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_. [Illustration: ARRANGEMENT OF LEAVES 1. Opposite. 2. Alternate. 3. Whorled.] 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. [Illustration: LEAF OF THE PANSY WITH TWO LARGE STIPULES.] 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).