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LEAF
*** Shopping-Tip: LEAF
:''This article is about the
plant organ. See
Leaf (disambiguation) for other meanings.''
In
botany, a '''leaf''' is an above-ground
plant Organ (anatomy) organ specialized for
photosynthesis. For this purpose, a leaf is typically flat (laminar) and thin, to expose the
chloroplast containing cells ('''''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 (physiology) 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 correctly referred to as
fronds.
Image:Leaf1web.jpg thumb|right|A leaf with laminar structure and pinnate venation
Image:Autumn leaf.jpg thumb|Fallen autumn leaves
Leaf anatomy
A structurally complete leaf of an
flowering plant 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. In some species, paired stipules are not obvious or are absent altogether; a petiole may be absent; or the blade may not be laminar (flattened). 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).
Image:Leaf_anatomy.svg center|600px|Diagram of leaf internal anatomy
Epidermis
The
epidermis (botany) epidermis is the outer multi-layered group of cells covering the leaf. It forms the boundary between the plant and the external world. The epidermis serves several functions: protection against water loss, regulation of gas exchange, secretion of metabolic compounds, and (in some species) absorption of water. 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
transparency (optics) transparent (epidermal cells lack chloroplasts) and coated on the outer side 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 tissue includes several differentiated cell types: epidermal cells, guard cells, subsidiary cells, and epidermal hairs (trichomes). The epidermal cells are the most numerous, largest, and least specialized. These are typically more elongated in the leaves of
monocots than in those of
dicots.
The epidermis is covered with
pores called '''''
stomata''''' (sing., ''stoma''), 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. Typically, the stomata are more numerous over the abaxial (lower) epidermis than the (adaxial) upper epidermis.
'''
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). This "assimilation tissue" is the primary location of photosynthesis in the plant. The products of photosynthesis are called '''assimilates'''.
In ferns and most flowering plants the mesophyll is divided into two layers:
*An upper '''
palisade cell palisade layer''' of tightly packed, vertically elongated cells, one to two cells thick, directly beneath the adaxial epidermis. Its cells contain many more chloroplasts than the spongy layer. These long cylindrical cells are regularly arranged in one to five rows. Cylindrical cells, with the chloroplasts close to the walls of the cell, can take optimal advantage of light. The slight separation of the cells provides maximum
absorption of carbon dioxide. This separation must be minimal to afford
capillary action for water distribution. In order to adapt to their different environment (such as sun or shade), plants had to adapt this structure to obtain optimal result. Sun leaves have a multi-layered palisade layer, while shade leaves or older leaves closer to the soil, are single-layered.
*Beneath the palisade layer is the '''spongy layer'''. The cells of the spongy layer are more rounded and not so tightly packed. There are large intercellular air spaces. These cells contain less chloroplasts than those of the palisade layer.
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. Even an epidermis and a mesophyll may be lacking. Instead for their gaseous exchanges they use a homogeneous '''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''' cells. Plants that lack chlorophyll cannot photosynthesize.
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 (colour) 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.
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 veins are made up of:
*
xylem, which brings water from the stem into the leaf.
*
phloem, which usually moves sap out, the latter containing the glucose produced by photosynthesis in the leaf.
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 morphology
image:leaf macrophotography.jpg thumb|Underside view of leaf
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. Other plant parts like stems or roots are non-determinant, and will continue to grow as long as they have the resources to do so.
Image:Leaves_opposite.jpg thumb|The leaves on this plant are arranged in pairs opposite one another, with successive pairs at right angles to each other ("decussate") along the red stem. Note developing buds in the axils of these leaves.
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 [http://wikibooks.org/wiki/Botany:_Leaves_(forms) Wikibooks].
Basic leaf types
*
Ferns have '''fronds'''.
*
Conifer leaves are typically needle-, awl-, or scale-shaped
*
Flowering plant Angiosperm (flowering plant) leaves: the standard form includes ''stipules'', ''petiole'', and ''lamina''.
*
Microphyll leaves.
*
Sheath leaves (type found in most
Poaceae grasses).
* Other specialized leaves.
Arrangement 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'''. There is a regularity in these angles and they follow the numbers in a
Fibonacci series: 1/2, 2/3, 3/5, 5/8, 8/13, 13/21, 21/34, 34/55, 55/89. This series tends to a limit of 360° x 34/89 = 137.52 or 137° 30', an angle known mathematically as the 'golden angle'. 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. Mathematics is the science of discovering numerical relationships and applying formulae to these relationships. 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; '''decussate''' if, as typical, each successive pair is rotated 90° going along the stem; or '''distichous''' if not rotated, but two-ranked (in the same plane).
* '''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).
Image:Closeuppineneedlessm.jpg right|thumb|Leaves of the Norway Spruce (''Picea abies'') are needle-shaped and the arrangement is spiral
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. There is no rachis, e.g. ''
Cannabis'' (hemp) and ''
Aesculus'' (buckeyes).
