Leaf (redirected from Plant leaves)

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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leaf

Structures of a leaf

(credit: © Merriam-Webster Inc.)

Any flattened, green outgrowth from the stem of a vascular plant. Leaves manufacture oxygen and glucose, which nourishes and sustains both plants and animals. Leaves and stem tissue grow from the same apical bud. A typical leaf has a broad, expanded blade (lamina), attached to the stem by a stalklike petiole. The leaf may be simple (a single blade), compound (separate leaflets), or reduced to a spine or scale. The edge (margin) may be smooth or jagged. Veins transport materials to and from the leaf tissues, radiating from the petiole through the blade. They are arranged in a netlike pattern in dicot leaves and are parallel in monocot leaves (see cotyledon). The leaf's outer layer (epidermis) protects the interior (mesophyll), whose soft-walled, unspecialized green cells (parenchyma) produce carbohydrate food by photosynthesis. In autumn the green chlorophyll pigments of deciduous leaves break down, revealing other pigment colors (yellow to red), and the leaves drop off the tree. Leaf scars that form during wound healing after the leaves drop are useful for identifying winter twigs. In conifers, evergreen needles, which are a type of leaf, persist for two or three years.

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In database management, the last node of a tree.

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Leaf

A lateral appendage which is borne on a plant stem at a node (joint) and which usually has a bud in its axil. In most plants, leaves are flattened in form, although they may be nearly cylindrical with a sheathing base as in onion. Leaves usually contain chlorophyll and are the principal organs in which the important processes of photosynthesis and transpiration occur.

Morphology

A complete dicotyledon leaf consists of three parts: the expanded portion or blade; the petiole which supports the blades; and the leaf base. Stipules are small appendages that arise as outgrowths of the leaf base and are attached at the base of the petiole. The leaves of monocotyledons may have a petiole and a blade, or they may be linear in shape without differentiation into these parts; in either case the leaf base usually encircles the stem. The leaves of grasses consist of a linear blade attached to the stem by an encircling sheath.

Leaves are borne on a stem in a definite fixed order, or phyllotaxy, according to species (Fig. 1). For identification purposes, leaves are classified according to type (Fig. 2) and shape (Fig. 3), and types of margins (Fig. 4), tips, and bases (Fig. 5). The arrangement of the veins, or vascular bundles, of a leaf is called venation (Fig. 6). The main longitudinal veins are usually interconnected with small veins. Reticulate venation is most common in dicotyledons, parallel venation in monocotyledons.

Leaf arrangement

Leaf types

Leaf shapes

Leaf margins of various types

Leaf tips and bases

Leaf venation

Surfaces of leaves provide many characteristics that are used in identification. A surface is glabrous if it is smooth or free from hairs; glaucous if covered with a whitish, waxy material, or “bloom”; scabrous if rough or harsh to the touch; pubescent, a general term for surfaces that are hairy; puberulent if covered with very fine, downlike hairs; villous if covered with long, soft, shaggy hairs; hirsute if the hairs are short, erect, and stiff; and hispid if they are dense, bristly, and harshly stiff.

The texture may be described as succulent when the leaf is fleshy and juicy; hyaline if it is thin and almost wholly transparent; chartaceous if papery and opaque but thin; scarious if thin and dry, appearing shriveled; and coriaceous if tough, thickish, and leathery.

Leaves may be fugacious, failing nearly as soon as formed; deciduous, failing at the end of the growing season; marcescent, withering at the end of the growing season but not falling until toward spring; or persistent, remaining on the stem for more than one season, the plant thus being evergreen. See Deciduous plants, Evergreen plants

Anatomy

The foliage leaf is the chief photosynthetic organ of most vascular plants. Although leaves vary greatly in size and form, they share the same basic organization of internal tissues and have similar developmental pathways. Like the stem and root, leaves consist of three basic tissue systems: the dermal tissue system, the vascular tissue system, and the ground tissue system. However, unlike stems and roots which usually have radial symmetry, the leaf blade usually shows dorsiventral symmetry, with vascular and other tissues being arranged in a flat plane.

Stems and roots have apical meristems and are thus characterized by indeterminate growth; leaves lack apical meristems, and therefore have determinate growth. Because leaves are more or less ephemeral organs and do not function in the structural support of the plant, they usually lack secondary growth and are composed largely of primary tissue only. See Apical meristem, Root (botany), Stem

The internal organization of the leaf is well adapted for its major functions of photosynthesis, gas exchange, and transpiration. The photosynthetic cells, or chlorenchyma tissue, are normally arranged in horizontal layers, which facilitates maximum interception of the Sun's radiation. The vascular tissues form an extensive network throughout the leaf so that no photosynthetic cell is far from a source of water, and carbohydrates produced by the chlorenchyma cells need travel only a short distance to reach the phloem in order to be transported out of the leaf (Fig. 7). The epidermal tissue forms a continuous covering over the leaf so that undue water loss is reduced, while at the same time the exchange of carbon dioxide and oxygen is controlled. See Epidermis (plant), Parenchyma, Phloem, Xylem

Three-dimensional diagram of internal structure of a typical dicotyledon leaf

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1.	(language)	LEAF - 1. LISP Extended Algebraic Facility. 2. "LEAF: A Language which Integrates Logic, Equations and Functions", R. Barbuti et al in Logic Programming, Functions Relations and Equations, D. DeGroot et al eds, P-H 1986, pp.201-238.
2.	(mathematics, data)	leaf - (Or "terminal node") In a tree, a node which has no daughter.