37. Leaves are
sessile, when the blade rests on the stem without the intervention of a petiole.
amplexicaul or stem-clasping, when the sessile base of the blade clasps the stem horizontally.
perfoliate, when the base of the blade not only clasps the stem, but closes round it on the opposite side, so that the stem appears to pierce through the blade.
decurrent, when the edges of the leaf are continued down the stem so as to form raised lines or narrow appendages, called wings.
sheathing, when the base of the blade, or of the more or less expanded petiole, forms a vertical sheath round the stem for some distance above the node.
38. Leaves and flowers are called radical, when inserted on a rhizome or stock, or so close to the base of the stem as to appear to proceed from the root, rhizome, or stock; cauline, when inserted on a distinct stem. Radical leaves are rosulate when they spread in a circle on the ground.
39. Leaves are
simple and entire, when the blade consists of a single piece, with the margin nowhere indented, simple being used in opposition to compound, entire in opposition to dentate, lobed, or divided.
ciliate, when bordered with thick hairs or fine hair-like teeth.
dentate or toothed, when the margin is only cut a little way in, into what have been compared to teeth. Such leaves are serrate, when the teeth are regular and pointed like the teeth of a saw; crenate, when regular and blunt or rounded (compared to the battlements of a tower); serrulate, and crenulate, when the serratures or crenatures are small; sinuate, when the teeth are broad, not deep, and irregular (compared to bays of the coast); wavy or undulate, when the edges are not flat, but bent up and down (compared to the waves of the sea).
lobed or cleft, when more deeply indented or divided, but so that the incisions do not reach the midrib or petiole. The portions thus divided take the name of lobes. When the lobes are narrow and very irregular, the leaves are said to be laciniate. The spaces between the teeth or lobes are called sinuses.
divided or dissected, when the incisions reach the midrib or petiole, but the parts so divided off, called segments, do not separate from the petiole, even when the leaf falls, without tearing.
compound, when divided to the midrib or petiole, and the parts so divided off, called leaflets, separate, at least at the fall of the leaf, from the petiole, as the whole leaf does from the stem, without tearing. The common stalk upon which the leaflets are inserted is called the common petiole or the rhachis; the separate stalk of each leaflet is a petiolule.
40. Leaves are more or less marked by veins, which, starting from the stalk, diverge or branch as the blade widens, and spread all over it more or less visibly. The principal ones, when prominent, are often called ribs or nerves, the smaller branches only then retaining the name of veins, or the latter are termed veinlets. The smaller veins are often connected together like the meshes of a net, they are then said to anastomose, and the leaf is said to be reticulate or net-veined. When one principal vein runs direct from the stalk towards the summit of the leaf, it is called the midrib. When several start from the stalk, diverge slightly without branching, and converge again towards the summit, they are said to be parallel, although not mathematically so. When 3 or 5 or more ribs or nerves diverge from the base, the leaf is said to be 3-nerved, 5-nerved, etc., but if the lateral ones diverge from the midrib a little above the base, the leaf is triplinerved, quintuplinerved, etc. The arrangement of the veins of a leaf is called their venation.
41. The Leaflets, Segments, Lobes, or Veins of leaves are
pinnate (feathered), when there are several succeeding each other on each side of the midrib or petiole, compared to the branches of a feather. A pinnately lobed or divided leaf is called lyrate when the terminal lobe or segment is much larger and broader than the lateral ones, compared, by a stretch of imagination, to a lyre; runcinate, when the lateral lobes are curved backwards towards the base of the leaf; pectinate, when the lateral lobes are numerous, narrow, and regular, like the teeth of a comb.
