Leaf - body, used, water, process, plants, chemical, form, reaction, carbon
Leaf
Leaf
Photo by: Subbotina Anna
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
Bay laurel leaves. (Reproduced by permission of
Field Mark Publications
.)
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).
A scanning electron micrograph of open stomata on the underside of a           rose leaf. Stomata are breathing pores scattered over the leaf surface that regulate the exchange of gases between the leaf's interior           and the atmosphere. (Reproduced by permission of
Photo Researchers, Inc.
)
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.