| 词条 | Leaf | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 释义 |
A leaf is an organ of a vascular plant and is the principal lateral appendage of the stem.{{sfn|Esau|2006}} The leaves and stem together form the shoot.{{sfn|Cutter|1969}} Leaves are collectively referred to as foliage, as in "autumn foliage".{{sfn|Haupt|1953}}{{sfn|Mauseth|2009}} A leaf is a thin, dorsiventrally flattened organ, usually borne above ground and specialized for photosynthesis. In most leaves, the primary photosynthetic tissue, the palisade mesophyll, is located on the upper side of the blade or lamina of the leaf{{sfn|Esau|2006}} but in some species, including the mature foliage of Eucalyptus,{{sfn|James et al|1999}} palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves have distinct upper surface ({{vanchor|adaxial}}) and lower surface ({{vanchor|abaxial}}) that differ in colour, hairiness, the number of stomata (pores that intake and output gases), the amount and structure of epicuticular wax and other features. Leaves can have many different shapes, sizes and textures. The broad, flat leaves with complex venation of flowering plants are known as megaphylls and the species that bear them, the majority, as broad-leaved or megaphyllous plants. In the clubmosses, with different evolutionary origins, the leaves are simple (with only a single vein) and are known as microphylls.{{sfn|Stewart|Rothwell|1993}} Some leaves, such as bulb scales, are not above ground. In many aquatic species the leaves are submerged in water. Succulent plants often have thick juicy leaves, but some leaves are without major photosynthetic function and may be dead at maturity, as in some cataphylls and spines. Furthermore, several kinds of leaf-like structures found in vascular plants are not totally homologous with them. Examples include flattened plant stems called phylloclades and cladodes, and flattened leaf stems called phyllodes which differ from leaves both in their structure and origin.{{sfn|Mauseth|2009}}{{sfn|Cooney-Sovetts|Sattler|1987}} Some structures of non-vascular plants look and function much like leaves. Examples include the phyllids of mosses and liverworts. General characteristicsLeaves are the most important organs of most vascular plants.{{sfn|Tsukaya|2013}} Green plants are autotrophic, meaning that they do not obtain food from other living things but instead create their own food by photosynthesis. They capture the energy in sunlight and use it to make simple sugars, such as glucose and sucrose, from carbon dioxide and water. The sugars are then stored as starch, further processed by chemical synthesis into more complex organic molecules such as proteins or cellulose, the basic structural material in plant cell walls, or metabolised by cellular respiration to provide chemical energy to run cellular processes. The leaves draw water from the ground in the transpiration stream through a vascular conducting system known as xylem and obtain carbon dioxide from the atmosphere by diffusion through openings called stomata in the outer covering layer of the leaf (epidermis), while leaves are orientated to maximise their exposure to sunlight. Once sugar has been synthesized, it needs to be transported to areas of active growth such as the plant shoots and roots. Vascular plants transport sucrose in a special tissue called the phloem. The phloem and xylem are parallel to each other but the transport of materials is usually in opposite directions. Within the leaf these vascular systems branch (ramify) to form veins which supply as much of the leaf as possible, ensuring that cells carrying out photosynthesis are close to the transportation system.{{sfn|Feugier|2006}} Typically leaves are broad, flat and thin (dorsiventrally flattened), thereby maximising the surface area directly exposed to light and enabling the light to penetrate the tissues and reach the chloroplasts, thus promoting photosynthesis. They are arranged on the plant so as to expose their surfaces to light as efficiently as possible without shading each other, but there are many exceptions and complications. For instance plants adapted to windy conditions may have pendent leaves, such as in many willows and eucalyptss. The flat, or laminar, shape also maximises thermal contact with the surrounding air, promoting cooling. Functionally, in addition to carrying out photosynthesis, the leaf is the principal site of transpiration, providing the energy required to draw the transpiration stream up from the roots, and guttation. Many gymnosperms have thin needle-like or scale-like leaves that can be advantageous in cold climates with frequent snow and frost.{{sfn|Purcell|2016}} These are interpreted as reduced from megaphyllous leaves of their Devonian ancestors.{{sfn|Stewart|Rothwell|1993}} Some leaf forms are adapted to modulate the amount of light they absorb to avoid or mitigate excessive heat, ultraviolet damage, or desiccation, or to sacrifice light-absorption efficiency in favour of protection from herbivory. For xerophytes the major constraint is not light flux or intensity, but drought.{{sfn|Willert et al|1992}} Some window plants such as Fenestraria species and some Haworthia species such as Haworthia tesselata and Haworthia truncata are examples of xerophytes.{{sfn|Bayer|1982}} and Bulbine mesembryanthemoides.{{sfn|Marloth|1913–1932}} Leaves also function to store chemical energy and water (especially in succulents) and may become specialised organs serving other functions, such as tendrils of peas and other legumes, the protective spines of cacti and the insect traps in carnivorous plants such as Nepenthes and Sarracenia.{{sfn|Simpson|2011|loc=p. 356}} Leaves are the fundamental structural units from which cones are constructed in gymnosperms (each cone scale is a modified megaphyll leaf known as a sporophyll){{sfn|Stewart|Rothwell|1993}}{{rp|408}} and from which flowers are constructed in flowering plants.{{sfn|Stewart|Rothwell|1993}}{{rp|445}} The internal organisation of most kinds of leaves has evolved to maximise exposure of the photosynthetic organelles, the chloroplasts, to light and to increase the absorption of carbon dioxide while at the same time controlling water loss. Their surfaces are waterproofed by the plant cuticle and gas exchange between the mesophyll cells and the atmosphere is controlled by minute (length and width measured in tens of µm) openings called stomata which open or close to regulate the rate exchange of carbon dioxide, oxygen, and water vapour into and out of the internal intercellular space system. Stomatal opening is controlled by the turgor pressure in a pair of guard cells that surround the stomatal aperture. In any square centimeter of a plant leaf there may be from 1,000 to 100,000 stomata.