The main photosynthetic organ of green plants, divided into a blade (lamina) and a stalk (petiole). The lamina is usually broad and thin, to present maximum surface area to sunlight and allow easy diffusion of gases and water vapour to and from the leaf. It is composed of several distinct layers of tissues: the epidermis protects the inner tissues - the palisade layer, which is the primary site of photosynthesis, and the spongy mesophyll, which has large air spaces and is the primary site of gas exchange. A network of vascular tissue, the veins, transports water and sap to and from the leaf. The epidermis secretes a waxy cuticle, mostly impervious to water and gases which enter and leave via pores (stomata) concentrated in the lower surface of the leaf. The main source of water loss for a plant is due to transpiration via the leaves. This is minimized by the waxy cuticle, and by the opening and closing of the stomata in response to changes in humidity. Other modifications to reduce water-loss are found particularly in plants from dry or cold regions, including reduction in leaf size, inrolled margins to protect stomata, and regular shedding of leaves during unfavourable seasons. Some plants have replaced their leaves entirely with less vulnerable photosynthetic organs, such as green stems. Leaves range from a few mm to 20 m/65 ft in length, exhibit a great variety of shapes, and may be entire, toothed, lobed, or completely divided into separate leaflets. These characters and the arrangement of the leaves on the stem are diagnostic for many plant groups. Leaves may also have specialized functions, such as water-storage in succulents, traps in carnivorous plants, and tendrils in climbers.
In botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat (laminar) and thin, to expose the cells containing chloroplast (chlorenchyma tissue) 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. Leaves are prominent in the human diet as leaf vegetables.
Leaf anatomy
A structurally complete leaf of an angiosperm consists 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 stem is called the leaf axil. Not every species produces leaves with all of these structural parts. The tremendous variety shown in leaf structure (anatomy) from species to species is presented in detail below under Leaf types, arrangements, and forms.
A leaf is considered to be a plant organ, typically consisting of the following tissues:
An epidermis that covers the upper and lower surfaces An interior chlorenchyma called the mesophyll An arrangement of veins (the vascular tissue).Epidermis
The epidermis is the outer multi-layered group of cells covering the leaf. Most leaves show dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions.
The epidermis is usually transparent (epidermal cells lack chloroplasts) and coated on the outer side with a waxy cuticle that prevents water loss. and is thicker on leaves from dry climates as compared with those from wet climates.
The epidermis tissue includes several differentiated cell types: epidermal cells, guard cells, subsidiary cells, and epidermal hairs (trichomes). These are typically more elongated in the leaves of monocots than in those of dicots.
The epidermis is covered with pores called stomata, part of a stoma complex consisting of a pore surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts. The stoma complex regulates the exchange of gases and water vapor between the outside air and the interior of the leaf.
Trichomes or hairs grow out from the epidermis in many species.
Mesophyll
Most of the interior of the leaf between the upper and lower layers of epidermis is a parenchyma (ground tissue) or chlorenchyma tissue called the mesophyll (= middle leaf).
In ferns and most flowering plants the mesophyll is divided into two layers:
An upper palisade layer of tightly packed, vertically elongated cells, one to two cells thick, directly beneath the adaxial epidermis. Sun leaves have a multi-layered palisade layer, while shade leaves or older leaves closer to the soil, are single-layered.The pores or stomata of the epidermis open into substomatal chambers, connecting to air spaces between the spongy layer cells.
These two different layers of the mesophyll are absent in many aquatic and marsh plants.
Leaves are normally green in color, which comes from chlorophyll found in plastids in the chlorenchyma cells.
Leaves in temperate, boreal, and seasonally dry zones may be seasonally deciduous (falling off or dying for the inclement season). This mechanism to shed leaves is called abscission. After the leaf is shed, a leaf scar develops on the twig.
Veins
The veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll.
The veins are made up of:
xylem, which brings water from the roots into the leaf. phloem, which usually moves sap out, the latter containing the glucose produced by photosynthesis in the leaf.Leaf morphology
External leaf characteristics (such as shape, margin, hairs, etc.) are important for identifying plant species, and botanists have developed a rich terminology for describing leaf characteristics. These structures are a part of what makes leaves determinant, they grow and achieve a specific pattern and shape, then stop.
Leaves may be classified in many different ways, and the type is usually characteristic of a species, although some species produce more than one type of leaf. The terminology associated with describing leaf morphology is presented (with illustrations) at Wikibooks.
