CHAP. VI, 
OF LEAVES. 
325 
most cases it consists of a thin plate of cellular tissue pierced 
by air vessels and woody tissue, and enclosed within a hollow 
empty stratum of cells forming epidermis. Beneath the upper 
epidermis the component bladders of the parenchyma are 
compactly arranged perpendicular to the plane of the epi- 
dermis, and have but a small quantity of air cavities among 
them. Beneath the lower epidermis the parenchyma is loosely 
arranged parallel with it, and is full of air chambers com- 
municating with the stomates. The epidermis prevents too 
rapid an evaporation 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 epidermis is composed of not less than three layers of 
thick-sided cells. To furnish leaves with the means of parting 
with superfluous moisture, at periods when the epidermis 
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 
epidermis and stomates would be useless, and accordingly 
neither exists. For the purpose of exposing the fluids con- 
tained in leaves to the influence of air, the epidermis would 
frequently offer an insufficient degree of surface. In order, 
therefore, to increase the quantity of surface exposed, the 
tissue of the leaf is cavernous, each stomate opening into a 
cavity beneath it, which is connected with multitudes of in- 
tercellular 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 epidermis, 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 perform a 
function analogous to that of the stomach in animals, digest- 
Y 3 