{{sfn|Krogh|2010}} The shape and structure of leaves vary considerably from species to species of plant, depending largely on their adaptation to climate and available light, but also to other factors such as grazing animals (such as deer), available nutrients, and ecological competition from other plants. Considerable changes in leaf type occur within species too, for example as a plant matures; as a case in point Eucalyptus species commonly have isobilateral, pendent leaves when mature and dominating their neighbours; however, such trees tend to have erect or horizontal dorsiventral leaves as seedlings, when their growth is limited by the available light.{{sfn|James |Bell|2000}} Other factors include the need to balance water loss at high temperature and low humidity against the need to absorb atmospheric carbon dioxide. In most plants leaves also are the primary organs responsible for transpiration and guttation (beads of fluid forming at leaf margins). Leaves can also store food and water, and are modified accordingly to meet these functions, for example in the leaves of succulent plants and in bulb scales. The concentration of photosynthetic structures in leaves requires that they be richer in protein, minerals, and sugars than, say, woody stem tissues. Accordingly, leaves are prominent in the diet of many animals. Correspondingly, leaves represent heavy investment on the part of the plants bearing them, and their retention or disposition are the subject of elaborate strategies for dealing with pest pressures, seasonal conditions, and protective measures such as the growth of thorns and the production of phytoliths, lignins, tannins and poisons. Deciduous plants in frigid or cold temperate regions typically shed their leaves in autumn, whereas in areas with a severe dry season, some plants may shed their leaves until the dry season ends. In either case the shed leaves may be expected to contribute their retained nutrients to the soil where they fall. In contrast, many other non-seasonal plants, such as palms and conifers, retain their leaves for long periods; Welwitschia retains its two main leaves throughout a lifetime that may exceed a thousand years. The leaf-like organs of Bryophytes (e.g., mosses and liverworts), known as phyllids, differ morphologically from the leaves of vascular plants in that they lack vascular tissue, are usually only a single cell thick and have no cuticle stomata or internal system of intercellular spaces. Simple, vascularised leaves (microphylls) first evolved as enations, extensions of the stem, in clubmosses such as Baragwanathia during the Silurian period. True leaves or euphylls of larger size and with more complex venation did not become widespread in other groups until the Devonian period, by which time the carbon dioxide concentration in the atmosphere had dropped significantly. This occurred independently in several separate lineages of vascular plants, in progymnosperms like Archaeopteris, in Sphenopsida, ferns and later in the gymnosperms and angiosperms. Euphylls are also referred to as macrophylls or megaphylls (large leaves).{{sfn|Stewart|Rothwell|1993}} {{vanchor|Morphology (large-scale features)}}{{see also|Glossary of leaf morphology}}A structurally complete leaf of an angiosperm consists of a petiole (leaf stalk), a lamina (leaf blade), and stipules (small structures located to either side of the base of the petiole). Not every species produces leaves with all of these structural components. Stipules may be conspicuous (e.g. beans and roses), soon falling or otherwise not obvious as in Moraceae or absent altogether as in the Magnoliaceae. 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 morphology. The petiole mechanically links the leaf to the plant and provides the route for transfer of water and sugars to and from the leaf. The lamina is typically the location of the majority of photosynthesis. The upper (adaxial) angle between a leaf and a stem is known as the axil of the leaf. It is often the location of a bud. Structures located there are called "axillary". External leaf characteristics, such as shape, margin, hairs, the petiole, and the presence of stipules and glands, are frequently important for identifying plants to family, genus or species levels, and botanists have developed a rich terminology for describing leaf characteristics. Leaves almost always have determinate growth. They grow to a specific pattern and shape and then stop. Other plant parts like stems or roots have non-determinate growth, and will usually continue to grow as long as they have the resources to do so. The type of leaf is usually characteristic of a species (monomorphic), although some species produce more than one type of leaf (dimorphic or polymorphic). The longest leaves are those of the Raffia palm, R. regalis which may be up to {{convert|25|m|ft|abbr=on}} long and {{convert|3|m|ft|abbr=on}} wide.{{sfn|Hallé|1977}} The terminology associated with the description of leaf morphology is presented, in illustrated form, at Wikibooks. Where leaves are basal, and lie on the ground, they are referred to as prostrate. Basic leaf types{{anchor|Basic leaf types}}
Arrangement on the stem{{main article|Phyllotaxis}}Different terms are usually used to describe the arrangement of leaves on the stem (phyllotaxis):
As a stem grows, leaves tend to appear arranged around the stem in a way that optimizes yield of light. In essence, leaves form a helix pattern centered around the stem, 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 sequence: 1/2, 2/3, 3/5, 5/8, 8/13, 13/21, 21/34, 34/55, 55/89. This series tends to the golden angle, which is approximately 360° × 34/89 ≈ 137.52° ≈ 137° 30′. In the series, the numerator indicates the number of complete turns or "gyres" until a leaf arrives at the initial position and the denominator indicates the number of leaves in the arrangement. This can be demonstrated by the following:
Divisions of the blade{{anchor|Divisions of the lamina (blade)}}Two basic forms of leaves can be described considering the way the blade (lamina) is divided. A simple leaf has an undivided blade. However, the leaf may be dissected to form 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 being separated along a main or secondary vein. The leaflets may have petiolules and stipels, the equivalents of the petioles and stipules of leaves. 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. The middle vein of a compound leaf or a frond, when it is present, is called a rachis.
