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87-432: A leaf is an organ of a vascular plant. Leaf or Leaves may also refer to: Leaf A leaf ( pl. : leaves ) is a principal appendage of the stem of a vascular plant , usually borne laterally above ground and specialized for photosynthesis . Leaves are collectively called foliage , as in "autumn foliage", while the leaves, stem, flower, and fruit collectively form the shoot system. In most leaves,

174-471: A are the partial pressures of water in the leaf and in the ambient air respectively, P is atmospheric pressure, and r is stomatal resistance. The inverse of r is conductance to water vapor ( g ), so the equation can be rearranged to and solved for g : Photosynthetic CO 2 assimilation ( A ) can be calculated from where C a and C i are the atmospheric and sub-stomatal partial pressures of CO 2 respectively . The rate of evaporation from

261-425: A phenotypic plasticity in response to [CO 2 ] atm that may have been an adaptive trait in the evolution of plant respiration and function. Predicting how stomata perform during adaptation is useful for understanding the productivity of plant systems for both natural and agricultural systems . Plant breeders and farmers are beginning to work together using evolutionary and participatory plant breeding to find

348-447: A proton pump drives protons (H ) from the guard cells. This means that the cells' electrical potential becomes increasingly negative. The negative potential opens potassium voltage-gated channels and so an uptake of potassium ions (K ) occurs. To maintain this internal negative voltage so that entry of potassium ions does not stop, negative ions balance the influx of potassium. In some cases, chloride ions enter, while in other plants

435-408: A decussate pattern, in which each node rotates by 1/4 (90°) as in the herb basil . The leaves of tricussate plants such as Nerium oleander form a triple helix. The leaves of some plants do not form helices. In some plants, the divergence angle changes as the plant grows. In orixate phyllotaxis, named after Orixa japonica , the divergence angle is not constant. Instead, it is periodic and follows

522-482: A great degree of variation in size and frequency about species and genotypes. White ash and white birch leaves had fewer stomata but larger in size. On the other hand sugar maple and silver maple had small stomata that were more numerous. Different classifications of stoma types exist. One that is widely used is based on the types that Julien Joseph Vesque introduced in 1889, was further developed by Metcalfe and Chalk, and later complemented by other authors. It

609-562: A high carbon dioxide affinity, phosphoenolpyruvate carboxylase (PEPcase). Retrieving the products of carbon fixation from PEPCase is an energy-intensive process, however. As a result, the PEPCase alternative is preferable only where water is limiting but light is plentiful, or where high temperatures increase the solubility of oxygen relative to that of carbon dioxide, magnifying RuBisCo's oxygenation problem. A group of mostly desert plants called "C.A.M." plants ( crassulacean acid metabolism , after

696-457: A leaf can be determined using a photosynthesis system . These scientific instruments measure the amount of water vapour leaving the leaf and the vapor pressure of the ambient air. Photosynthetic systems may calculate water use efficiency ( A / E ), g , intrinsic water use efficiency ( A / g ), and C i . These scientific instruments are commonly used by plant physiologists to measure CO 2 uptake and thus measure photosynthetic rate. There

783-504: A petiole like structure. Pseudopetioles occur in some monocotyledons including bananas , palms and bamboos . 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 (apetiolate), or the blade may not be laminar (flattened). The petiole mechanically links the leaf to

870-463: A plant matures; as a case in point Eucalyptus species commonly have isobilateral, pendent leaves when mature and dominating their neighbors; however, such trees tend to have erect or horizontal dorsiventral leaves as seedlings, when their growth is limited by the available light. 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

957-419: A regular organization at the cellular scale. Specialized cells that differ markedly from surrounding cells, and which often synthesize specialized products such as crystals, are termed idioblasts . The 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

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1044-475: A scaffolding matrix imparting mechanical rigidity to leaves. Leaves are normally extensively vascularized 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. The major tissue systems present are These three tissue systems typically form

1131-541: 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 heavily morphologically from

1218-411: A significant effect on stomatal closure of its leaves. There are different mechanisms of stomatal closure. Low humidity stresses guard cells causing turgor loss, termed hydropassive closure. Hydroactive closure is contrasted as the whole leaf affected by drought stress, believed to be most likely triggered by abscisic acid . It is expected that [CO 2 ] atm will reach 500–1000 ppm by 2100. 96% of

1305-402: A single (sometimes more) primary vein in the centre of the leaf, referred to as the midrib or costa, which is continuous with the vasculature of the petiole. The secondary veins, also known as second order veins or lateral veins, branch off from the midrib and extend toward the leaf margins. These often terminate in a hydathode , a secretory organ, at the margin. In turn, smaller veins branch from

1392-573: A single leaf grows from each node, and when the stem is held straight, the leaves form a helix . The divergence angle is often represented as a fraction of a full rotation around the stem. A rotation fraction of 1/2 (a divergence angle of 180°) produces an alternate arrangement, such as in Gasteria or the fan-aloe Kumara plicatilis . Rotation fractions of 1/3 (divergence angles of 120°) occur in beech and hazel . Oak and apricot rotate by 2/5, sunflowers, poplar, and pear by 3/8, and in willow and almond

1479-523: A single point. 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 variety of climatic conditions. Although it

1566-409: 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 (sometimes referred to as nerves) constitute one of the most visible features of leaves. The veins in a leaf represent