Basic leaf types
Ferns have fronds. Conifer leaves are typically needle-, awl-, or scale-shaped Angiosperm (flowering plant) leaves: the standard form includes stipules, petiole, and lamina. Lycophytes have microphyll leaves. Sheath leaves (type found in most grasses). Other specialized leavesArrangement on the stem
As a stem grows, leaves tend to appear arranged around the stem in a way that optimizes yield of light. In essence, leaves come off the stem in a spiral pattern, either clockwise or counterclockwise, with (depending upon the species) the same angle of divergence. In the series, the numerator gives the number of complete turns or gyres until the leaf arrives at the initial position. The denominator gives the number of leaves in the arrangement. This can be demonstrated by the following:
alternate leaves have an angle of 180° (or 1/2) 120° (or 1/3) : three leaves in one circle 144° (or 2/5) : five leaves in two gyres 135° (or 3/8) : eight leaves in three gyres.The fact that an arrangement of anything in nature can be described by a mathematical formula is not in itself mysterious. The formulae themselves can provide clues to the underlying physiological processes that, in this case, determine where the next leaf bud will form in the elongating stem. However, we can more easily describe the arrangement of leaves using the following terms:
Alternate — leaf attachments singular at nodes, and leaves alternate direction, to a greater or lesser degree, along the stem. Opposite — leaf attachments paired at each node; Whorled — three or more leaves attach at each point or node on the stem. As with opposite leaves, successive whorls may or may not be decussate, rotated by half the angle between the leaves in the whorl (i.e., successive whorls of three rotated 60°, whorls of four rotated 45°, etc). Note: opposite leaves may appear whorled near the tip of the stem. Rosulate — leaves form a rosette ( = a cluster of leaves growing in crowded circles from a common center).Divisions of the lamina (blade)
Two basic forms of leaves can be described considering the way the blade is divided. A simple leaf has an undivided blade. However, the leaf shape may be one of lobes, but the gaps between lobes do not reach to the main vein. A compound leaf has a fully subdivided blade, each leaflet of the blade separated along a main or secondary vein. Because each leaflet can appear to be a "simple leaf", it is important to recognize where the petiole occurs to identify a compound leaf. Compound leaves are a characteristic of some families of higher plants, such as the Fabaceae.
Palmately compound leaves have the leaflets radiating from the end of the petiole, like fingers off the palm of a hand. Pinnately compound leaves have the leaflets arranged along the main or mid-vein (called a rachis in this case). Bipinnately compound leaves are twice divided: the leaflets are arranged along a secondary vein that is one of several branching off the rachis. trifoliate: a pinnate leaf with just three leaflets, e.g. Characteristics of the petiole Petiolated leaves have a petiole. In peltate leaves, the petiole attaches to the blade inside from the blade margin. Sessile or clasping leaves do not have a petiole. In sessile leaves the blade attaches directly to the stem. In clasping leaves, the blade partially or wholly surrounds the stem, giving the impression that the shoot grows through the leaf such as in Claytonia perfoliata of the purslane family (Portulacaceae).In some Acacia species, such as the Koa Tree (Acacia koa), the petioles are expanded or broadened and function like leaf blades; There may or may not be normal pinnate leaves at the tip of the phyllode.
Characteristics of the stipule A stipule, present on the leaves of many dicotyledons, is an appendage on each side at the base of the petiole, resembling a small leaf. They may be lasting and not be shed (a stipulate leaf, such as in roses and beans); or be shed as the leaf expands, leaving a stipule scar on the twig (an exstipulate leaf). rhubarb, encircling the petiole base interpetiolar : between the petioles of two opposite leaves. intrapetiolar : between the petiole and the subtending stemVenation (arrangement of the veins)
There are two subtypes of venation, craspedodromus (the major veins stretch up to the margin of the leaf) and camptodromous (major veins come close to the margin, but bend before they get to it). the leaf has usually one main vein (called the mid-vein), with veinlets, smaller veins branching off laterally, usually somewhat parallel to each other; several main veins diverge from near the leaf base where the petiole attaches, and radiate toward the edge of the leaf; Parallel-veined, parallel-ribbed, parallel-nerved, penniparallel — veins run parallel most the length of the leaf, from the base to the apex.
Leaf terminology
ShapeSee Leaf shape
Margins (edge)
The leaf margin is characteristic for a genus and aids in determining the species.
Tip of the leaf
acuminate: long-pointed, prolonged into a narrow, tapering point in a concave manner.Base of the leaf
acuminate: coming to a sharp, narrow, prolonged point.Surface of the leaf
The surface of a leaf can be described by several botanical terms:
farinose: bearing farina;The leaf surface is also host to a large variety of microorganisms;
Hairiness (trichomes)
Leaves can show several degrees of hairiness.
Adaptations
In the course of evolution, leaves adapted to different environments in the following ways:
A certain surface structure avoids moistening by rain and contaminations (Lotus effect). Sliced leaves reduce wind resistance. Hairs on the leaf surface trap humidity in dry climates and creates a large boundary layer and reduces water loss. Waxy leaf surfaces reduce water loss. Shiny leaves deflect the sun's rays. Reductions of leaf sizes accompanied by a transfer of the photosynthetic functions to the stems reduces water loss. In more or less opaque or buried in the soil leaves translucent windows filter the light before the photosynthetis takes place at the inner leaf surfaces (e.g. Thicker leaves store water (leaf succulents). Aromatic oils, poisons or pheromones produced by leaf borne glands deter herbivores (e.g.
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