Characteristics of the petiolePetiolated leaves have a petiole (leaf stalk), and are said to be petiolate. Sessile (epetiolate) leaves have no petiole and the blade attaches directly to the stem. Subpetiolate leaves are nearly petiolate or have an extremely short petiole and may appear to be sessile. In clasping or decurrent leaves, the blade partially surrounds the stem. When the leaf base completely surrounds the stem, the leaves are said to be perfoliate, such as in Eupatorium perfoliatum. In peltate leaves, the petiole attaches to the blade inside the blade margin. 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. 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. Stipules 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".
Veins{{see also|#Venation|#Vascular tissue}}Veins (sometimes referred to as nerves) constitute one of the more visible leaf traits or characteristics. The veins in a leaf represent the vascular structure of the organ, extending into the leaf via the petiole and provide transportation of water and nutrients between leaf and stem, and play a crucial role in the maintenance of leaf water status and photosynthetic capacity.They also play a role in the mechanical support of the leaf.{{sfn|Rolland-Lagan et al|2009}}{{sfn|Walls|2011}} Within the lamina of the leaf, while some vascular plants possess only a single vein, in most this vasculature generally divides (ramifies) according to a variety of patterns (venation) and form cylindrical bundles, usually lying in the median plane of the mesophyll, between the two layers of epidermis.{{sfn|Dickison|2000}} This pattern is often specific to taxa, and of which angiosperms possess two main types, parallel and reticulate (net like). In general, parallel venation is typical of monocots, while reticulate is more typical of eudicots and magnoliids ("dicots"), though there are many exceptions.{{sfn|Rudall|2007}}{{sfn|Dickison|2000}}[1] The vein or veins entering the leaf from the petiole are called primary or first order veins. The veins branching from these are secondary or second order veins. These primary and secondary veins are considered major veins or lower order veins, though some authors include third order.{{sfn|Sack|Scoffoni|2013}} Each subsequent branching is sequentially numbered, and these are the higher order veins, each branching being associated with a narrower vein diameter.{{sfn|Roth-Nebelsick et al|2001}} In parallel veined leaves, the primary veins run parallel and equidistant to each other for most of the length of the leaf and then converge or fuse (anastomose) towards the apex. Usually many smaller minor veins interconnect these primary veins, but may terminate with very fine vein endings in the mesophyll. Minor veins are more typical of angiosperms, which may have as many as four higher orders.{{sfn|Sack|Scoffoni|2013}} In contrast, leaves with reticulate venation there is a single (sometimes more) primary vein in the centre of the leaf, referred to as the midrib or costa and is continuous with the vasculature of the petiole more proximally. The midrib then branches to a number of smaller secondary veins, also known as second order veins, that extend toward the leaf margins. These often terminate in a hydathode, a secretory organ, at the margin. In turn, smaller veins branch from the secondary veins, known as tertiary or third order (or higher order) veins, forming a dense reticulate pattern. The areas or islands of mesophyll lying between the higher order veins, are called areoles. Some of the smallest veins (veinlets) may have their endings in the areoles, a process known as areolation.{{sfn|Roth-Nebelsick et al|2001}} These minor veins act as the sites of exchange between the mesophyll and the plant's vascular system.{{sfn|Walls|2011}} Thus minor veins collect the products of photosynthesis (photosynthate) from the cells where it takes place, while major veins are responsible for its transport outside of the leaf. At the same time water is being transported in the opposite direction.{{sfn|Ueno et al|2006}}{{sfn|Rudall|2007}}{{sfn|Dickison|2000}} The number of vein endings is very variable, as is whether second order veins end at the margin, or link back to other veins.[1] There are many elaborate variations on the patterns that the leaf veins form, and these have functional implications. Of these, angiosperms have the greatest diversity.{{sfn|Sack|Scoffoni|2013}} Within these the major veins function as the support and distribution network for leaves and are correlated with leaf shape. For instance the parallel venation found in most monocots correlates with their elongated leaf shape and wide leaf base, while reticulate venation is seen in simple entire leaves, while digitate leaves typically have venation in which three or more primary veins diverge radially from a single point.{{sfn|Runions et al|2005}}{{sfn|Walls|2011}}{{sfn|Roth-Nebelsick et al|2001}}[3] In evolutionary terms, early emerging taxa tend to have dichotomous branching with reticulate systems emerging later. Veins appeared in the Permian period (299–252 mya), prior to the appearance of angiosperms in the Triassic (252–201 mya), during which vein hierarchy appeared enabling higher function, larger leaf size and adaption to a wider vaiety of climatic conditions.{{sfn|Sack|Scoffoni|2013}} Although it is the more complex pattern, branching veins appear to be plesiomorphic and in some form were present in ancient seed plants as long as 250 million years ago. A pseudo-reticulate venation that is actually a highly modified penniparallel one is an autapomorphy of some Melanthiaceae, which are monocots; e.g., Paris quadrifolia (True-lover's Knot). In leaves with reticulate venation, veins form a scaffolding matrix imparting mechanical rigidity to leaves.[1] Morphology changes within a single plant