* ''Pinnately compound'' leaves have the leaflets arranged along the main or ''mid-vein'' (called a '''rachis''' in this case).
**odd pinnate: with a terminal leaflet, e.g. ''
ash tree Fraxinus'' (ash).
**even pinnate: lacking a terminal leaflet, e.g. ''
Mahogany Swietenia'' (mahogany).
* ''Bipinnately compound'' leaves are twice divided: the leaflets are arranged along a secondary vein that is one of several branching off the ''rachis''. Each leaflet is called a ''pinnule''. The pinnules on one secondary vein are called ''pinna''; e.g. ''
Albizia'' (silk tree).
*''trifoliate'': a pinnate leaf with just three leaflets, e.g. ''
clover Trifolium'' (clover), ''
Laburnum'' (laburnum).
*''pinnatifid'': pinnately dissected to the midrib, but with the leaflets not entirely separate, e.g. some ''
Sorbus'' (whitebeams).
;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; these are called '''phyllodes'''. 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).
*The situation, arrangement, and structure of the stipules is called the ''stipulation''.
**free
**adnate : fused to the petiole base
**ochreate : provided with ochrea, or sheath-formed stipules, e.g.
rhubarb,
**encircling the petiole base
**interpetiolar : between the petioles of two opposite leaves.
**intrapetiolar : between the petiole and the subtending stem
Venation (arrangement of the veins)
Image:Palmate.jpg thumb|right|Palmate-veined leaf
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).
* Feather-veined, reticulate — the veins arise pinnately from a single mid-vein and subdivide into veinlets. These, in turn, form a complicated network. This type of venation is typical for
dicotyledons.
** Pinnate-netted, penniribbed, penninerved, penniveined; the leaf has usually one main vein (called the '''mid-vein'''), with '''veinlets''', smaller veins branching off laterally, usually somewhat parallel to each other; eg ''
Malus'' (apples).
** Three main veins originate from the base of the lamina, as in ''
Ceanothus''.
** Palmate-netted, palmate-veined, fan-veined; several main veins
divergence diverge from near the leaf base where the petiole attaches, and radiate toward the edge of the leaf; e.g. most
maple ''Acer'' (maples).
* Parallel-veined, parallel-ribbed, parallel-nerved, penniparallel — veins run
Parallel (geometry) parallel most the length of the leaf, from the base to the apex. ''Commissural'' veins (small veins) connect the major parallel veins. Typical for most
monocotyledons, such as
Poaceae grasses.
* Dichotomous — There are no dominant bundles, with the veins forking regularly by pairs; found in ''
Ginkgo'' and some
fern pteridophytes.
Leaf terminology
image:Leaf morphology no title.png thumb|right|300px|Chart illustrating leaf morphology terms
;Shape
See
Leaf shape
Margins (edge)
The leaf margin is characteristic for a genus and aids in determining the species.
*entire: even; with a smooth margin; without toothing
*ciliate: fringed with hairs
*crenate: wavy-toothed; dentate with rounded teeth, such as
beech ''Fagus'' (beech)
*dentate: toothed, such as
Chestnut ''Castanea'' (chestnut)
**coarse-toothed: with large teeth
**glandular toothed: with teeth that bear glands.
*denticulate: finely toothed
*doubly toothed: each tooth bearing smaller teeth, such as
elm ''Ulmus'' (elm)
*lobate: indented, with the indentations not reaching to the center, such as many
Oak ''Quercus'' (oaks)
**palmately lobed: indented with the indentations reaching to the center, such as
Hop (plant) ''Humulus'' (hop).
*serrate: saw-toothed with asymmetrical teeth pointing forward, such as
Nettle ''Urtica'' (nettle)
*serrulate: finely serrate
*sinuate: with deep, wave-like indentations; coarsely crenate, such as many ''
Rumex'' (docks)
*spiny: with stiff, sharp points, such as some
holly ''Ilex'' (hollies) and
thistle ''Cirsium'' (thistles).
Tip of the leaf
image:Leaves-scan.jpg thumb|right|200px|Leaves showing various morphologies. Clockwise from upper left: tripartite lobation, elliptic with serrulate margin, peltate with palmate venation, acuminate odd-pinnate (center), pinnatisect, lobed, elliptic with entire margin
* acuminate: long-pointed, prolonged into a narrow, tapering point in a concave manner.
* acute: ending in a sharp, but not prolonged point
* cuspidate: with a sharp, elongated, rigid tip; tipped with a cusp.
* emarginate: indented, with a shallow notch at the tip.
* mucronate: abruptly tipped with a small short point, as a continuation of the midrib; tipped with a mucro.
* mucronulate: mucronate, but with a smaller spine.
* obcordate: inversely heart-shaped, deeply notched at the top.
* obtuse: rounded or blunt
* truncate: ending abruptly with a flat end, that looks cut off.
Base of the leaf
* acuminate: coming to a sharp, narrow, prolonged point.
* acute: coming to a sharp, but not prolonged point.
