Leaf

Morphology

Blade

Venation

Anatomy

Epidermis

Mesophyll

Veins

Phyllotaxy

Evolution

Resources

A leaf is a plant organ that is an outgrowth of the stem, and has three main parts: the blade, a flattened terminal portion; the petiole, a basal stalk that connects the blade to the stem; and the stipules, small appendages at the base of the petiole. However, the leaves of many species lack one or more of these three parts.

Leaves function in photosynthesis, or the biological conversion of sunlight into chemical energy; in transpiration, or the transport of water from the plant by evaporation; and in cellular respiration, the oxidation of foods, and consequent synthesis of high-energy molecules.

There is great variety of leaf size and shape among different species of plants. Duckweeds are tiny aquatic plants with leaves that are less than 1 millimeter in diameter, the smallest of any species of vascular plant. Certain species of palm tees have the largest known leaves, more than 60 feet (18 m) in length.

Morphology

All leaves can be classified as simple or compound. A simple leaf has a single blade; a compound leaf consists of two or more separate blades, each of which is termed as leaflet. Compound leaves may be palmately compound, in which the separate leaflets originate from one point on the petiole, or pinnately compound, in which the leaflets originate from different points along a central stalk that extends from the petiole.

Blade

The size and shape of the blade are often characteristic of a species, and are useful in species identification. However, the leaf blades of some species, such as those of oaks (Quercus), exhibit great variation in size and shape, sometimes even on the same tree. Botanists use a large vocabulary of specialized terms to describe leaf outline, margin, apex, base, and vestiture (surface covering). For example, pine (Pinus) leaves are considered acicular, meaning they are shaped like a needle; aspen (Populus) leaves are considered ovate, meaning they resemble a two-dimensional projection of an egg; may apple (Podophyllum) leaves are considered peltate, meaning they are shaped like a shield, and are attached to the stalk on the lower leaf surface.

Venation

Venation is the pattern of veins in the leaf blade. The veins consist of vascular tissues important for the

transport of food and water. Leaf veins connect the blade to the petiole, and lead from the petiole to the stem. The two primary vascular tissues in leaf veins are xylem, which is important for transport of water and soluble ions into the leaf, and phloem, which is important for transport of carbohydrates (made by photosynthesis) from the leaf to the rest of the plant.

A leaf’s venation pattern is classified as reticulated, parallel, or dichotomous. In reticulated venation, the veins are arranged in a netlike pattern, in that they are interconnected like the strands of a net. Reticulated venation is the most common venation pattern, and occurs in the leaves of nearly all dicotyledonous Angiosperms, whose embryos have two cotyledons (seed leaves) as in flowering plants such as maple, oak, and rose. In parallel venation, the veins are smaller in size and parallel (or nearly parallel) to one another, although a series of smaller veins connects the large veins. Parallel venation occurs in the leaves of nearly all monocotyledonous Angiosperms, whose embryos have one cotyledon, as in flowering plants such as lilies and grasses. In dichotomous venation, the veins branch off from one another like the branches of a tree. This is the rarest venation pattern, and occurs in the leaves of some ferns and in the gymnospermtree, Ginkgo biloba.

Anatomy

Although the leaves of different plants vary in their overall shape, most are rather similar in their internal anatomy. Leaves generally consist of epidermal tissue on the upper and lower surfaces, vascular tissues in the veins, and less-differentiated mesophyll, or ground tissue, throughout the body. Some plant species have a special type of photosynthesis, known as C-4 photosynthesis, and their leaves have a unique internal anatomy. These highly specialized leaves are not considered here.

Epidermis

Epidermal cells are on the upper and lower surfaces of a leaf. They have two features that prevent evaporative water loss: they are packed densely together and 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. Epidermal cells often have large vacuoles that contain flavonoid pigments. Flavonoids generally absorb ultraviolet radiation, and may act as a sort of natural sunscreen for the internal layers of the leaf, by filtering out harmful ultraviolet radiation from the sun.

The leaf epidermis has small pores, called stomata, which open up for photosynthetic gas exchange and transpiration. Stomata are scattered throughout the epidermis, but are typically more numerous on the lower leaf surface. Each individual stoma (pore) is surrounded by a pair of specialized epidermal cells, called guard cells. In most species, the guard cells close their stomata during the night to prevent transpirational water loss, and open their stomata during the day so they can take up carbon dioxide for photosynthesis, and give off oxygen as a waste product.