Anatomy (medium and small scale)Medium-scale featuresLeaves are normally extensively vascularised and typically have networks of vascular bundles containing xylem, which supplies water for photosynthesis, and phloem, which transports the sugars produced by photosynthesis. Many leaves are covered in trichomes (small hairs) which have diverse structures and functions. Small-scale featuresThe major tissue systems present are
These three tissue systems typically form a regular organisation at the cellular scale. Specialised cells that differ markedly from surrounding cells, and which often synthesise specialised products such as crystals, are termed idioblasts.{{sfn|Cote|2009}} Major leaf tissuesEpidermisThe epidermis is the outer layer of cells covering the leaf. It is covered with a waxy cuticle which is impermeable to liquid water and water vapor and forms the boundary separating the plant's inner cells from the external world. The cuticle is in some cases thinner on the lower epidermis than on the upper epidermis, and is generally thicker on leaves from dry climates as compared with those from wet climates.[2] The epidermis serves several functions: protection against water loss by way of transpiration, regulation of gas exchange and secretion of metabolic compounds. Most leaves show dorsoventral anatomy: The upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions. The epidermis tissue includes several differentiated cell types; epidermal cells, epidermal hair cells (trichomes), cells in the stomatal complex; guard cells and subsidiary cells. The epidermal cells are the most numerous, largest, and least specialized and form the majority of the epidermis. They are typically more elongated in the leaves of monocots than in those of dicots. Chloroplasts are generally absent in epidermal cells, the exception being the guard cells of the stomata. The stomatal pores perforate the epidermis and are surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts, forming a specialized cell group known as the stomatal complex. The opening and closing of the stomatal aperture is controlled by the stomatal complex and regulates the exchange of gases and water vapor between the outside air and the interior of the leaf. Stomata therefore play the important role in allowing photosynthesis without letting the leaf dry out. In a typical leaf, the stomata are more numerous over the abaxial (lower) epidermis than the adaxial (upper) epidermis and are more numerous in plants from cooler climates. Mesophyll{{for|the term Mesophyll in the size classification of leaves|Leaf size}}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 (Greek for "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:
Leaves are normally green, due to chlorophyll in chloroplasts in the mesophyll cells. Plants that lack chlorophyll cannot photosynthesize. Vascular tissueThe veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll. The pattern of the veins is called venation. In angiosperms the venation is typically parallel in monocotyledons and forms an interconnecting network in broad-leaved plants. They were once thought to be typical examples of pattern formation through ramification, but they may instead exemplify a pattern formed in a stress tensor field.{{sfn|Couder et al|2002}}{{sfn|Corson et al|2009}}{{sfn|Laguna et al|2008}} A vein is made up of a vascular bundle. At the core of each bundle are clusters of two distinct types of conducting cells:
The xylem typically lies on the adaxial side of the vascular bundle and the phloem typically lies on the abaxial side. Both are embedded in a dense parenchyma tissue, called the sheath, which usually includes some structural collenchyma tissue. Leaf developmentAccording to Agnes Arber's partial-shoot theory of the leaf, leaves are partial shoots,{{sfn|Arber|1950}} being derived from leaf primordia of the shoot apex.{{sfn|Simpson|2011|loc=p. 356}} Compound leaves are closer to shoots than simple leaves. Developmental studies have shown that compound leaves, like shoots, may branch in three dimensions.{{sfn|Rutishauser|Sattler|1997}}{{sfn|Lacroix et al|2003}} On the basis of molecular genetics, Eckardt and Baum (2010) concluded that "it is now generally accepted that compound leaves express both leaf and shoot properties."{{sfn|Eckardt|Baum|2010}} EcologyBiomechanicsPlants respond and adapt to environmental factors, such as light and mechanical stress from wind. Leaves need to support their own mass and align themselves in such a way as to optimise their exposure to the sun, generally more or less horizontally. However horizontal alignment maximises exposure to bending forces and failure from stresses such as wind, snow, hail, falling debris, animals, and abrasion from surrounding foliage and plant structures. Overall leaves are relatively flimsy with regard to other plant structures such as stems, branches and roots.