for the sum- 

 mer'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 



FIG. 183. Magnified slice of the epidermis and superficial parenchyma of a Cactus, after 

 Schleiden ; exhibiting the epidermis greatly thickened by a stratified deposition in the cells : 

 and the 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 (91). 



FIG. 186. Similar section from another species of Cactus, passing through one of the sto- 

 mata, and the deep intercellular space beneath it. 



