The shape of the base and the apex of the leaf or leaflet is often characteristic. The base may be rounded (Fig. [104]), tapering (Fig. [93]), cordate or heart-shaped (Fig. [105]), truncate or squared as if cut off. The apex may be blunt or obtuse, acute or sharp, acuminate or long-pointed, truncate (Fig. [108]). Name examples.
The shape of the margin is also characteristic of each kind of leaf. The margin is entire when it is not indented or cut in any way (Figs. [99], [103]). When not entire, it may be undulate or wavy (Fig. [92]), serrate or saw-toothed (Fig. [105]), dentate or more coarsely notched (Fig. [95]), crenate or round-toothed, lobed, and the like. Give examples.
Fig. 109.—Different Forms of Leaves from one Plant of Ampelopsis.
Leaves on the same plant often differ greatly in form. Observe the different shapes of leaves on the young growths of mulberries (Fig. [2]) and wild grapes; also on vigorous squash and pumpkin vines. In some cases there may be simple and compound leaves on the same plant. This is marked in the so-called Boston ivy or ampelopsis (Fig. [109]), a vine that is used to cover brick and stone buildings. Different degrees of compounding, even in the same leaf, may often be found in honey locust. Remarkable differences in forms are seen by comparing seed-leaves with mature leaves of any plant (Fig. [30]).
The Leaf and its Environment.—The form and shape of the leaf often have direct relation to the place in which the leaf grows. Floating leaves are usually expanded and flat, and the petiole varies in length with the depth of the water. Submerged leaves are usually linear or thread-like, or are cut into very narrow divisions: thereby more surface is exposed, and possibly the leaves are less injured by moving water. Compare the sizes of the leaves on the ends of branches with those at the base of the branches or in the interior of the tree top. In dense foliage masses, the petioles of the lowermost or undermost leaves tend to elongate—to push the leaf to the light.
On the approach of winter the leaf usually ceases to work, and dies. It may drop, when it is said to be deciduous; or it may remain on the plant, when it is said to be persistent. If persistent leaves remain green during the winter, the plant is said to be evergreen. Give examples in each class. Most leaves fall by breaking off at the lower end of the petiole with a distinct joint or articulation. There are many leaves, however, that wither and hang on the plant until torn off by the wind; of such are the leaves of grasses, sedges, lilies, orchids, and other plants of the monocotyledons. Most leaves of this character are parallel-veined.
Leaves also die and fall from lack of light. Observe the yellow and weak leaves in a dense tree top or in any thicket. Why do the lower leaves die on house plants? Note the carpet of needles under the pines. All evergreens shed their leaves after a time. Counting back from the tip of a pine or spruce shoot, determine how many years the leaves persist. In some spruces a few leaves may be found on branches ten or more years old.
Arrangement of Leaves.—Most leaves have a regular position or arrangement on the stem. This position or direction is determined largely by exposure to sunlight. In temperate climates they usually hang in such a way that they receive the greatest amount of light. One leaf shades another to the least possible degree. If the plant were placed in a new position with reference to light, the leaves would make an effort to turn their blades.
When leaves are opposite the pairs usually alternate. That is, if one pair stands north and south, the next pair stands east and west. See the box elder shoot, on the left in Fig. [110]. One pair does not shade the pair beneath. The leaves are in four vertical ranks.