Mesophyll

Mesophyll cells constitute the main body of a leaf, occurring between the upper and lower epidermis. Typically, the leaves of temperate zone plants have two layers of mesophyll cells, the palisade mesophyll on the adaxial (upper) side, and the spongy mesophyll on the abaxial (lower) side. The palisade mesophyll is a layer of densely packed, columnar cells that contain many chloroplasts. This layer is responsible for most of the photosynthesis of leaves. The spongy mesophyll is composed of large, often odd-shaped photosynthetic cells separated from one another by large intercellular spaces that apparently facilitate the exchange of photosynthetic gases.

Veins

Veins, which penetrate the mesophyll layers of a leaf, consist of vascular tissue, xylem, and phloem, and connect the vascular tissue of the stem to the photo-synthetic cells of the mesophyll, via the petiole. Xylem cells mainly transport water and minerals from the roots to the leaves, and phloem cells mainly transport carbohydrates made by photosynthesis in the leaves to the rest of the plant. Typically, the xylem cells are on the upper side of the leaf vein, and the phloem cells are on the lower side.

Phyllotaxy

Phyllotaxy is the arrangement of leaves on a stem. As a stem grows at its apex, new leaf buds form along the stem by a highly controlled developmental process. Depending on the species, the leaf origins on the stem may be opposite (in which leaves arise in pairs on opposite sides of the stem), whorled (three or more leaves arise from the same locus on the stem), or alternate (leaves are arranged in a helix along the stem).

Most species have alternate leaves. This pattern is often called spiral phyllotaxy because a spiral is formed when an imaginary line is drawn that connects progressively older leaf origins on the stem. The divergence angle of successive leaves determines the developmental spiral of leaves and homologous plant organs, such as the individual florets of a sunflower, and has been intensively studied by botanists and mathematicians since the mid-1800s. Interestingly, the angle between successive leaves on a stem is often about 137.5 degrees, known as the Fibonacci angle.

Two major theories have been proposed to explain spiral phyllotaxy: available space theory and repulsion theory. Both propose that the siting of leaf nodes is determined by the position of existing leaf nodes. While the two are often portrayed as competing theories, they are not in fact mutually exclusive. Available space theory says that the physical space among existing leaf nodes determines where new leaves originate. Repulsion theory says that synthesis of a chemical growth inhibitor(s) at the apex of older leaf nodes determines the position of new leaves, and that new leaves form only where the concentration of the growth inhibitor(s) is below a certain threshold.

Evolution

About 380 million years ago, plants with vascular tissue first evolved a special type of leaf, referred to as a microphyll. A microphyll typically has a single mid-vein, and arises from a stem that does not have leaf gaps, in regions of parenchyma (i.e, unspecialized) tissue where the vascular strand leads into the leaf base. The microphyll may have originated as an outgrowth of a vascularized stem, or by evolutionary simplification of a complex branch system. The leaves of certain modern plants in the Lycopodophyta (Lycopods) and Sphenophyta (Horsetails) are classified as microphylls. Although the microphylls of these modern plants are quite small, some of their fossilized relatives had very large microphylls.

About 350 million years ago, plants first evolved megaphylls, the leaf type of modern seed plants and ferns. A megaphyll typically has a complex venation pattern, and arises from a stem that has leaf gaps, or regions of parenchyma tissue where the vascular strand leads into the leaf base. One theory proposes that megaphylls, as well as other plant organs, evolved by modification of branch systems. In other words, megaphylls may have evolved by the flattening of a three-dimensional branch system, and connection of the flattened branches with a cellular webbing.

Many botanists believe that the four different whorls of a flower (sepals, petals, stamens, and carpels) originated by evolutionary modification of the megaphylls of a free-sporing plant. Shortly after the evolutionary origin of the Angiosperms (flowering plants), there was a major division between the mono-cots (plants whose embryos have one cotyledon) and dicots (plants whose embryos have two cotyledons). The leaves of modern monocots, such as grasses and lilies, tend to be narrow and have parallel venation; the leaves of dicots tend to be wide, and have reticulated venation.

