CHAP. IV. OF LEAVES. 269 



shown, in speaking of the anatomy of a leaf, that in most cases 

 it consists of a thin plate of cellular tissue pierced by air 

 vessels and woody tissue, and inclosed within a hollow empty 

 stratum of cells forming cuticle. Beneath the upper cuticle 

 the component bladders of the cellular tissue are compactly ar- 

 ranged perpendicular to the plane of the cuticle, and have 

 but a small quantity of air-cavities among them. Beneath 

 the lower cuticle the bladders are loosely arranged parallel 

 with the cuticle, and are full of air chambers communicating 

 with the stomates. The cuticle prevents too rapid an evapor- 

 ation beneath the solar rays, and thickens when it is especially 

 necessary to control evaporation more powerfully than usual ; 

 thus in the Oleander, which has to exist beneath the fervid sun of 

 Barbary, in a parched country, the cuticle is composed of not 

 less than three layers of thick-sided cuticle. To furnish leaves 

 with the means of parting with superfluous moisture, at periods 

 when the cuticle offers too much resistance, there are stomates 

 which act like valves, and open to permit its passage : or when, 

 in dry weather, the stem does not supply fluid in sufficient 

 quantity from the soil for the nourishment of the leaves, these 

 same stomates open themselves at night, and allow the 

 entrance of atmospheric moisture, closing when the cavities of 

 the leaf are full. In submersed leaves, in which no variation 

 can take place in the condition of the medium in which they 

 float, both cuticle and stomates would be useless, and accord- 

 ingly neither exists. For the purpose of exposing the fluids 

 contained in the leaves to the influence of the air, the cuticle 

 would frequently offer an insufficient degree of surface. In 

 order, therefore, to increase the quantity of surface that is 

 exposed, the tissue of the leaf is cavernous, each stomate 

 opening into a cavity beneath it, which is connected with 

 multitudes of intercellular passages. But, as too much fluid 

 might be lost by evaporation in parts exposed to the sun, we 

 find that the cells of the upper stratum of parenchyma only 

 expose their ends to the cuticle, and interpose a barrier 

 between the direct rays of the sun and the more lax respiring 

 portion forming the under stratum. It is not improbable, 

 moreover, that those cells which form die upper stratum per- 

 form a function analogous to that of the stomach in animals, 



