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

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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.