Many modern plants have evolved complex and highly specialized leaves. For example, the insecteating organs of carnivorous plants, such as Venus Flytrap, Sundew, Pitcher Plant, and Bladderwort, are all highly specialized leaves. Dischidia rafflesiana, a tropical epiphyte, has among the most specialized leaves of any plant. Its tubular leaves collect forest debris and rainwater, providing a habitat for the colonies of ants that live inside. As the ants die, their bodies dissolve and special roots of the plant absorb

KEY TERMS

Cellular respiration —The oxidation of food within cells, involving consumption of oxygen and production of carbon dioxide, water, and chemical energy in the form of ATP (adenosine triphosphate).

Dicot —A plant whose embryo has two cotyledons (that is, seed leaves) and several other general characteristics.

Epiphyte —A plant that relies upon another plant, such as a tree, for physical support, but does not harm the host plant.

Leaf gap —A gap in the vascular system of a stem, just above where the stem’s vascular tissue has curved off into the petiole.

Megaphyll —A leaf type typical of seed plants and ferns, which has branched veins and is usually associated leaf gaps in the stem.

Microphyll —A leaf type typical of Sphenophyta (horsetails) and Lycopodophyta (lycopods) that is scale-like, not associated with leaf gaps in the stem, and usually has a single midvein.

Monocot —A plant whose embryo has a single cotyledon (seed leaf) and several other general characteristics.

Photosynthesis —The biological conversion of light energy into chemical energy.

Transpiration —The movement of water out of a plant, most of which occurs through the stomata of leaves.

the nutrients that are released, providing nourishment for the plant.

Resources

BOOKS

Attenborough, D. The Private Life of Plants. Princeton, NJ:

Princeton University Press, 1995.

Galston, A.W. Life Processes of Plants: Mechanisms for

Survival. New York: W.H. Freeman Press, 1993. Kaufman, P.B., et al. Plants: Their Biology and Importance.

Harp College Press, 1990.

Wilkins, M. Plant Watching. New York: Facts on File Inc.,

1988.

OTHER

Bellevue Community College, Science Division. “The Leaf” <http://scidiv.bcc.ctc.edu/rkr/biology203/labs/pdfs/Leaf.pdf> (accessed December 2, 2006)

Peter A. Ensiminger

Leaf

A leaf is a plant organ which is an outgrowth of the stem, and has three main parts: the blade, a flattened terminal portion; the petiole, a basal stalk which connects the blade to the stem; and the stipules, small appendages at the base of the petiole. However, the leaves of many species lack one or more of these three parts.

Leaves function in photosynthesis , or the biological conversion of light energy into chemical energy; in transpiration , or the transport of water from the plant by evaporation ; and in cellular respiration, the oxidation of foods, and consequent synthesis of high-energy molecules.

There is great variety of leaf size and shape among different species of plants. Duckweeds are tiny aquatic plants with leaves that are less than 1 millimeter in diameter, the smallest of any species of vascular plant. Certain species of palm tees have the largest known leaves, more than 60 ft (18 m) in length.

Morphology

All leaves can be classified as simple or compound. A simple leaf has a single blade, whereas a compound leaf consists of two or more separate blades, each of which is termed as leaflet. Compound leaves may be palmately compound, in which the separate leaflets originate from one point on the petiole, or pinnately compound, in which the leaflets originate from different points along a central stalk which extends from the petiole.

Blade

The size and shape of the blade are often characteristic of a species, and are useful in species identification. However, the leaf blades of some species, such as those of oaks (Quercus), exhibit great variation in size and shape, sometimes even when on the same tree . Botanists use a large vocabulary of specialized terms to describe the leaf outline, margin, apex, base, and vestiture (surface covering). For example, Pine (Pinus) leaves are considered acicular, meaning they are shaped like a needle; Aspen (Populus) leaves are considered ovate, meaning they resemble a two-dimensional projection of an egg; May apple (Podophyllum) leaves are considered peltate, meaning they are shaped like a shield, and are attached to the stalk on the lower leaf surface.

Venation

Venation is the pattern of veins in the blade of a leaf. The veins consist of vascular tissues which are important for the transport of food and water. Leaf veins connect the blade to the petiole, and lead from the petiole to the stem. The two primary vascular tissues in leaf veins are xylem, which is important for transport of water and soluble ions into the leaf, and phloem, which is important for transport of carbohydrates (made by photosynthesis) from the leaf to the rest of the plant.

