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leaf
chief food-manufacturing organ of a plant, a lateral outgrowth of the growing point of stem. The typical leaf consists of a stalk (the petiole) and a blade—the thin, flat, expanded portion (needlelike in most conifers) that is normally green in color because of the presence of the pigment chlorophyll. In many leaves, small processes called stipules occur at the base of the stalk and protect the bud; sometimes the stipule is large (as in the Japanese quince) and, if green, also manufactures food. The leaf blade is veined with sap-conducting tubes (xylem and phloem) with thick-walled supporting cells. The blade consists of an upper and a lower layer of closely fitted epidermal cells, including specialized paired guard cells that control the size of tiny pores, or stomata, for gaseous exchange and the release of water vapor (see transpiration ). The upper epidermis is usually coated with a waterproof cuticle and contains fewer stomata than the underside, if any at all. Between these two layers are large palisade and spongy cells, rich in chlorophyll for food manufacture (see photosynthesis ) and permeated with interconnecting air passages leading to the stomata. Leaves vary in size (up to 60 ft/18m long in some palms), shape, venation, color, and texture, and are classified as simple (one blade) or compound (divided into leaflets). The blade margins may be entire (smooth and unindented), toothed (with small sharp or wavy indentations), or lobed (with large indentations, or sinuses). In monocotyledonous plants, the veins are usually parallel; dicotyledons have leaves with reticulately branched veins that may be pinnate (with one central vein, the midrib, and smaller branching veins) or palmate (with several large veins branching from the leaf base into the blade). Pigments besides chlorophyll that give a leaf its characteristic color are the carotenoids (orange-red and yellow), the anthocyanins (red, purple, and blue), and the tannins (brown). White results from the absence of pigments. In deciduous plants, a layer of cells forms the abscission tissue at the base of the stalk in the autumn, cutting off the flow of sap; the unstable chlorophyll disintegrates and, in a temperate zone, the remaining pigments are displayed to produce colorful fall foliage. When these cells dry up completely, the leaf falls. Evergreen plants usually produce new leaves as soon as the old ones fall; the leaves of most conifers remain on the tree from 2 to 10 years (in some species up to 20 years). Leaves may be modified or specialized for protection (spines and bud scales), climbing (tendrils), trapping insects (as in pitcher plants), water storage (as in succulents), or food storage (bulb scales and, in the embryo plantlet, cotyledons).
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