Leaf Guide, Meaning , Facts, Information and Description
This article is about the leaf, a plant organ. See Leaf (disambiguation) for other meanings.In botany, a leaf is an aboveground 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.
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2 Leaf Types, Arrangements, and Forms 3 Leaf shapes 4 Leaf margins 5 Leaf tips 6 Leaf bases 7 Hairiness 8 Leaf surfaces 9 Adaptations 10 See Also 11 External link |
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).
Not all species produce leaves with all of these parts. In some species, stipules are not obvious; or a petiole may be absent. The blade is not always laminar (flattened).
The point at which the petiole attaches to the plant stem is called the leaf axil.
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 species.
1) An epidermis that covers the upper and lower surfaces;
2) an interior chlorenchyma called the mesophyll
3) veins (the vascular tissue).
The epidermis has different 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 transparent (cells lack chloroplasts) and coated on the outer surface with a waxy cuticle 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 pores 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 oxygen and carbon dioxide 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 vapor 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 plants from the sedum family (Crassulaceae) in arid regions. To prevent extensive water loss, they open their stomata during the night.
Stomata lack in some underwater plants.
Trichomes or hairs grow out from the epidermis in many species.
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). This assimilation tissue is made of chloroplast and is the primary photosynthetic tissue of the plant. The products of this photosynthesis are called assimilates. There also a few plants with mesophyll which lack chloroplast, and therefore also lack assimilation.
In ferns and most flowering plants it is divided into two layers :
This two different layers of the mesophyll are lacking in many aquatic and marsh plants. Even an epidermis and a mesophyll may be lacking. Instead they use for their gaseous exchanges a homogenous aerenchyma (thin-walled cells separated by large gas-filled spaces). Their stomata are situated at the upper surface.
Leaves are normally green in color, which comes from chlorophyll found in plastids in the chlorenchyma.
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.
In cold autumns they sometimes turn yellow, bright orange or red as various accessory pigments (carotenoids and anthocyanins) are revealed when the tree responds to cold and reduced sunlight by curtailing chlorophyll production.
The veins are made up of :
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.
Leaves are set in a particular arrangement around the stem in order to gain an optimal yield of light. Subsequent leaves are arranged in spirals, clockwise or counterclockwise, with always the same angle of divergence. There is a certain regularity in these angles : they follow the numbers in a Fibonacci-series : 1/2 , 1/3 , 2/5 , 3/8 , 5/13 , 8/21 , 13/34, 21/55, 34/89 ....
This series tends to a limit of 360° x 34/89 = 137,52 or 137° 30'. This angle is known mathematically as 'the golden angle'.
The numerator gives the number of gyres, till the leaf arrives at the same initial position. The denominator gives the number of leaves in this arrangement. This can easily be seen :
This is an Article on Leaf. Page Contains Information, Facts Details or Explanation Guide About Leaf Leaf structure
A leaf typically consists of the following tissues: Epidermis
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.Mesophyll
The pores or stomata of the epidermis open into substomatal chambers, the spaces between the spongy layer cells.Veins
The veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll. They are typical examples of pattern formation through ramification.
The xylem typically lies over the phloem. Both are embedded in a dense parenchyma tissue (= ground tissue), called pith, with usually some structural collenchyma tissue present.Leaf Types, Arrangements, and Forms
Leaves of the Norway Spruce (Picea abies)
are needle-shaped and the arrangement
is whorled
In some acacia species, such as the Koa Tree (Acacia koa), the petioles are called phyllodes. They are expanded or broadened and they function like leaf blades. At their tip grow the pinnate leaves.
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).
Leaf margins
The leaf margin is characteristic for a genus and aids in determining the species.
Leaf tips
Leaf bases
Hairiness
Leaves can show several degrees of hairiness. The meaning of several of the following terms can partially overlap.
Leaf surfaces
The surface of a leaf can be described by several botanical terms :
Adaptations
In order to survive in a harsh environment, leaves can adapt in the following ways:
See Also
External link