Many of the roughly 275,000 different species of plants that produce leaves can be distinguished from one another by their leaves alone. The variety of shapes, sizes, and textures of leaves seems to be almost infinite. The leaves of some of the smaller duckweeds are less than 1 millimeter (0.04 inch) wide. The mature leaves of the Seychelles Island palm can be 6 meters (20 feet) long, and the floating leaves of a giant water lily, which reach 2 meters (6.5 feet) in diameter (Fig. 7.3), can support, without sinking, weights of more than 45 kilograms (100 pounds) distributed over their surface. In lilies, pines, ferns, and many other plants, different forms of leaves (e.g., tiny papery scales; colored leaves called bracts; spines) may be produced, along with typical photosynthetic leaves, on the same plant.
In addition to flattened, variously shaped, colored, spinelike leaves and those of various textures, there are others that are tubular, feathery, cup shaped, or needlelike; in
fact, leaves may assume virtually any form (Fig. 7.4). They may be smooth or hairy, slippery or sticky, waxy or glossy, pleasantly fragrant or foul smelling, edible or poisonous. They also may be of almost every color of the rainbow and of exquisite beauty, especially when viewed with a microscope.
Leaves are attached to stems at regions called nodes, with stem regions between nodes being known as intern-odes. The arrangement of leaves on a stem ( phyllotaxy) in a given species of plant generally occurs in one of three ways. In most species, leaves are attached alternately or in a spiral along a stem, with one leaf per node, in what is called an alternate arrangement. In some plants, two leaves may be attached at each node, providing an opposite arrangement. When three or more leaves occur at a node, they are said to be whorled.
The arrangement of veins in a leaf or leaflet blade (venation) may also be either pinnate or palmate. In pinnately veined leaves, there is one primary vein called the midvein, which is included within an enlarged midrib; secondary veins branch from the midvein. In palmately veined leaves, several primary veins fan out from the base of the blade. The primary veins are more or less parallel to one another in monocots (Fig. 7.5) and diverge from one another in various ways in dicots (see Fig. 7.9). The branching arrangement of veins in dicots is called netted, or reticulate venation. In a few leaves (e.g., those of Ginkgo), no midvein or other large veins are present. Instead, the veins fork evenly and progressively from the base of the blade to the opposite margin. This is called dichotomous venation (Fig. 7.4K).
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