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 usvial ; 
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 the upper stratum per- 
form a function analogous to that of the stomach in animals. 