{{sfn|Read|Stokes|2006}} Both leaf blade and petiole structure influence the leaf's response to forces such as wind, allowing a degree of repositioning to minimise drag and damage, as opposed to resistance. Leaf movement like this may also increase turbulence of the air close to the surface of the leaf, which thins the boundary layer of air immediately adjacent to the surface, increasing the capacity for gas and heat exchange, as well as photosynthesis. Strong wind forces may result in diminished leaf number and surface area, which while reducing drag, involves a trade off of also reduces photosynthesis. Thus, leaf design may involve compromise between carbon gain, thermoregulation and water loss on the one hand, and the cost of sustaining both static and dynamic loads. In vascular plants, perpendicular forces are spread over a larger area and are relatively flexible in both bending and torsion, enabling elastic deforming without damage.{{sfn|Read|Stokes|2006}} Many leaves rely on hydrostatic support arranged around a skeleton of vascular tissue for their strength, which depends on maintaining leaf water status. Both the mechanics and architecture of the leaf reflect the need for transportation and support. Read and Stokes (2006) consider two basic models, the "hydrostatic" and "I-beam leaf" form (see Fig 1).{{sfn|Read|Stokes|2006}} Hydrostatic leaves such as in Prostanthera lasianthos are large and thin, and may involve the need for multiple leaves rather single large leaves because of the amount of veins needed to support the periphery of large leaves. But large leaf size favours efficiency in photosynthesis and water conservation, involving further trade offs. On the other hand, I-beam leaves such as Banksia marginata involve specialised structures to stiffen them. These I-beams are formed from bundle sheath extensions of sclerenchyma meeting stiffened sub-epidermal layers. This shifts the balance from reliance on hydrostatic pressure to structural support, an obvious advantage where water is relatively scarce. {{sfn|Read|Stokes|2006}} Long narrow leaves bend more easily than ovate leaf blades of the same area. Monocots typically have such linear leaves that maximise surface area while minimising self-shading. In these a high proportion of longitudinal main veins provide additional support.{{sfn|Read|Stokes|2006}}Interactions with other organismsAlthough not as nutritious as other organs such as fruit, leaves provide a food source for many organisms. The leaf is a vital source of energy production for the plant, and plants have evolved protection against animals that consume leaves, such as tannins, chemicals which hinder the digestion of proteins and have an unpleasant taste. Animals that are specialized to eat leaves are known as folivores. Some species have cryptic adaptations by which they use leaves in avoiding predators. For example, the caterpillars of some leaf-roller moths will create a small home in the leaf by folding it over themselves. Some sawflies similarly roll the leaves of their food plants into tubes. Females of the Attelabidae, so-called leaf-rolling weevils, lay their eggs into leaves that they then roll up as means of protection. Other herbivores and their predators mimic the appearance of the leaf. Reptiles such as some chameleons, and insects such as some katydids, also mimic the oscillating movements of leaves in the wind, moving from side to side or back and forth while evading a possible threat. Seasonal leaf lossLeaves 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. When the leaf is shed, it leaves a leaf scar on the twig. In cold autumns, they sometimes change color, and turn yellow, bright-orange, or red, as various accessory pigments (carotenoids and xanthophylls) are revealed when the tree responds to cold and reduced sunlight by curtailing chlorophyll production. Red anthocyanin pigments are now thought to be produced in the leaf as it dies, possibly to mask the yellow hue left when the chlorophyll is lost—yellow leaves appear to attract herbivores such as aphids.{{sfn|Doring et al|2009}} Optical masking of chlorophyll by anthocyanins reduces risk of photo-oxidative damage to leaf cells as they senesce, which otherwise may lower the efficiency of nutrient retrieval from senescing autumn leaves.{{sfn|Feild et al|2001}} Evolutionary adaptationIn the course of evolution, leaves have adapted to different environments in the following ways:
Terminology{{see also|Glossary of leaf morphology|Glossary of plant morphology|Glossary of botanical terms}}Shape{{main article|Glossary of leaf shapes}}Edge (margin){{anchor|Margins (edge)}}
Apex (tip){{anchor|Tip of the leaf}}
Base{{anchor|Base of the leaf}}
Surface{{anchor|Surface of the leaf}}
The leaf surface is also host to a large variety of microorganisms; in this context it is referred to as the phyllosphere. Hairiness{{anchor|Hairiness (trichomes)}}"Hairs" on plants are properly called trichomes. Leaves can show several degrees of hairiness. The meaning of several of the following terms can overlap.
