Leaf

I

Introduction

Leaf, part of a plant that serves primarily as the plant's food-making organ in a process called photosynthesis. Leaves take part in other plant functions as well, including transpiration and guttation, both of which remove excess water from the plant, and respiration, the process by which a plant obtains oxygen and energy. Leaves also may store food and water and provide structural support.

A leaf is an extension of a plant's stem. Although most leaves are flat, broad, or bladelike, they also may be many other shapes, including round, oval, or feathery. In general, the leaves of trees such as hardwoods tend to be broad and relatively large, and the leaves of conifers, or cone-bearing trees, are usually small and needlelike in shape. In size, leaves range from only several millimeters (a fraction of an inch) long, as in the water plant Elodea, to 15 to 18 m (15 to 60 ft) long, as in some palm trees.

Green leaves derive their color from a green pigment called chlorophyll. The presence of additional pigments causes other leaf colors such as red in coleus and purple in cabbage. In temperate regions of the world, the leaves of some plants change color in autumn. Leaves of most garden plants turn yellow in the autumn, but those of many trees take on brilliant orange or red colors.

Most plants whose leaves change color also lose their leaves in the autumn. Such plants are called deciduous. In other plants, such as laurels and pines, the leaves do not change color and do not fall off in autumn. Such plants are called evergreens.

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II

The Parts of a Typical Leaf

The typical green leaf is called a foliage leaf. It usually consists of two basic parts: a petiole and a blade.

The petiole is a stalklike structure that supports the leaf blade on the stem. It also serves as a passageway between the stem and the blade for water and nutrients. Another function of the petiole is to move the leaf into the best position for receiving sunlight. Most petioles are long, narrow, and cylindrical.

Many plants, such as grasses and corn, do not have petioles. In these plants the base of the blade is attached directly to the stem—the base encircles the stem as a sheath. Such leaves are called sessile leaves.

The leaf blade is usually a thin, flat structure. Its margins, or edges, may be smooth, as in the dogwood; jagged or toothed, as in the elm; or lobed, as in the oak and maple. The surface of the blade may be smooth, fuzzy, sticky, dull, or shiny. In most plants the leaves have a single blade and are referred to as simple. In other plants, such as clover, the blade is divided into separate leaflets. This kind of leaf is called a compound leaf. Most of the functions carried on by leaves take place in the blade.

A

Epidermis

The blade consists of an upper and lower epidermis and a spongy layer of tissue, called the mesophyll. Running through the mesophyll is a branching system of veins.

The epidermis is the leaf blade's skin. It is a thin, usually transparent, colorless layer of cells that covers both the upper and lower surfaces of the blade. The epidermis prevents the leaf from losing excessive amounts of water and protects it against injury.

In most plants the epidermis is covered with cutin, a waxy substance secreted by the epidermal cells. The layer of cutin, called the cuticle, is responsible for the glossy appearance of some leaves. The cuticle gives the leaf additional protection by slowing down the rate at which water is lost. Generally, the cuticle is thinner on the epidermis on the underside of the leaf than on the upper epidermis, which is exposed to the sun.

In many kinds of leaves, hairs grow from the epidermis. The soft hairs of plants such as the mullein give the leaves a woolly or feltlike texture. In some plants the epidermal hairs secrete fluids. For example, in geraniums and petunias the hairs secrete a fluid that gives the leaves a clammy texture. The strong-smelling oils of the peppermint and spearmint plants come from epidermal hairs. In other plants, such as the nettle, the epidermal hairs are stiff and contain a poisonous fluid that produces a skin irritation when a person is pricked by them.

B

Guard Cells

Scattered throughout the epidermis are pairs of bean-shaped cells, called guard cells. Guard cells contain chloroplasts, which are tiny granules filled with the green pigment chlorophyll. Chlorophyll gives leaves their characteristic green color. Chloroplasts enable leaves to carry on photosynthesis because they are able to absorb carbon dioxide and sunlight, which are required for the food-making process. In response to heat and light, each pair of guard cells pulls apart, and a tiny pore forms between them. The pores, called stomata, open to the outside atmosphere.

When the stomata are open, carbon dioxide and oxygen pass either in or out—when carbon dioxide enters, it takes part in photosynthesis, the food-making process that releases oxygen as a waste product. This oxygen passes out of the leaf. At the same time, oxygen also enters the leaf, where it takes part in respiration, a process that forms carbon dioxide as a waste product. This carbon dioxide passes out through the stomata. Water also passes out of the open stomata in the form of a vapor. This process is called transpiration. Generally, there are more stomata on the under surface of a leaf than on the upper surface. This prevents water from evaporating too quickly or in excessive amounts from the leaf's upper side, which is exposed to the sun. Stomata close at night, providing another level of water conservation.

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