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Plant, any member of the plant kingdom, comprising about 260,000 known species of mosses, liverworts, ferns, herbaceous and woody plants, bushes, vines, trees, and various other forms that mantle the Earth and are also found in its waters. Plants range in size and complexity from small, nonvascular mosses, which depend on direct contact with surface water, to giant sequoia trees, the largest living organisms, which can draw water and minerals through their vascular systems to elevations of more than 100 m (330 ft).. read more

Parts of Typical Leaf

Parts of 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. See leaf mechanism.

C Water Pores

In addition to the stomata, many kinds of leaves have large specialized water pores in their epidermis. These pores, called hydathodes, permit guttation, the process by which a plant loses liquid water. Unlike the stomata, hydathodes remain open all the time.

Guttation takes place only when water is being rapidly absorbed by the roots, such as after a heavy rainfall, and when transpiration slows down, as on cool, humid nights. When these conditions occur together, droplets of water can be seen on the leaf early in the morning before they evaporate in the heat of the day. Unlike dew, which condenses on leaves from water vapor in the air and covers the entire leaf surface, guttation droplets form only on the edges and tips of leaves. Generally, the droplets are noticeable only on the leaves of strawberries and a few other kinds of plants.

D Mesophyll

The mesophyll, sandwiched between the upper and lower epidermis, consists of many thin-walled cells that are usually arranged in two layers. The palisade layer is next to the upper epidermis. It consists of cylindrical cells that are packed closely together. Next to the palisade layer and making up most of the thickness of the leaf blade is the spongy layer. The spongy layer consists of roundish cells that are packed loosely together and have numerous air spaces between them. In most plants the spongy layer extends down to the lower epidermis. However, in certain grasses, irises, and other plants whose leaves grow straight up and down, the spongy layer is wedged between two palisade layers of mesophyll. Like the guard cells, all the cells of the mesophyll contain chloroplasts.

E Veins

Running through the middle of the mesophyll and branching out to all of its cells are veins. The veins extend into the petiole and connect with other veins in the stem of the plant. A major function of the veins is to help support the leaf blade. Each type of plant has a characteristic pattern of veins forming lines and ridges in the blade.

The veins of a leaf are made up of two specialized tissues, xylem and phloem. Xylem usually forms the upper half of the vein. It consists of tubular open-ended cells that are arranged end to end. The walls of the cells are thick and rigid. Xylem conducts water and dissolved minerals to the leaf blade from the rest of the plant.

Phloem lies on the underside of the vein. It is made up of thin-walled tubular cells with tiny openings at their ends, somewhat like a sieve. These cells are also arranged end to end. Phloem carries food manufactured in the blade to the rest of the plant.

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