* auriculate: ear-shaped
* cordate: heart-shaped with the norch away from the stem.
*
cuneate: wedge-shaped.
* hastate: shaped like an halberd and with the basal lobes pointing outward.
* oblique: slanting.
* reniform: kidney-shaped but rounder and broader than long.
* rounded: curving shape.
* sagittate: shaped like an arrowhead and with the acute basal lobes pointing downward.
* truncate: ending abruptly with a flat end, that looks cut off.
Surface of the leaf
The surface of a leaf can be described by several botanical terms:
*farinose: bearing farina; mealy, covered with a waxy, whitish powder.
*glabrous: smooth, not hairy.
*glaucous: with a whitish bloom; covered with a very fine, bluish-white powder.
*glutinous: sticky, viscid.
*papillate, papillose: bearing papillae (minute, nipple-shaped protuberances).
*pubescent: covered with erect hairs (especially soft and short ones)
*punctate: marked with dots; dotted with depressions or with translucent glands or colored dots.
*rugose: deeply wrinkled; with veins clearly visible.
*scurfy: covered with tiny, broad scalelike particles.
*tuberculate: covered with tubercles; covered with warty prominences.
*verrucose: warted, with warty outgrowths.
*viscid, viscous: covered with thick, sticky secretions.
Hairiness (trichomes)
Leaves can show several degrees of hairiness. The meaning of several of the following terms can overlap. See also :
Trichome.
*glabrous: no hairs of any kind present.
*arachnoid, arachnose: with many fine, entangled hairs giving a cobwebby appearance.
*barbellate: with finely barbed hairs (barbellae).
*bearded: with long, stiff hairs.
*bristly: with stiff hair-like prickles.
*canescent: hoary with dense grayish-white pubescence.
*ciliate: marginally fringed with short hairs (cilia).
*ciliolate: minutely ciliate.
*floccose: with flocks of soft, woolly hairs, which tend to rub off.
*glandular: with a gland at the tip of the hair.
*hirsute: with rather rough or stiff hairs.
*hispid: with rigid, bristly hairs.
*hispidulous: minutely hispid.
*hoary: with a fine, close grayish-white pubescence.
*lanate, lanose: with woolly hairs.
*pilose: with soft, clearly separated hairs.
*puberulent, puberulous: with fine, minute hairs.
*pubescent: with soft, short and erect hairs.
*scabrous, scabrid: rough to the touch
*sericeous: silky appearance through fine, straight and appressed (lying close and flat) hairs.
*silky: with adpressed, soft and straight pubescence.
*stellate, stelliform: with star-shaped hairs.
*strigose: with appressed, sharp, straight and stiff hairs.
*tomentose: densely pubescent with matted, soft white woolly hairs.
**cano-tomentose: between canescent and tomentose
**felted-tomentose: woolly and matted with curly hairs.
*villous: with long and soft hairs, usually curved.
*woolly: with long, soft and tortuous or matted hairs.
Adaptations
Image:Inflorescence 1.jpg Poinsettia.html" title="Meaning of thumb thumb|right|300px|The leaves of [[Poinsettia have evolved a red pigmentation in order to attract insects and birds to the central flowers, an adaptive function normally served by
petals..html" title="Meaning of right|300px|The leaves of [[Poinsettia">thumb|right|300px|The leaves of [[Poinsettia have evolved a red pigmentation in order to attract insects and birds to the central flowers, an adaptive function normally served by
petals.">right|300px|The leaves of [[Poinsettia">thumb|right|300px|The leaves of [[Poinsettia have evolved a red pigmentation in order to attract insects and birds to the central flowers, an adaptive function normally served by
petals.
In order to survive in a harsh
natural environment environment, leaves can adapt in the following ways:
* Hairs develop on the leaf surface to trap humidity in dry climates, creating a large boundary layer to lessen water loss
* Leaves rustle to move humidity away from the surface reducing the boundary layer resistance between the leaf and the air.
* Plant prickles are modified clusters of epidermal hairs
*
Waxy leaf surfaces form to prevent water loss
* Small, shiny leaves to deflect the
sun's rays
* Thicker leaves to store water (e.g.
rhubarb)
* Change to spines instead of laminar (blade) leaves (e.g.
cactus)
* Shrink (to
phyllodes) or disappear (with the appearance of
cladodes), as photosynthetic functions are transferred to the leaf stem (
Acacia species)
* Change shape to deflect wind or reduce wind resistance
* Leaves to trap insects (e.g.
pitcher plant)
* Change to bulb parts to store food (e.g.
onion)
* Produce aromatic oils to deter herbivores (e.g.
eucalypts)
* Protect as
Spine (botany) spines, which are modified leaves.
* Change Color to attract
insects,
birds.(e.g.
Poinsettia)
See also
*
Cuneate
*
Leaf blower
*
Vernation
External links
{{commonscat|Leaves}}
-
Position and Arrangement
Category:Photosynthesis
Category:Plant physiology
Category:plant morphology
Category:Plant anatomy
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