The venation pattern of a leaf is classified as reticulated, parallel , or dichotomous. In reticulated venation, the veins are arranged in a net-like pattern, in that they are all interconnected like the strands of a net. Reticulated venation is the most common venation pattern, and occurs in the leaves of nearly all dicotyledonous Angiosperms, whose embryos have two cotyledons (seed leaves) as in flowering plants such as Maple, Oak, and Rose. In parallel venation, the veins are all smaller in size and parallel or nearly parallel to one another, although a series of smaller veins connects the large veins. Parallel venation occurs in the leaves of nearly all monocotyledonous Angiosperms, whose embryos have one cotyledon, as in flowering plants such as lilies and grasses . In dichotomous venation, the veins branch off from one another like the branches of a tree. This is the rarest venation pattern, and occurs in the leaves of some ferns and in the gymnospermtree, Ginkgo biloba.

Anatomy

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, vascular tissues in the veins, and less-differentiated mesophyll, or ground tissue, throughout the body. Some plant species have a special type of photosynthesis, known as C-4 photosynthesis, and their leaves have a unique internal anatomy. These highly specialized leaves are not considered here.

Epidermis

Epidermal cells are on the upper and lower surfaces of a leaf. They have two features which prevent evaporative water loss: 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 which are several cells thick. Epidermal cells often have large vacuoles which contain flavonoid pigments. Flavonoids generally absorb ultraviolet radiation , and may act as a sort of natural sunscreen for the internal layers of the leaf, by filtering out harmful ultraviolet radiation from the sun .

The leaf epidermis has small pores, called stomata, which open up for photosynthetic gas exchange and transpiration. Stomata are scattered throughout the epidermis, but are typically more numerous on the lower leaf surface. Each individual stoma (pore) is surrounded by a pair of specialized epidermal cells, called guard cells. In most species, the guard cells close their stomata during the night to prevent transpirational water loss, and open their stomata during the day so they can take up carbon

dioxide for photosynthesis, and give off oxygen as a waste product.

Mesophyll

Mesophyll cells constitute the main body of a leaf, occurring between the upper and lower epidermis. Typically, the leaves of temperate-zone plants have two layers of mesophyll cells, the palisade mesophyll on the adaxial (upper) side, and the spongy mesophyll on the abaxial (lower) side. The palisade mesophyll is a layer of densely packed, columnar cells which contain many chloroplasts. This layer is responsible for most of the photosynthesis of leaves. The spongy mesophyll is composed of large, often odd-shaped, photosynthetic cells separated from one another by large, intercellular spaces. The intercellular spaces apparently facilitate the exchange of photosynthetic gases.

Veins

Veins penetrate the mesophyll layers of a leaf. Veins consist of vascular tissue, xylem, and phloem, and connect the vascular tissue of the stem to the photosynthetic cells of the mesophyll, via the petiole. Xylem cells mainly transport water and minerals from the roots to the leaves, and phloem cells mainly transport carbohydrates made by photosynthesis in the leaves to the rest of the plant. Typically, the xylem cells are on the upper side of the leaf vein, and the phloem cells are on the lower side.

Phyllotaxy

Phyllotaxy is the arrangement of leaves on a stem. As a stem grows at its apex, new leaf buds form along the stem by a highly controlled developmental process. Depending on the species, the leaf origins on the stem may be opposite (in which leaves arise in pairs on opposite sides of the stem), whorled (three or more leaves arise from the same locus on the stem), or alternate (leaves are arranged in a helix along the stem).

Most species have alternate leaves. This pattern is often called spiral phyllotaxy because a spiral is formed when an imaginary line is drawn which connects progressively older leaf origins on the stem. The divergence angle of successive leaves determines the developmental spiral of leaves and homologous plant organs, such as the individual florets of a sunflower, and has been intensively studied by botanists and mathematicians since the mid-1800s. Interestingly, the angle between successive leaves on a stem is often about 137.5 degrees, known as the Fibonaci angle.

Two major theories have been proposed to explain spiral phyllotaxy: ava ble space theory and repulsion theory. Both propose that the siting of leaf nodes is determined by the position of existing leaf nodes. While the two are often portrayed as competing theories, they are not in fact mutually exclusive. Available space theory says that the physical space among existing leaf nodes determines where new leaves originate. Repulsion theory says that synthesis of a chemical growth inhibitor(s) at the apex of older leaf nodes determines the position of new leaves, and that new leaves form only where the concentration of the growth inhibitor(s) is below a certain threshold.

