Introduction to Leaves

Leaves, organs that grow from the stems of the higher plants. Leaves range in length from less than one inch (a few millimeters) in duckweeds to more than 50 feet (15 m) in some plants. The chief functions of leaves are to make food for plants and to free them of excess moisture. In some plants leaves also trap insects, which are used as food. Some plants reproduce themselves by means of leaves.

The leaves of many plants are important foods. Cabbage and lettuce, for example, are eaten by humans, and the leaves of grasses and other plants nourish cattle, sheep, and horses. The leafstalks of some plants, such as rhubarb, celery, and celtuce, also are foods. Bay leaves and the leaves of parsley, sage, marjoram, and many other plants are herbs used to flavor foods.

Tea leaves and tobacco leaves are the bases of great industries. Flavorings and perfumes are obtained from the leaves of peppermint, spearmint, wintergreen, and many other plants. The leaves of many plants, including belladonna, coca, and digitalis, yield useful drugs. Dyes are obtained from the leaves of henna and indigo plants. Leaves also provide material for clothing, basketry, and roofing.

Arrangement On Stems

Leaves grow from the joints, or nodes, of stems. The upper angle between a leaf and a stem is called the axil of the leaf.

Leaves are said to be opposite when they grow from opposite sides of a node. Maple and buckeye leaves grow in this way. Leaves are called alternate or spiraled when they grow singly from nodes at different points around the stem. The American elm provides an example. Leaves that grow in groups of three or more at a node are called whorled. This arrangement is less common than the other two. The catalpa is a plant with this type of leaves.

The Parts of A Leaf

A typical leaf consists of a leafstalk, or petiole; and an expanded blade, or lamina. The petiole connects the blade to the stem. (Leaves that do not have petioles are called sessile.) In some plants a pair of stipules grows from the petiole, below the blade. These are mere shreds in some plants, and large and leaflike in others. In some plants they have developed into spines or tendrils.

A broad leafA broad leaf consists of a leafstalk, or petiole; and an expanded blade, or lamina.
The Petiole

Tiny tubes within the petiole carry a watery solution of minerals to the blade, and conduct food from the blade to the stem. As a rule, petioles turn blades toward the light, but in plants such as the compass plant they turn them away from it.

The Blade

is usually broad, flat, and thin. It is hairlike in some grasses, tubular in onions, scalelike in cedars, and needlelike in pines. Sedums and a number of other plants have fleshy blades that store water.

Within the blade, the tubes that pass through the petiole branch out into a network of veins. The veins carry liquids to and from the cells within the blade. They also help to keep the blade extended.

There are two basic ways in which the veins are arranged within a leafparallel and netted.

Parallel

Parallel veins, such as those in the leaves of most grasses, run side by side from the base to the tip of the blade. Pinnately parallel veins, such as those of banana leaves, branch out from a central vein running the length of the leaf. The branches are parallel to each other in the same way that the barbs of a feather are parallel.

Netted

In pinnately netted leaves the veins extend outward from a central vein, like branches of a tree, but are not parallel to each other. Elms and oaks have such arrangements. In palmately netted leaves, such as those of the sycamore and maple, the largest veins branch out somewhat like outspread fingers. From each large branch there are smaller branches.

The Forms of Leaves

Leaves axe described as simple or compound, depending on whether they have one or a number of blades. They are described also by their shapes and their margins (edges).

Simple Leaves

A simple leaf, or the leaflet of a compound leaf, may be lance-shaped, oval, elliptical, heart-shaped, or any of a large number of other shapes. The margin may be entire (smooth and unindented, as in corn); serrated (saw-toothed, as apple and elm leaves); waved (having rounded projections, as in the chestnut oak); or lobed (with deep indentations, as in oak and maple leaves). Pinnately lobed leaves (such as those of the oak) have indentations pointing toward the central vein. Palmately lobed leaves (maple leaves, for example) have indentations that point toward the base of the blade.

Compound Leaves

A compound leaf consists of a number of leaflets. The leaflets look like leaves, but buds do not grow in their axils, as they do in the axils of true leaves. In a pinnately compound leaf the leaflets grow from both sides of the petiole, as the barbs grow from the shaft of a feather. The leaflets of a palmately compound leaf grow from the tip of the petiole. Doubly compound leaves have leaflets made up of still other leaflets.

Structure of A Leaf

A leaf consists of a thin outer skin, or epidermis, covering cells that make up the mesophyll (middle leaf). An intricate network of veins runs through the mesophyll.

The Epidermis

consists of a single layer of cells. Its outer surface, or cuticle, is coated with a waxy substance called cutin. Cutin helps to prevent evaporation from the mesophyll. In many plants certain cells of the epidermis grow into hairs. A hair consists of single cells arranged end to end in either single or branching threads. The hairs of tobacco and of a number of other plants secrete a sticky substance.

