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In botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin, to expose the chloroplast containing cells (chlorenchyma) to light over a broad area, and to allow light to penetrate fully into the tissues.
Leaves are also the sites in most plants where respiration, transpiration, and guttation take place. Leaves can store food and water, and are modified in some plants for other purposes.
The comparable structures of ferns are referred to as fronds.
A structurally complete leaf of an angiosperm consists typically of a petiole (leaf stem), a lamina (leaf blade), and stipules (small processes located to either side of the base of the petiole).
The point at which the petiole attaches to the plant stem is called the leaf axil.
Not all species produce leaves with all of these parts. In some species, stipules are not obvious or present, or the petiole may be absent. The blade is not always laminar (flattened).
A leaf has a dorsiventral anatomy, in other words the upper surface and the underside have a different construction and different functions.
The external leaf characteristics (such as shape, margin, hairs, etc.) are important for identifying plant speciesThis article discusses biological species. Also see combinatorial species for the mathematical meaning of the term. Species is also a movie by Roger Donaldson. In English "species" is both singular and plural. The word " specie" is unrelated and is used t.
A leaf typically consists of the following tissues:
The epidermis is the outer multi-layered group of cells covering the leaf blade. It is the boundary between the plant and its surroundings. The structure of the epidermis of the upper surface and the underside of the leaf may be different.
The epidermis has several functions: absorption of water, protection against water loss, gas exchange through photosynthesis, secretion of metabolic compounds, and protection against outside influences.
The layer is usually transparentIn optics, transparency is the property of being transparent or allowing light to pass. The opposite property is opacity . Though transparency usually refers to visible light in common usage, it can actually refer to any type of radiation. For example, fl (cells lack chloroplasts) and coated on the outer surface with a waxy cuticleIn botany the cuticle is the waxy covering produced by the epidermal cells of leaves to protect the plant from excessive water loss. The cuticle is thicker in plants living in dry climates than in those from wet climates. The cuticle is mostly composed of that prevents water loss. The cuticle may be thinner on the lower epidermis than on the upper epidermis; and is thicker on leaves from dry climates as compared with those from wet climates.
The epidermis contains differentiated cell types; epidermal cells, guard cells, subsidiary cells and the epidermal hairs (trichomes).
The epidermal cells are the largest and the least specialized group. They are elongated in the leaves of monocots.
The epidermis is covered with poreA pore in general, is some form of opening, usually very small. Pores can be found on many organisms, such as in plants, animals, and humans. More commonly, in talking about the skin, a pore is an opening that secretes sebaceous oil to lubricate and protes called stomata (sing., stoma). It is part of the stoma complex: the stoma, enclosed at each side by chloroplast-containing guard cells, and two to four subsidiary cells (lacking chloroplast). These stomata enable oxygenOxygen is the chemical element in the periodic table that has the symbol O and atomic number 8. The element is very common, found not only on Earth but throughout the universe. Molecular oxygen (O, often called free oxygen on Earth is thermodynamically un and carbon dioxideCarbon dioxide is an atmospheric gas composed of one carbon and two oxygen atoms. One of the best known of chemical compounds, it is frequently called by its formula: :CO (pronunciation: "see oh two") Carbon dioxide results from the combustion of organic to move in and out of the leaf. There is a substomatal chamber (an air space) below the stoma. These pores are more numerous over the abaxial (lower) epidermis than the (adaxial) upper epidermis in most leaves. Water vapour also passes out of the stomata during transpiration.
To conserve water, the paired guard cells at each side of the stomata may force the opening or the closing of the stomata. This happens through a differential thickening of the walls of the guard cells. When they become inflated in a humid surrounding, the internal water pressure (turgor pressure) opens up the stoma. The inverse happens, as the guard cells lose water pressure on a warm day, regulating transpiration and the flow of carbon dioxide.
The opening and closing of the stomata is also triggered by light. They are open during daytime and closed at night. The reverse happens with some plants from arid regions, such as some of the sedum family ( Crassulaceae); to prevent extensive water loss, they open their stomata during the night.
Stomata are not found in some underwater plants.
Trichomes or hairs grow out from the epidermis in many species.