The petiole may be absent in a leaf, and when it is the leaves are termed sessile, or sitting leaves. These leaves sometimes coil round the stem, and are called “amplexicaul,” or stalk embracers. These are simple leaves. Compound leaves are composed of several blades or laminæ on a stalk, and are seldom sessile. Simple Leaves are almost innumerable in form and variety. Leaves may be equal or unequal, acicular or linear, rounded or oval, cordate or obcordate, reniform or sagittate, perfoliate or connate, crisp, whorled or truncate, retuse, acuminate or mucronate. The margins of simple leaves again are entire, deft, notched, crenated, crenulated, sinuous, or dentated. They are pinnatifid, multifid, or lobed, according to the divisions of the leaf.
Compound leaves are also divided into classes. The pinnate, as the rose-leaf, the clover trefoil. There are “doubly pinnate,” the digitate, as in the horse chestnut. Compound and simple leaves can be readily distinguished by inspection, for the former are “articulated” to the stalk and can be separated, but the simple leaves will be torn, for they are confluent throughout.
Leaves are evergreen or deciduous, accordingly as they retain or shed themselves. The ordinary leaf is deciduous; the fir and the yew and the imported laurel are evergreens. We have very few of these as natives of England, the ivy, yew, and fir being the three most common. Sometimes a plant peculiar to Killarney, and known as the arbutus, is included in the list. But the Scotch fir and the yew are distinctly native evergreens.
The detailed characteristics of leaves must be passed over until we come to the fly-catching leaves—such as Venus’ fly-trap, the droseras, and nepenthes, which appear to catch and devour insects for food. The Venus’ fly-trap may be examined, and we shall find the leaves covered with tiny and very sensitive hairs. Often a fly happens to alight upon the leaf, which is extended in a most innocent manner (see illustration). As soon as the fly settles the leaves close, and the digits lock tightly together, thus preventing the escape of the prey. The droseras, sarracenias, and nepenthes also kill their food. The sarracenias form curious cups, into which insects are enticed in search of fluid, and then, as in the case of the house-haunting cockroach, they cannot get out again. The nepenthes have a cup and lid for insect-catching, and within the cup a liquid is secreted.
We will close this portion of our subject with a quotation from a recent article upon botany referring to leaf arrangement. The writer says:—
Fig. 765.—Leaf of Dionæ.
