STRUCTURE OF AERIAL LEAVES. 143 
the hard parts, which by their ramification form what are called 
the veins or nerves. 
A. The Petiole.—This when present consists of fibro-vascular 
tissue (fig. 276), surrounded by parenchyma, and the whole 
covered by epidermis, which commonly contains a variable 
number of stomata, and is frequently furnished with hairs and 
other epidermal appendages. The parenchyma immediately 
below the epidermis is sometimes specially modified and forms 
the tissue known as collenchyma (fig. 95), which is one form 
of what has been termed the hypoderma (see pages 48 and 145), 
The fibro-vascular tissue varies in its nature in the leaves of 
the different classes of plants, being merely prolongations of 
that of the three kinds of stems already fully described. 
Thus in Dicotyledons the fibro-vascular tissue (jig. 276) com- 
monly consists of spiral, and pitted, annular, or some other 
vessels, and also of sieve-tubes, and 
wood and liber-cells, that is, of the 
same elements essentially as the wood 
and liber—the spiral vessels and the 
other structures belonging to the 
xylem being placed above those of the 
phloém or liber. 
B. The Lamina.—The whole of 
the lamina is covered by the epider- 
mis, which is furnished with stomata | 
in the manner already described. | 
The stomata are, however, almost | 
confined to that portion of the epider- 
mis which corresponds to the paren- 
chyma of the leaf. The epidermis is 
also frequently furnished with various 
appendages, as Hairs, Glands, and 
their several modifications. The epi- 
dermis with its stomata and appen- 
dages having been already fully de- 
scribed under their respective heads, 
Fic. 276. 
Fig. 276. Fibro-vascular tissue 
passing from a branch, 3}, of 
an herbaceous Dicotyledon 
into the petiole, p, surrounded 
by parenchyma. «a, Articu- 
it now remains only to allude to the 
fibro-vascular and parenchymatous 
systems of the lamina which are 
situated between the epidermis of its 
lation between the petiole 
and the branch from which it 
arises, ¢, 7. Spiral and annu- 
lar vessels. 7,7. Wood-cells, 
Z, 1. Liber cells. 
upper and lower surfaces. 
1. Fibro-vascular System.—This is in direct connexion with 
that of the stem or branch in the three great divisions of plants 
respectively. We shall direct our attention more especially to 
that of the leaves of Dicotyledons. The fibro-vascular system 
of such plants in by far the majority of cases consists of an 
upper layer which is in connexion with the fibro-vascular system 
of the wood and petiole when present (fig. 276, t, f); and of a 
lower which is continuous with the liber (/). The upper layer 
