CHAP. n. 
LEAVES. 
123 
observations are in accordance with his : an opinion, there- 
fore, which some botanists have entertained, that spiral ves- 
sels terminate in the stomates (D. C. Organogr. p. 272. &c.), 
must be abandoned. At the margin of a leaf the epidermis 
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 are found upon various parts of the epidermis : 
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 
then alike in appearance, and are both equally furnished with 
stomates. In succulent leaves they are said to be either alto- 
gether absent or very rare ; but this is not exactly the fact. 
They are fewer and smaller, and perhaps more imperfect, in 
succulent than in other parts, but by no means absent. Ac- 
cording to the observations of De Candolle [Organogr, p. 272.), 
they are, in the Orange and Mesembryanthemum, as ten in 
the former to one in the latter. 
I have remarked [Bot. Reg, 1540.) the singular fact, that 
certain plants have the power of forming stomates on the 
upper surface of their leaves, if from any cause their leaves 
are inverted. Thus the stomates are usually upon the under 
side of leaves, where also the veins are more prominent, and 
hairs appear exclusively, if hairs are found upon only one 
of the two surfaces. In Alstrdmeria that side of the leaves 
which is organically the undermost becomes, in consequence 
of a twist in the petiole, the uppermost, and that side which 
is borne uppermost is turned undermost ; and then the organic 
underside, being turned uppermost, has no stomates ; while 
the organic upper side, being turned downwards, although un- 
der other circumstances it would have neither stomates, hairs, 
nor elevated veins, acquires all those characters in consequence 
of its inversion. A very curious observation, in connection 
with this subject, has been made by Mirbel, in his memoir 
upon the structure of Marchantia polymorpha. 
The young bulbs by which this plant is multiplied are ori- 
ginally so homogeneous in structure, that there is no apparent 