Evolution

About 380 million years ago, plants with vascular tissue first evolved a special type of leaf, referred to as a microphyll. A microphyll typically has a single midvein, and arises from a stem which does not have leaf gaps, in regions of parenchyma (i.e, unspecialized) tissue where the vascular strand leads into the leaf base. The microphyll may have originated as an outgrowth of a vascularized stem, or by evolutionary simplification of a complex branch system. The leaves of certain modern plants in the Lycopodophyta (Lycopods) and Sphenophyta (Horsetails ) are classified as microphylls. Although the microphylls of these modern plants are quite small, some of their fossilized relatives had very large microphylls.

About 350 million years ago, plants first evolved megaphylls, the leaf type of modern seed plants and ferns. A megaphyll typically has a complex venation pattern, and arises from a stem which has leaf gaps, or regions of parenchyma tissue where the vascular strand leads into the leaf base. One theory proposes that megaphylls, as well as other plant organs, evolved by modification of branch systems. In other words, megaphylls may have evolved by the flattening of a three-dimensional branch system, and connection of the flattened branches with a cellular webbing.

Many botanists believe that the four different whorls of a flower (sepals, petals, stamens, and carpels) originated by evolutionary modification of the megaphylls of a free-sporing plant. Shortly after the evolutionary origin of the Angiosperms (flowering plants), there was a major division between the monocots (plants whose embryos have one cotyledon) and dicots (plants whose embryos have two cotyledons). The leaves of modern monocots, such as grasses and lilies, tend to be narrow and have parallel venation; the leaves of dicots tend to be wide, and have reticulated venation.

Many modern plants have evolved complex and highly specialized leaves. For example, the insect-eating organs of carnivorous plants , such as Venus Flytrap, Sundew, Pitcher Plant, and Bladderwort, are all highly specialized leaves. Dischidia rafflesiana, a tropical epiphyte, has among the most specialized leaves of any plant. Its leaves are tubular in shape and they collect forest debris and rain water, providing a habitat for the colonies of ants which live inside. As the ants die, their bodies dissolve and special roots of the plant absorb the nutrients that are released, providing nourishment for the plant.

Resources

books

Attenborough, D. The Private Life of Plants. Princeton University Press, 1995.

Galston, A.W. Life Processes of Plants: Mechanisms for Survival. W.H. Freeman Press, 1993.

Kaufman, P.B., et al. Plants: Their Biology and Importance. Harp College Press, 1990.

Wilkins, M. Plant Watching. Facts on File Inc., 1988.

Peter A. Ensiminger

KEY TERMS

Cellular respiration

—The oxidation of food within cells, involving consumption of oxygen and production of carbon dioxide, water, and chemical energy in the form of ATP (adenosine triphosphate).

Dicot

—A plant whose embryo has two cotyledons (that is, seed leaves) and several other general characteristics.

Epiphyte

—A plant which relies upon another plant, such as a tree, for physical support, but does not harm the host plant.

Leaf gap

—A gap in the vascular system of a stem, just above where the stem's vascular tissue has curved off into the petiole.

Megaphyll

—A leaf type typical of seed plants and ferns, which has branched veins and is usually associated leaf gaps in the stem.

Microphyll

—A leaf type typical of Sphenophyta (horsetails) and Lycopodophyta (lycopods) which is scale-like, not associated with leaf gaps in the stem, and usually has a single midvein.

Monocot

—A plant whose embryo has a single cotyledon (seed leaf) and several other general characteristics.

Photosynthesis

—The biological conversion of light energy into chemical energy.

Transpiration

—The movement of water out of a plant, most of which occurs through the stomata of leaves.

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 ]

Leaf

A leaf is the main energy-capturing and food-producing organ of most plants. Nearly every plant on Earth owes its continued existence to its leaves, which collect energy from sunlight and convert it into food through a process known as photosynthesis. Leaves may vary widely in size and shape, but all are designed primarily to capture as much light as possible without drying out.

