140 THE LEAF OR PHYLLOME, 
Section 3. Tue LEAF oR PHYLLOME. 
1. GENERAL DESCRIPTION AND PARTS OF THE LEAF, 
THE leaf may be defined as a lateral development of the 
stem or branch. In the lowest leaf-bearing plants, as Mosses, 
it consists entirely of parenchyma; but in the higher classes 
of plants the leaf usually contains, in addition to the paren- 
chyma, a framework or skeleton, consisting of wood-cells or 
liber-cells, or both, and vessels of different kinds, all of which 
structures are in direct connexion with similar parts of the 
stem or branch. We distinguish therefore, in such leaves, as 
in the stem and branch, both a parenchymatous and a fibro- 
vascular system—the former constituting the soft parts, and 
the latter the hard parts, which act as a mechanical support to 
the leaf, and, by their ramification, form what are called veins 
or nerves. The leaf is therefore an appendicular organ of the 
stem, but it differs from the latter organ in the order of its 
development ; for while in the stem or branch the apex is the 
youngest part, the reverse is the case in the leaf, where the 
apex is first formed and consequently the oldest, and is gradu- 
ally pushed outwards by the formation of the other parts between 
it and the stem. 
The leaves are usually of a green colour and of a more or 
less flattened nature; but in the Stonecrop, Aloes, and many 
other plants, they are thick and fleshy, when they are said to be 
succulent. In other cases, as in the scales of the bud, the thin 
membranous coverings of tunicated bulbs and corms, the fleshy 
scales of bulbs, and the leaves of Broom-rapes, &c., they are 
colourless, or of a yellowish or brownish colour, and of simple 
structure ; they are then termed scales or cataphyllary leaves, 
the ordinary leaves being called foliage leaves. 
The part of the stem or branch from which a leaf arises is 
called a node, and the space between two nodes an iternode. 
The portion of the leaf next the stem is termed its base, the 
opposite extremity the apex, and the lines connecting the base 
and apex the margins. The leaf being commonly of a flattened 
nature, has only two surfaces; but when succulent it has 
frequently more than two surfaces. The terms upper and 
lower are applied to the two surfaces of ordinary leaves, because 
in by far the greater number of plants such leaves are placed 
horizontally, so that one surface is turned upwards, and the 
other downwards. There are certain leaves, however, which 
are placed vertically, as those of some species of Acacia and 
Eucalyptus, in which case the margins are turned upwards and 
downwards instead of the surfaces. The angle formed by the 
union of the upper surface of the leaf with the stem is called the 
axil, and everything which arises out of that point is said to be 