Timing
VenationClassification{{multiple image | header = Hickey primary venation types| align = right | direction = vertical | width = 200 | float = none| image1 =Ostrya virginiana1.jpg| caption1 = 1. Pinnate venation, Ostrya virginiana| alt1 = |image2=Tulip Leaves AWL.JPG|thumb|caption2=2. Parallel venation, Iris |image3=Maianthemum bifolium 2.JPG|caption3=3. Campylodromous venation, Maianthemum bifolium |image4=Starr 031118-0115 Miconia calvescens.jpg|caption4=4. Acrodrous venation (basal), Miconia calvescens |image5=Puttali (Tamil- பூத்தாளி) (5656476463).jpg|caption5=5. Actinodromous venation (suprabasal), Givotia moluccana |image6=Platanus orientalis leaf.JPG|caption6=6. Palinactodromous venation, Platanus orientalis }} A number of different classification systems of the patterns of leaf veins (venation or veination) have been described,[1] starting with Ettingshausen (1861),{{sfn|Ettingshausen|1861}} together with many different descriptive terms, and the terminology has been described as "formidable".[1] One of the commonest among these is the Hickey system, originally developed for "dicotyledons" and using a number of Ettingshausen's terms derived from Greek (1973–1979):{{sfn|Hickey|1973}}{{sfn|Hickey|Wolfe|1975}}{{sfn|Hickey|1979}} (see also: Simpson Figure 9.12, p. 468)[1] Hickey system
| Craspedodromous (Greek: kraspedon - edge, dromos - running)}}{{defn | The major veins reach to the margin of the leaf.}}{{term | Camptodromous}}{{defn | Major veins extend close to the margin, but bend before they intersect with the margin.}}{{term | Hyphodromous}}{{defn | All secondary veins are absent, rudimentary or concealed}}{{glossary end}} These in turn have a number of further subtypes such as eucamptodromous, where secondary veins curve near the margin without joining adjacent secondary veins.{{multiple image | header = Pinnate | align = center | direction = horizontal | width = 95 | float = none | image1 =Leaf morphology - venation Hickey 1973 - craspedodromous simple.svg | caption1 = Craspedodromous | image2= Leaf morphology - venation Hickey 1973 - camptodromous eucamptodromous.svg | caption2= Camptodromous | image3= Leaf morphology - venation Hickey 1973 - hyphodromous.svg | caption3= Hyphodromous }}
| header = Parallelodromous | align = center | direction = horizontal | width = 75 | float = none | image1 = Leaf morphology - venation Hickey 1973 - parallelodromous.svg }}
| header = Campylodromous | align = center | direction = horizontal | width = 75 | float = none | image1 =Leaf morphology - venation Hickey 1973 - campylodromous.svg }}
| header = Acrodromous | align = center | direction = horizontal | width = 75 | float = none | image1 = Leaf morphology - venation Hickey 1973 - acrodromous imperfect basal.svg | caption1 = Imperfect basal | image2 = Leaf morphology - venation Hickey 1973 - acrodromous imperfect suprabasal.svg | caption2=Imperfect suprabasal | image3=Leaf morphology - venation Hickey 1973 - acrodromous perfect basal.svg | caption3=Perfect basal | image4=Leaf morphology - venation Hickey 1973 - acrodromous perfect suprabasal.svg | caption4=Perfect suprabasal }}
| header = Actinodromous | align = center | direction = horizontal | width = 75 | float = none | image1 = Leaf morphology - venation Hickey 1973 - actinodromous imperfect marginal.svg | caption1 = Imperfect marginal | image2=Leaf morphology - venation Hickey 1973 - actinodromous imperfect reticulate.svg | caption2=Imperfect reticulate }}
| header = Palinactodromous | align = center | direction = horizontal | width = 75 | float = none | image1 =Leaf morphology - venation Hickey 1973 - palinactinodromous.svg }} Types 4–6 may similarly be subclassified as basal (primaries joined at the base of the blade) or suprabasal (diverging above the blade base), and perfect or imperfect, but also flabellate. At about the same time, Melville (1976) described a system applicable to all Angiosperms and using Latin and English terminology.{{sfn|Melville|1976}} Melville also had six divisions, based on the order in which veins develop.
A modified form of the Hickey system was later incorporated into the Smithsonian classification (1999) which proposed seven main types of venation, based on the architecture of the primary veins, adding Flabellate as an additional main type. Further classification was then made on the basis of secondary veins, with 12 further types, such as;
terms which had been used as subtypes in the original Hickey system.{{sfn|Leaf Architecture Working Group|1999}} {{multiple image| header = Secondary venation patterns | align = center | direction = horizontal | width = 95 | float = none | image1 = Leaf morphology - venation Hickey 1973 - camptodromous brochidodromous.svg | caption1 = Brochidodromous | image2=Leaf morphology - venation Hickey 1973 - craspedodromous simple.svg | caption2 = Craspedodromous | image3=Leaf morphology - venation Hickey 1973 - camptodromous eucamptodromous.svg | caption3=Eucamptodromous | image4=Leaf morphology - venation Hickey 1973 - camptodromous cladodromous.svg | caption4=Cladodromous }}{{multiple image | align = center | image1 =Hildegardia migeodii - leaf shape (8307117710).jpg | caption1 = Brochidodromous Hildegardia migeodii | width1={{#expr: (150 * 1900 /1425) round 0}} | image2=Celtis occidentalis (18).JPG | caption2=Craspedodromous Celtis occidentalis | width2={{#expr: (150 * 2736 /3192) round 0}} | image3=Cornus officinalis 02.JPG | caption3=Eucamptodromous Cornus officinalis | width3={{#expr: (150 * 2448/3264) round 0}} | image4=Rhus ovata 1.jpg | caption4=Cladodromous Rhus ovata | width4={{#expr: (150 * 1500 /1155) round 0}} }} Further descriptions included the higher order, or minor veins and the patterns of areoles (see Leaf Architecture Working Group, Figures 28–29).{{sfn|Leaf Architecture Working Group|1999}}
