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Leaf

Leaf

A leaf is a plant's principal organ of photosynthesis, the process by which sunlight is used to form foods from carbon dioxide and water. Leaves also help in the process of transpiration, or the loss of water vapor from a plant.

A typical leaf is an outgrowth of a stem and has two main parts: the blade (flattened portion) and the petiole (pronounced PET-ee-ole; the stalk connecting the blade to the stem). Some leaves also have stipules, small

paired outgrowths at the base of the petiole. Scientists are not quite sure of the function of stipules.

Leaf size and shape differ widely among different species of plants. Duckweeds are tiny aquatic plants with leaves that are less than 0.04 inch (1 millimeter) in diameter, the smallest of any plant species. Certain species of palm trees have the largest known leaves, more than 230 feet (70 meters) in length.

Words to Know

Abscission layer: Barrier of special cells created at the base of petioles in autumn.

Blade: Flattened part of a leaf.

Chloroplasts: Small structures that contain chlorophyll and in which the process of photosynthesis takes place.

Margin: Outer edge of a blade.

Midrib: Single main vein running down the center of a blade.

Petiole: Stalk connecting the blade of a leaf to the stem.

Phloem: Plant tissue consisting of elongated cells that transport carbohydrates and other nutrients.

Photosynthesis: Process by which a plant uses sunlight to form foods from carbon dioxide and water.

Stomata: Pores in the epidermis of leaves.

Transpiration: Evaporation of water in the form of water vapor from the stomata.

Xylem: Plant tissue consisting of elongated cells that transport water and mineral nutrients.

Leaf arrangement

A leaf can be classified as simple or compound according to its arrangement. A simple leaf has a single blade. A compound leaf consists of two or more separate blades, each of which is termed a leaflet. Each leaflet can be borne at one point or at intervals on each side of a stalk. Compound leaves with leaflets originating from the same point on the petiole (like fingers of an outstretched hand) are called palmately compound. Compound leaves with leaflets originating from different points along a central stalk are called pinnately compound.

All leaves, no matter their shape, are attached to the stem in one of three ways: opposite, alternate, or whorled. Opposite leaves are those growing in pairs opposite or across from each other on the stem. Alternate leaves are attached on alternate sides of the stem. Whorled leaves are three or more leaves growing around the stem at the same spot. Most plant species have alternate leaves.

Blade

The outer edge of a blade is called the margin. An entire margin is one that is smooth and has no indentations. A toothed margin has small or wavy indentations. A lobed margin has large indentations (called sinuses) and large projections (called lobes).

Venation is the pattern of veins in the blade of a leaf. A single main vein running down the center of a blade is called a midrib. Several main veins are referred to as principle veins. A network of smaller veins branch off from a midrib or a principle vein.

All veins transport nutrients and water in and out of the leaves. The two primary tissues in leaf veins are xylem (pronounced ZY-lem) and phloem (pronounced FLOW-em). Xylem cells mainly transport water and mineral nutrients from the roots to the leaves. Phloem cells mainly transport carbohydrates (made by photosynthesis) from the leaves to the rest of the plant. Typically, xylem cells are on the upper side of the leaf vein and phloem cells are on the lower side.

Internal anatomy of leaves

Although the leaves of different plants vary in their overall shape, most leaves are rather similar in their internal anatomy. Leaves generally consist of epidermal tissue on the upper and lower surfaces and mesophyll tissue throughout the body.

Epidermal cells have two features that prevent the plant from losing water: they are packed densely together and they are covered by a cuticle (a waxy layer secreted by the cells). The epidermis usually consists of a single layer of cells, although the specialized leaves of some desert plants have epidermal layers that are several cells thick. The epidermis contains small pores called stomata, which are mostly found on the lower leaf surface. Each individual stoma (pore) is surrounded by a pair of specialized guard cells. In most species, the guard cells close their stomata during the night (and during times of drought) to prevent water loss. During the day, the guard cells open their stomata so they can take in carbon dioxide for photosynthesis and give off oxygen as a waste product.

The mesophyll layer is divided into two parts: palisade cells and spongy cells. Palisade cells are densely packed, elongated cells lying directly beneath the upper epidermis. These cells house chloroplasts, small structures that contain chlorophyll and in which the process of photosynthesis takes place. Spongy cells are large, often odd-shaped cells lying underneath palisade cells. They are loosely packed to allow gases (carbon dioxide, oxygen, and water vapor) to move freely between them.

Leaves in autumn

Leaves are green in summer because they contain the pigment chlorophyll, which absorbs all the wavelengths of sunlight except for green (sunlight or white light comprises all the colors of the visible spectrum: red, orange, yellow, green, blue, indigo, and violet). In addition to chlorophyll, leaves contain carotenoid (pronounced kuh-ROT-in-oid) pigments, which appear orange-yellow. In autumn, plants create a barrier of special cells, called the abscission (pronounced ab-SI-zhen) layer, at the base of the petiole. Moisture and nutrients from the plant are cut off and the leaf begins to die. Chlorophyll is very unstable and begins to break down quickly. The carotenoid pigments, which are more stable, remain in the leaf after the chlorophyll has faded, giving the plant a vibrant yellow or gold appearance.