The epidermis is pierced by a vast number of tiny pores called stomata (singular: stoma), from the Greek word for mouths. The stomata occur chiefly on the undersides of leaves. In the olive and many other plants they occur only on the lower surface; and in still other plants, such as the water lily, they are found only on the upper surface.

Each stoma is enclosed by a pair of guard cells shaped somewhat like beans. When they absorb moisture the guard cells swell up and curve away from each other, thus opening the stoma. When they lose moisture they collapse and close the stoma.

The chief function of the guard cells is to regulate the breathing of the leaf by admitting air and letting other gases escape. They open the stomata in daylight, when the leaf needs air with which to make food, and close them at night. When the leaf wilts the guard cells collapse, thus reducing loss of moisture.

The Mesophyll

is made up of thin-walled cells containing chlorophyll, the green substance that begins the food-making in plants. In most plants the mesophyll cells form two types of tissue: palisade tissue and spongy tissue. The palisade tissue is made up of palisade cells, cylindrical cells set at right angles to the surface of the leaf. Palisade tissue is usually found under the upper surface of leaves. In some erect leaves it occurs under both surfaces.

The spongy tissue consists of irregularly shaped cells. It contains many air spaces that lead to the stomata. Air enters the spaces through the stomata, and waste gases escape from the spaces through the stomata.

Veins extend throughout the mesophyll. They provide passage for fluids and help support the leaf. The veins are continuations of tiny tubes that extend from the stem through the petiole. They supply the cells of the mesophyll with minerals dissolved in water, which they need to make food, and carry food back to the stem.

Functions of Leaves

The process by which plants make food from carbon dioxide, water, and energy of sunlight is called photosynthesis. This process is begun by the chlorophyll. In the higher plants photosynthesis occurs chiefly in the leaves, in which chlorophyll is contained in the mesophyll.

Plants use only a tiny fraction of the water they obtain through their roots. The remainder they give off chiefly as water vapor, through their stomata, by the process of transpiration. Plants transpire huge quantities of water. A large apple tree can give off an average of 10 gallons (38 l) a day.

Some Specialized Leaves

Every seed contains one or more cotyledons, or seed leaves. Cotyledons digest the food stored in seeds and supply it to seedlings, nourishing them until the first true leaves appear. In some plants the cotyledons contain chlorophyll and make food by photosynthesis.

Bud scales are leaves that protect buds. The leaves of cacti and some other plants have developed into spines. Tendrils have developed from certain leaves in the sweet pea and a number of other plants. A leaf that grows at the base of a flower stalk, flower, or cluster of flowers is called a bract. In the dogwood, poinsettia, calla lily, and many other plants the bracts have developed into showy, flowerlike growths.

Cut leaves, placed in soil or some other medium, can be used to reproduce African violets, begonias, gloxinias, and certain other plants. Leaves of the Venus's flytrap, sundew, and pitcher plant trap insects and other small animals, which the plants use as food.

Why Leaves Fall

In temperate regions most trees and shrubs shed their leaves in autumn in response to shorter days, decreased sunlight, and cooler temperatures. Two chemicalsabscisic acid, a hormone, and ethyleneappear in increased quantities in the leaf. They cause structural changes that, in effect, kill the leaf. As a result, transpiration stops and water that would have been lost as vapor through pores in the leaves flows back into the branches, trunk, and roots. Thus, the shedding of leaves conserves water during winter when it is difficult for plants to obtain moisture from the frozen soil.

In early autumn a layer of cells at the base of the petiole softens, dries, and becomes corky, separating from the stem. The leaf is then held only by the tiny tubes that pass through the petiole. When these are broken by wind, rain, or frost, the leaf falls.

Why Leaves Change Color

Most leaves have a colorless epidermis; their green color is due to the chlorophyll, which contains a green pigment, within the mesophyll. In plants such as the coleus, however, the cells of the epidermis contain other pigments. The leaves are the colors of those pigments rather than green.

Leaves that are normally green turn yellow, brown, orange, gold, red, blue, or purple when they die. This change is caused by the disappearance of chlorophyll from the leaves. Decreasing sunlight reduces the amount of chlorophyll and slows photosynthesis in the leaf. The receding chlorophyll reveals pigments that were already in the mesophyll but were masked by larger amounts of chlorophyll. The yellow, brown, orange, and gold colors are produced by three groups of pigments: xanthophylls, (yellow), tannins (brown), and carotenes (yellowish orange to light red).

Blues, purples, and deep reds are produced by pigments called anthocyanins. Anthocyanins are produced by the action of sunlight on sugar and other substances within the leaves. They are produced in great quantities in the sugar-rich leaves of sugar maples. These are among the most brilliantly colored of all autumn leaves. Much sunlight is needed during autumn to produce bright colors. An early frost is also necessary to produce bright colors since it prevents the sugary fluid in the leaves from draining into the stems.