| header = Flabellate | align = center | direction = horizontal | width = 75 | float = none | image1 =Leaf morphology - venation Hickey 1973 - flabellate.svg }} Analyses of vein patterns often fall into consideration of the vein orders, primary vein type, secondary vein type (major veins), and minor vein density. A number of authors have adopted simplified versions of these schemes.{{sfn|Judd et al| 2007}}[1] At its simplest the primary vein types can be considered in three or four groups depending on the plant divisions being considered;
where palmate refers to multiple primary veins that radiate from the petiole, as opposed to branching from the central main vein in the pinnate form, and encompasses both of Hickey types 4 and 5, which are preserved as subtypes; e.g., palmate-acrodromous (see National Park Service Leaf Guide).{{sfn|Florissant Leaf Key|2016}}
| header = Palmate | align = center | direction = horizontal | float = none | image1 = Leaf morphology venation palmate.png }} Other systemsAlternatively, Simpson uses:[1]
| image1 =Horsetail vegeative stem.JPG| caption1 = Equisetum: Reduced microphyllous leaves (L) arising in whorl from node| alt1 = |width1 =175 |height1 = 528 | image2 =Ginkgo biloba scanned leaf.jpg|caption2= Ginkgo biloba: Dichotomous venation |width2 =580 |height2 = 599 }} However, these simplified systems allow for further division into multiple subtypes. Simpson,[1] (and others){{sfn|Berg|2007}} divides parallel and netted (and some use only these two terms for Angiosperms)[18] on the basis of the number of primary veins (costa) as follows;
| Penni-parallel (pinnate, pinnate parallel, unicostate parallel)}}{{defn | Single central prominent midrib, secondary veins from this arise perpendicularly to it and run parallel to each other towards the margin or tip, but do not join (anastomose). The term unicostate refers to the prominence of the single midrib (costa) running the length of the leaf from base to apex. e.g. Zingiberales, such as Bananas etc.}}{{term | Palmate-parallel (multicostate parallel)}}{{defn | Several equally prominent primary veins arising from a single point at the base and running parallel towards tip or margin. The term multicostate refers to having more than one prominent main vein. e.g. "fan" (palmate) palms (Arecaceae){{glossary}}{{term | Multicostate parallel convergent}}{{defn | Mid-veins converge at apex e.g. Bambusa arundinacea {{=}} B. bambos (Aracaceae), Eichornia}}{{term | Multicostate parallel divergent}}{{defn | Mid-veins diverge more or less parallel towards the margin e.g. Borassus (Poaceae), fan palms}}{{glossary end}}}}{{glossary end}}
}}{{term | Pinnately (veined, netted, unicostate reticulate)}}{{defn | Single prominent midrib running from base to apex, secondary veins arising on both sides along the length of the primary midrib, running towards the margin or apex (tip), with a network of smaller veinlets forming a reticulum (mesh or network). e.g. Mangifera, Ficus religiosa, Psidium guajava, Hibiscus rosa-sinensis, Salix alba}}{{term | Palmately (multicostate reticulate)}}{{defn | More than one primary veins arising from a single point, running from base to apex. e.g. Liquidambar styraciflua This may be further subdivided;{{glossary}}{{term | Multicostate convergent}}{{defn | Major veins diverge from origin at base then converge towards the tip. e.g. Zizyphus, Smilax, Cinnamomum}}{{term | Multicostate divergent}}{{defn | All major veins diverge towards the tip. e.g. Gossypium, Cucurbita, Carica papaya, Ricinus communis}}{{glossary end}}}}{{term | Ternately (ternate-netted)}}{{defn | Three primary veins, as above, e.g. (see) Ceanothus leucodermis,[19] C. tomentosus,[20] Encelia farinosa }}{{glossary end}} {{multiple image | header = Simpson venation patterns | align = center | direction = | total_width= 800 | float = |perrow=4| image1 = Maranta leuconeura var. erythroneura1.jpg | caption1 = Maranta leuconeura var. erythroneura (Zingiberales): Penni-parallel | width1={{#expr: (150 * 800 /600) round 0}} | image2= Coccothrinax argentea kz2.JPG | caption2= Coccothrinax argentea (Arecaceae): Palmate-parallel | width2={{#expr: (150 * 800 /577) round 0}} | image5 = Salix alba leaf.jpg | caption5 = Salix alba: Pinnately netted | width5={{#expr: (150 * 138/598) round 0}} | image6= Liquidambar feuilles FR 2013.jpg | caption6= Liquidambar styraciflua: Palmately netted | width6={{#expr: (150 * 800/549) round 0}} | image3=Plantarum indigenarum et exoticarum icones ad vivum coloratae, oder, Sammlung nach der Natur gemalter Abbildungen inn- und ausländlischer Pflanzen, für Liebhaber und Beflissene der Botanik (15902604278).jpg | caption3=Bambusa bambos: Multicostate parallel convergent | width3={{#expr: (150 * 361/598) round 0}} | image4=SrahSrangTree.jpg | caption4= Borassus sp.: Multicostate parallel divergent | width4={{#expr: (150 * 450/600) round 0}} | image7=(Ziziphus jujuba) Foliage at Ammuguda 01.jpg | caption7= Ziziphus jujuba: Multicostate palmate convergent | width7={{#expr: (150 * 664/600) round 0}} | image8=Starr 050128-3307 Gossypium tomentosum.jpg | caption8= Gossypium tomentosum: Multicostate palmate divergent | width8={{#expr: (150 * 179/240) round 0}} }} These complex systems are not used much in morphological descriptions of taxa, but have usefulness in plant identification, [1] although criticized as being unduly burdened with jargon.[22]An older, even simpler system, used in some flora{{sfn|Cullen et al|2011}} uses only two categories, open and closed.
