CHAP. II. 
LEAVES. 
99 
tes; but in submersed leaves the parenchyma is naked, no 
cuticle overlaying it. 
The general nature of the parenchymatous part of leaves 
has been very well explained, both by Link and others, and 
figured by Mohl, firstly in 1828 ( Uher die Poren des Pflanzen- 
zellgewehes, tab. i. fig. 4, &c.), and afterwards in his very ela- 
borate enquiry into the Anatomy of Palms. But the most 
complete account is that of Adolphe Brongniart, in 1830 
{Annales des Sc., vol. xxi. p. 420.), of which the principal 
part of what follows is an abstract. 
The cuticle is a layer of bladders adhering firmly to each 
other, and sometimes but slightly to the subjacent tissue, from 
which they are entirely different in form and nature : in form, 
for the cellules are depressed, and, in consequence of the variety 
of outline that they present, form meshes either regular or 
irregular; and in nature, because these bladders are perfectly 
transparent, colourless, and probably filled with air, — for the 
manner in which light passes through them proves that they 
do not contain dense fluid. They scarcely ever contain any 
organic particles, and are probably but little permeable either 
to fluids or gaseous matter; while, on the other hand, the 
bladders of the subjacent parenchyma are filled with the green 
substance that determines the colour of the leaf. The cuticle 
is not always formed of a single layer of bladders, but in some 
cases consists of two, or even three. No trace whatever is 
discoverable of vessels either terminating in or beneath the 
cuticle ; Brongniart states this most explicitly, and my own 
observations are entirely in accordance with his : an opinion, 
therefore, which some botanists have entertained, that spiral 
vessels terminate in the stomates (D. C. Organogr, p. 272, 
&c.), must hereafter be abandoned. At the margin of a leaf 
the cuticle is generally harder than elsewhere, and sometimes 
becomes so indurated as to assume a flinty texture, as in the 
Aloe, and many other plants. 
Stomates (p. 39,) are found upon various parts of the cu- 
ticle : in some plants only on that of the under side of leaves, 
in others on the upper also ; in floating leaves upon the latter 
only. When leaves are so turned that their margins are 
directed towards the earth and the heavens, the two faces are 
H 2 