The red autumn color of certain plants comes from a purple-red pigment known as anthocyanin (pronounced an-tho-SIGH-a-nin). Unlike carotenoids, anthocyanins are not present in a leaf during the summer. They are produced only after a leaf starts to die. During the autumn cycle of warm days and cool nights, sugars remaining in the leaf undergo a chemical reaction, producing anthocyanins.

[See also Photosynthesis ]

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leaf

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

leaf / lēf/ • n. (pl. leaves / lēvz/ ) 1. a flattened structure of a higher plant, typically green and bladelike, that is attached to a stem directly or via a stalk. Leaves are the main organs of photosynthesis and transpiration.Compare with compound leaf, leaflet. See also illustration at tree. ∎  any of a number of similar plant structures, e.g., bracts, sepals, and petals. ∎  foliage regarded collectively. ∎  the state of having leaves: the trees are still in leaf. ∎  the leaves of tobacco or tea: [as adj.] leaf tea. 2. a thing that resembles a leaf in being flat and thin, typically something that is one of two or more similar items forming a set or stack. ∎  a single thickness of paper, esp. in a book with each side forming a page. ∎  [with adj.] gold, silver, or other specified metal in the form of very thin foil. ∎  the hinged part or flap of a door, shutter, or table. ∎  an extra section inserted to extend a table. ∎  the inner or outer part of a cavity wall or double-glazed window. ∎  any of the stacked metal strips that form a leaf spring. • v. [intr.] 1. (of a plant, esp. a deciduous one in spring) put out new leaves. 2. (leaf through) turn over (the pages of a book or the papers in a pile), reading them quickly or casually: he leafed through the stack of notes. PHRASES: shake (or tremble) like a leaf (of a person) tremble greatly, esp. from fear. take a leaf out of someone's booksee book. turn over a new leafsee turn.DERIVATIVES: leaf·age / ˈlēfij/ n. leafed adj. (see leaved). leaf·less adj. leaf·like / -līk/ adj. ORIGIN: Old English lēaf, of Germanic origin; related to Dutch loof and German Laub.

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leaf

leaf A flattened structure that develops from a superficial group of tissues, the leaf buttress, on the side of the stem apex. Each leaf has a lateral bud in its axil. Leaves are arranged in a definite pattern (see phyllotaxis) and usually show limited growth. Each consists of a broad flat lamina (leaf blade) and a leaf base, which attaches the leaf to the stem; a leaf stalk (petiole) may also be present. The leaves of bryophytes are simple appendages, which are not homologous with the leaves of vascular plants as they develop on the gametophyte generation. Md1506

Leaves show considerable variation in size, shape, arrangement of veins, type of attachment to the stem, and texture. They may be simple or divided into leaflets, i.e. compound (see illustration). Types of leaf include: cotyledons (seed leaves); scale leaves, which lack chlorophyll and develop on rhizomes or protect the inner leaves of a bud; foliage leaves, which are the main organs for photosynthesis and transpiration; and bracts and floral leaves, such as sepals, petals, stamens, and carpels, which are specialized for reproduction.

Leaves may be modified for special purposes. For example the leaf bases of bulbs are swollen with food to survive the winter. In some plants leaves are reduced to spines for protection and their photosynthetic function is carried out by another organ, such as a cladode.

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leaf

leaf fall of the leaf autumn, an expression first recorded in Roger Ascham's Toxophilus (1545). See also the Fall.
take a leaf out of a person's book base one's conduct on the behaviour of another person, as if by following rules written for them on a page.
turn over a new leaf improve one's conduct or performance and put unsatisfactory behaviour behind one, as by turning over the page of a book. Recorded from the late 16th century, it now always means to alter for the better, but could previously also mean just to alter or even alter for the worse.

See also fig leaf.

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leaf

leaf A thin, usually green, expanded organ borne at a node on the stem of a plant, typically comprising a petiole (stalk) and blade (lamina) and subtending a bud in the axil of the petiole. The leaves are the main site of photosynthesis. Sometimes, in classification, the term is restricted to the leaves that are diploid structures of the sporophyte generation.

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leaf

leaf.
1. Part of a door, panel, or shutter that folds, i.e. is hung on hinges or pivoted.

2. One of two skins of brick or block forming a cavity-wall.

3. Ornament derived from the leaves of plants, such as the acanthus, bay, laurel, olive, palm, or other plant. See water-leaf.

4. Very thin finish, such as a veneer, or gilding.

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leaf

leaf Part of a plant, an organ that contains the green pigment chlorophyll and is involved in photosynthesis and transpiration. It usually consists of a blade and a stalk (petiole), which attaches it to a stem or twig. Most leaves are simple (undivided), but some are compound (divided into leaflets).

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leaf

leaf part of a plant; fold of paper. OE. lēaf, corr. to OS. lōf (Du. loof), OHG. loup (G. laub). ON. lauf, Goth. laufs :- Gmc. *lauƀaz, -am, of which there are no certain cognates.

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leaf

leaf Short for leaf node. See also tree.

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leaf

leafaperitif, beef, belief, brief, chief, enfeoff, fief, grief, interleaf, leaf, Leif, lief, Mazar-e-Sharif, misbelief, motif, naif, O'Keeffe, reef, seif, Sharif, sheaf, shereef, sportif, Tenerife, thief •tea leaf • fig leaf • bas-relief • flyleaf •drop-leaf • broadleaf • cloverleaf •massif • leitmotif

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