Other descriptive termsThere are also many other descriptive terms, often with very specialised usage and confined to specific taxonomic groups.{{sfn|Neotropikey|2017}} The conspicuousness of veins depends on a number of features. These include the width of the veins, their prominence in relation to the lamina surface and the degree of opacity of the surface, which may hide finer veins. In this regard, veins are called obscure and the order of veins that are obscured and whether upper, lower or both surfaces, further specified.{{sfn|Oxford herbaria glossary|2017}}[12] Terms that describe vein prominence include bullate, channelled, flat, guttered, impressed, prominent and recessed (Fig. 6.1 Hawthorne & Lawrence 2013).[22][26] Veins may show different types of prominence in different areas of the leaf. For instance Pimenta racemosa has a channelled midrib on the upper surfae, but this is prominent on the lower surface.[22] Describing vein prominence:
| image1 = Blattadern-wein-P7089798-PS.jpg | caption1 = Vitis vinifera Bullate | width1={{#expr: (150 * 800 /600) round 0}} | image2= Flickr - João de Deus Medeiros - Clidemia hirta.jpg | caption2= Clidemia hirta Channeled | width2={{#expr: (150 * 800 /600) round 0}} | image3=Cornus mas (3).jpg | caption3=Cornus mas Impressed | width3={{#expr: (150 * 800/435) round 0}} | image4=Berberis gagnepainii thorn.jpg | caption4= Berberis gagnepainii Obscure (under surface) | width4={{#expr: (150 * 495/393) round 0}} | image5= Spathiphyllum cannifolium kz2.jpg | caption5 = Spathiphyllum cannifolium Prominent | width5={{#expr: (150 * 415/600) round 0}} | image6= Viburnum plicatum var plicatum4.jpg | caption6= Viburnum plicatum Recessed | width6={{#expr: (150 * 445/599) round 0}} }} Describing other features:
Diagrams of venation patterns
Size{{main article|Leaf size}}The terms megaphyll, macrophyll, mesophyll, notophyll, microphyll, nanophyll and leptophyll are used to describe leaf sizes (in descending order), in a classification devised in 1934 by Christen C. Raunkiær and since modified by others.{{sfn|Whitten et al|1997}} See also{{div col|colwidth=30em}}
References1. ^{{Cite journal | doi=10.4028/www.scientific.net/AMR.1141.57|title = Insights from the Plant World: A Fractal Analysis Approach to Tune Mechanical Rigidity of Scaffolding Matrix in Thin Films| journal=Advanced Materials Research| volume=1141| pages=57–64|year = 2016|last1 = Bagchi|first1 = Debjani| last2=Dasgupta| first2=Avik| last3=Gondaliya| first3=Amit D.| last4=Rajput| first4=Kishore S.}} [5][6][7][8][9][10][11][12][13][14][15][16]2. ^{{Cite journal |jstor = 3220956|title = The Relation of Leaf Structure to Physical Factors|journal = Transactions of the American Microscopical Society|volume = 26|pages = 19–98|last1 = Clements|first1 = Edith Schwartz|year = 1905|doi = 10.2307/3220956}} 3. ^[https://web.archive.org/web/20131203015400/http://www.kew.org/Glossary/hysteranthous.htm?prefix=h Kew Glossary: Hysteranthous] 4. ^[https://web.archive.org/web/20131203015354/http://www.kew.org/Glossary/synanthous.htm?prefix=s Kew Glossary: Synanthous] 5. ^1 {{harvnb|Cullen et al|2011|loc=[https://books.google.com/books?id=zKOyo9qv2HsC&pg=PA398 Berberis gagnepainii vol. II p. 398]}} 6. ^1 2 3 {{harvnb|Hawthorne|Lawrence|2013|loc=[https://books.google.com/books?id=CNFuyOVTSf4C&pg=PA135 Leaf venation pp. 135–136]}} 7. ^1 {{harvnb|Hemsley|Poole|2004|loc=[https://books.google.com/books?id=7Eub0D4QWXIC&pg=PA254 Leaf morphology and drying p. 254]}} 8. ^1 2 3 4 5 {{harvnb|Kling et al|2005|loc=Leaf Venation}} 9. ^1 {{harvnb|Kwantlen|2015|loc=[https://plantdatabase.kpu.ca/plant/plantDetail/1640 Spathiphyllum cannifolium]}} 10. ^1 {{harvnb|Oxford herbaria glossary|2017|loc=Vein prominence}} 11. ^1 2 {{harvnb|Hughes|2017|loc=Pimenta racemosa}} 12. ^1 {{harvnb|Massey|Murphy|1996|loc=Surface-Venation-Texure}} 13. ^1 2 3 4 5 6 7 8 9 {{harvnb|Simpson|2011|loc=[https://books.google.com/books?id=dj8KRImgyf4C&pg=PA465 Leaf venation pp. 465–468]}} 14. ^1 {{harvnb|Simpson|2017|loc=Ceanothus leucodermis}} 15. ^1 {{harvnb|Simpson|2017|loc=Ceanothus tomentosus}} 16. ^1 {{harvnb|Angiosperm Morphology|2017|loc=Venation}} }} Bibliography{{refbegin|30em}}Books and chapters
Articles and theses
Websites
External links{{commons category multi|Leaves|Leaf veins}}{{wiktionary|leaf|position = }}{{Botany}}{{Authority control}} 5 : Leaves|Plant anatomy|Plant morphology|Plant physiology|Photosynthesis |
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