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The arrangement of the leaves on the stem is a matter of great importance for purposes of identification. Especially should it be noted whether the leaves are opposite, alternate, whorled (arranged in circles round the stem), or radical (apparently starting direct from the root).

Some leaves have smaller leaves or scales at their bases, that is, at the points where they are attached to the stem of the plant. Such leaves or scales are termed stipules. They are often so well developed that they are as conspicuous as the ordinary foliage leaves, and in such instances they perform the functions of the latter. The presence and character of the stipules should always be noted. A leaf without stipules is said to be exstipulate.

Arrangement of Leaves

1. Opposite. 2. Alternate. 3. Whorled.

Leaf of the Pansy with Two Large Stipules.

A leaf usually consists of two distinct parts—the petiole or stalk, and the lamina or blade. Some, however, have no petiole, but the blade is in direct contact with the stem. These leaves are said to be sessile, and some of them clasp the stem, or even extend downwards on the stem, forming a wing or a sheath.

A leaf is said to be simple when the blade is in one continuous whole, even though it may be very deeply divided; but when the blade is cut into distinct parts by incisions that extend quite into the midrib (the continuation of the stalk to the tip of the leaf), the leaf is compound.

The student must be careful to distinguish between compound leaves and little branches or twigs bearing several simple leaves, for they are often very similar in general appearance. The compound leaf may always be known by the total absence of buds, and often by the presence of one or more stipules at the base of its stalk; while a branch bearing a similar appearance usually has a terminal bud, also buds in the exils of its leaves, and never any stipules at the point where it originates. The distinct parts of compound leaves are termed leaflets.

Attention to the form and character of the leaf is often of as much importance as the observation of the flower in the determination of species. Not only should we note the general shape of the leaf, but also the character of its surface, its margin, and its apex. The surface may be smooth, hairy, downy, velvety, shaggy, rough, wrinkled or dotted. The margin is said to be entire when it is not broken by incisions of any kind. If not entire it may be toothed, serrate (sawlike), crenate or wavy. Sometimes it happens that the teeth bear still smaller teeth, in which case the margin is said to be doubly toothed; or, if the teeth are sawlike, it is doubly serrate. As regards the apex, it is generally sufficient to note whether it is acute (sharp), obtuse (blunt), or bifid (divided into two).

Margins of Leaves

1. Entire. 2. Serrate or sawlike. 3. Doubly serrate. 4. Dentate or toothed. 5. Crenate. 6. Doubly crenate. 7. Sinuate or wavy.

It is not necessary to describe separately all the principal forms of simple and compound leaves. These are illustrated, and the student should either make himself acquainted with the terms applied to the different shapes, or refer, as occasion requires, to the illustrations. Concerning the compound leaves, however, their segments are themselves sometimes divided after the manner of the whole, and even the secondary segments may be similarly cut. Thus, if the segments of a pinnate leaf are themselves pinnately compound, the leaf is said to be bi-pinnate; and, if the secondary segments are also compound, it is a tri-pinnate leaf.