Leaf

A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants.^[1][2]

Typically a leaf is a thin, flattened organ borne above ground and specialized for photosynthesis, but many types of leaves are adapted in ways almost unrecognisable in those terms: some are not flat (for example many succulent leaves and conifers), some are not above ground (such asbulb scales), and some are without major photosynthetic function (consider for example cataphylls, spines, and cotyledons).

Conversely, many structures of non-vascular plants, or even of some lichens, which are not plants at all (in the sense of being members of the kingdom Plantae), do look and function much like leaves. Furthermore, several structures found in vascular plants look like leaves but are not totally homologous with leaves; they differ from typical leaves in their structures and origins. Examples include phyllodes, cladodes, andphylloclades.^[2][3]

According to Agnes Arber's partial-shoot theory of the leaf, leaves are partial shoots.^[4] Compound leaves are closer to shoots than simple leaves. Developmental studies have shown that compound leaves, like shoots, may branch in three dimensions.^[5][6] On the basis of molecular genetics, Eckardt and Baum (2010) concluded that "is is now generally accepted that compound leaves express both leaf and shoot properties."

Parts of a leaf

The Blade, or lamina, is the broad, flat part of the leaf. Photosynthesis occurs in the blade, which has many green food-making cells. Leaf blades differ from one another in several ways: (1) the types of edges, (2) the patterns of the veins, and (3) the number of blades per leaf.

The Types of Edges. Almost all narrow, grasslike leaves and needles leaves have a blade with a smooth edge, as do many broadleaf plants, particularly those that are native to warm climates. The rubber plant, a common house plant, is a good example of such a plant.

The leaves of many temperate broadleaf plants have small, jagged points called teeth along the blade edge. Birch and elm trees have such leaves. Some plants havehydathodes, tiny valvelike structures that can release excess water from the leaf. The teeth of young leaves on many plants, including cottonwood and pin cherry trees, bear tiny glads. These glands produce liquids that protect the young leaf from plant-eating insects.

Some temperate broadleaf plants -- including sassafras trees and certain mulberry and oak trees -- have lobed leaves. The edge of such a leaf looks as if large bites have been taken out of it. This lobing helps heat escape from the leaf.

The Patterns of the Veins. Veins carry food and water in a leaf. They also support the blade, much as the metal ribs support the fabric of an open umbrella.

In most broad leaves, the veins form a netlike pattern, with several large veins connected by smaller ones. The smallest veins supply every part of the blade with water. They also collect the food made by the green cells.

There are two main types of net-vein patterns -- pinnate (featherlike) and palmate (palmlike or handlike). Pinnately veined leaves have one large central vein, called the midrib, which extends from the base of the blade to its tip. Other large veins branch off on each side of the midrib. The leaves of beech, birch, and elm trees have such a vein pattern. A palmately veined leaf has several main veins of about equal size, all of which extend from a common point at the base of the blade. The vein patterns of maple, sweet gum, and sycamore leaves are palmate.

Narrow leaves and needle leaves are not net-veined. Narrow leaves have a parallel-vein pattern. Several large veins run alongside one another from the base of the blade to the tip. Small crossveins connect the large veins. Needle leaves are so small that they have only one or two veins running through the center of the blade.

The Petiole is the stemlike part of the leaf that joins the blade to the stem. Within a petiole are tiny tubes that connect with the veins in the blade. Some of the tubes carry water into the leaf. Others carry away food that the leaf has made. In many trees and shrubs, the petioles bend in such a way that the blades receive the most sunlight, thus assuring that few leaves are shaded by other leaves. The petiole also provides a flexible "handle" that enables the blade to twist in the wind and so avoid damage.

In some plants, the petioles are much larger than the stems to which they are attached. For example, the parts we eat of celery and rhubarb plants are petioles. In contrast, the leaves of some soft-stemmed plants, particularly grasses, have no petioles.

The Stipules are two small flaps that grow at the base of the petiole of some plants. In some plants, the stipules grow quickly, enclosing and protecting the young blade as it develops. Some stipules, such as those of willows and certain cherry trees, produce substances that prevent insects from attacking the developing leaf.

In many plants the stipules drop off after the blade has developed, but garden peas and a few other kinds of plants have large stipule.