100 THE VEGETABLE WORLD. 
rectilinear and figuring as a square, sometimes irregular and 
sinuous, as in Fig. 125. The granular contents of these cellules 
are neither very apparent nor very important, but we find in them 
betimes an aqueous liquid variously coloured. The cellular 
elements of this epidermous membrane are intimately united and 
pressed one against the other, in such a manner as to give it a cer- 
tain solidity and power of resistance. 
The cells present an exterior wall 
in connection with the air, which 
is much thicker than the lateral or 
interior walls. Some of these cells 
are sometimes observed to be elon- 
gated, ramified, partitioned off, as it 
were, so as to constitute hairs or 
down of various forms. 
The epidermic membrane is not 
continuous or perfectly close. It 
resents, on the contrary, from 
space to space, small openings formed by the separation of two 
cells. These openings being elastic, and capable of expansion oT 
compression, according to exterior circumstances, are intended to 
exhale the gaseous and vapoury products of perspiration in the 
plant, and also to absorb the gas and moisture of the atmosphere, 
the function of the cellular tissue of the epidermis being to 
transmit fluids in all directions. They bear the name of stomates, 
from the Greek word oropa; 
“mouth.” A stomates of Cycas 
y, under the microscope is seen i2 
Fig. 126. 
The stomates are most abundant 
on the lower surface of leaves. 
Their number varies much according to the plant ; and the smaller 
they are, the more numerous. In the Pink they present four 
thousand in the space of a square inch; the Iris twelve thousand, 
and the Lilac a hundred and twenty thousand. The epidermis 
which covers and protects the parenchyma of the leaf is itself covered 
with an extremely delicate pellucid membrane, whose structure is, 
almost inappreciable, the discovery of which we owe to M. Adrien 
Fig. 125.—Structure of the epidermis of a leaf. 