PARTS OF A LEAF

Leaves are attached to and supported by the plant stem, which provides the leaves with water and inorganic nutrients from the soil. Most leaves have two main parts: the blade and the petiole. The blade, or lamina, is the broad, flattened surface of the leaf that absorbs radiant energy from the sun. The blade is attached to the stem by a stalk called a petiole that also supports it. The blade is made up of two layers of cells—a tight, outer layer of cells called the epidermis, and a thicker, inner layer of mesophyll cells. The epidermis is covered by a waxy coating called a cuticle that helps cut down water loss from the leaf. It is in the inner mesophyll cells where photosynthesis is carried out.

The petiole not only joins the leaf to the stem, but contains tiny tubes that connect with veins inside the blade. Besides strengthening the blade, the veins' main purpose is to act as pipelines and transport water and food to and from its cells. A large vein called the midrib usually runs along the center of the leaf and smaller branching veins run out to its edges.

Edges of leaves can differ greatly, and while narrow leaves like grass have smooth edges, many broadleaf blades have jagged points called teeth at their edges. In some plants, these teeth act as valves and release excess water, while in others they function as tiny glands producing a liquid that repels insects. Leaves also contain a stoma (plural, stomata), which is critically important to the leaves' operations. Because the wax coating of its blade is not porous, leaves have developed special openings that allow gases (carbon dioxide and oxygen) to be exchanged and water to be released. A stoma is similar to a tiny slit that opens or closes by the action of two guard cells on either side. These cells can change shape and make the stoma open or close. This is essential during photosynthesis when the plant must take in carbon dioxide (and give off oxygen as a by-product). The stomata also enables a plant to regulate how much water it loses. When the stomata are open, they allow water to escape into the atmosphere. To minimize water loss, stomata tend to close at night when photosynthesis is not occurring, and open during the day when rapid gas exchange is necessary. During unusually dry conditions the stomata may close to prevent wilting, and photosynthesis is reduced.

LEAVES ARE ESSENTIAL TO PLANTS

Leaves are like a food factory for a plant since they begin with raw materials and process them internally to produce glucose, which the plant uses for growth, development, and reproduction. The plant itself can also become food for primary consumers that eat parts of the plant and obtain the energy the plant has stored. Within the leaf's internal structures called chloroplasts are the plant's main light-absorbing compound called chlorophyll. It is this pigment that gives plants their typically green color. Photosynthesis begins as the leaf lets carbon dioxide in through its stomata and obtains water from its roots through its veins. When sunlight strikes the chlorophyll in the chloroplasts, light energy splits the water into hydrogen

and oxygen. Hydrogen combines with carbon dioxide to make the simple sugar glucose, and oxygen is released through the stomata as a byproduct. All of this occurs within the leaf at the cellular level.

HOW LEAVES DEVELOP

At the very beginning of their existence, leaves are contained in embryo form within the seed and are called a cotyledon. Once the seed germinates, or sprouts, the cotyledon emerges and eventually becomes the first true leaf. As the plant matures, more leaves are formed from buds that formed on the stem. Once the bud begins to unfold and open, the leaf begins its growth period and reaches full size anywhere from one to several weeks. The mature leaf turns a deep green and begins to make food for itself and the rest of the plant. Although a leaf contains other colors, they are masked by the chlorophyll (a green pigment). As autumn approaches, however, the plant releases a hormone and the chlorophyll starts to break down and eventually disappears from the leaves, allowing the remaining colors of yellow, orange, or red to finally be seen. Once the chlorophyll breaks down, the leaf no longer makes food, its veins become plugged, and it soon withers and dies. A layer of cells grow across the base of its petiole, shutting it off from the stem, and the leaf soon dries, twists in the wind, and breaks off. When a tree's dead leaves fall to the ground, they take away some of the waste products the tree produced. They also eventually become food for bacteria and decay on the ground, adding essential humus or organic matter to the soil and offering new nourishment for other plants to use.

Leaves are vital to life on Earth. Since they are the actual site where photosynthesis occurs, they are the first link in the food chain (the series of stages energy goes through in the form of food), providing food to animals. They are not only the factories of the primary producers (plants), but they help make the air breathable for animals. Without the oxygen that plants give off during photosynthesis, Earth's supply of breathable oxygen might be eventually used up. People also use leaves for many products, from tea and herbs, to lettuce and spinach, to drugs like digitalis and tobacco.

[See alsoPhotosynthesis; Plant Anatomy; Plants ]

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.