1653-405: 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

1740-550: A stoma. This meristemoid then divides asymmetrically one to three times before differentiating into a guard mother cell. The guard mother cell then makes one symmetrical division, which forms a pair of guard cells. Cell division is inhibited in some cells so there is always at least one cell between stomata. Stomatal patterning is controlled by the interaction of many signal transduction components such as EPF (Epidermal Patterning Factor), ERL (ERecta Like) and YODA (a putative MAP kinase kinase kinase ). Mutations in any one of

1827-483: A variety of patterns (venation) and form cylindrical bundles, usually lying in the median plane of the mesophyll , between the two layers of epidermis . 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. The vein or veins entering

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1914-454: Is also coordinated by the cellular peptide signal called stomagen, which signals the activation of the SPCH, resulting in increased number of stomata. Environmental and hormonal factors can affect stomatal development. Light increases stomatal development in plants; while, plants grown in the dark have a lower amount of stomata. Auxin represses stomatal development by affecting their development at

2001-513: Is based on the size, shape and arrangement of the subsidiary cells that surround the two guard cells. They distinguish for dicots : In monocots , several different types of stomata occur such as: In ferns , four different types are distinguished: Stomatal crypts are sunken areas of the leaf epidermis which form a chamber-like structure that contains one or more stomata and sometimes trichomes or accumulations of wax . Stomatal crypts can be an adaption to drought and dry climate conditions when

2088-405: Is because the light response of stomata to blue light is independent of other leaf components like chlorophyll . Guard cell protoplasts swell under blue light provided there is sufficient availability of potassium . Multiple studies have found support that increasing potassium concentrations may increase stomatal opening in the mornings, before the photosynthesis process starts, but that later in

2175-455: Is called a stipe in ferns . The lamina is the expanded, flat component of the leaf which contains the chloroplasts . The sheath is a structure, typically at the base that fully or partially clasps the stem above the node, where the leaf is attached. Leaf sheathes typically occur in Poaceae (grasses) and Apiaceae (umbellifers). Between the sheath and the lamina, there may be a pseudopetiole ,

2262-574: Is dependent on the diffusion resistance provided by the stomatal pores and also on the humidity gradient between the leaf's internal air spaces and the outside air. Stomatal resistance (or its inverse, stomatal conductance ) can therefore be calculated from the transpiration rate and humidity gradient. This allows scientists to investigate how stomata respond to changes in environmental conditions, such as light intensity and concentrations of gases such as water vapor, carbon dioxide, and ozone . Evaporation ( E ) can be calculated as where e i and e

2349-567: Is little evidence of the evolution of stomata in the fossil record, but they had appeared in land plants by the middle of the Silurian period. They may have evolved by the modification of conceptacles from plants' alga-like ancestors. However, the evolution of stomata must have happened at the same time as the waxy cuticle was evolving – these two traits together constituted a major advantage for early terrestrial plants. There are three major epidermal cell types which all ultimately derive from

2436-436: Is one way plants have responded to the increase in concentration of atmospheric CO 2 ([CO 2 ] atm ). Although changes in [CO 2 ] atm response is the least understood mechanistically, this stomatal response has begun to plateau where it is soon expected to impact transpiration and photosynthesis processes in plants. Drought inhibits stomatal opening, but research on soybeans suggests moderate drought does not have

2523-441: Is severely limited by the capacity to store fixed carbon in the vacuoles, so it is preferable only when water is severely limited. However, most plants do not have CAM and must therefore open and close their stomata during the daytime, in response to changing conditions, such as light intensity, humidity, and carbon dioxide concentration. When conditions are conducive to stomatal opening (e.g., high light intensity and high humidity),

2610-406: 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

2697-477: Is used in photosynthesis , passes through stomata by gaseous diffusion . Water vapour diffuses through the stomata into the atmosphere as part of a process called transpiration . Stomata are present in the sporophyte generation of the vast majority of land plants , with the exception of liverworts , as well as some mosses and hornworts . In vascular plants the number, size and distribution of stomata varies widely. Dicotyledons usually have more stomata on

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2784-494: 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

2871-424: The gymnosperms and angiosperms . Euphylls are also referred to as macrophylls or megaphylls (large leaves). A structurally complete leaf of an angiosperm consists of a petiole (leaf stalk), a lamina (leaf blade), stipules (small structures located to either side of the base of the petiole) and a sheath. Not every species produces leaves with all of these structural components. The proximal stalk or petiole

2958-405: The leaf are saturated with water vapour , which exits the leaf through the stomata in a process known as transpiration . Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapour. Ordinarily, carbon dioxide is fixed to ribulose 1,5-bisphosphate (RuBP) by the enzyme RuBisCO in mesophyll cells exposed directly to the air spaces inside the leaf. This exacerbates

3045-551: The phyllids of mosses and liverworts . Leaves are the most important organs of most vascular plants. 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 ,

3132-399: 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

3219-408: The abaxial (lower) epidermis than the adaxial (upper) epidermis and are more numerous in plants from cooler climates. Stomata The term is usually used collectively to refer to the entire stomatal complex, consisting of the paired guard cells and the pore itself, which is referred to as the stomatal aperture. Air, containing oxygen , which is used in respiration , and carbon dioxide , which

3306-679: The amount of light they absorb to avoid or mitigate excessive heat, ultraviolet damage, or desiccation, or to sacrifice light-absorption efficiency in favor of protection from herbivory. For xerophytes the major constraint is not light flux or intensity , but drought. Some window plants such as Fenestraria species and some Haworthia species such as Haworthia tesselata and Haworthia truncata are examples of xerophytes. and Bulbine mesembryanthemoides . Leaves also function to store chemical energy and water (especially in succulents ) and may become specialized organs serving other functions, such as tendrils of peas and other legumes,

3393-512: The atmosphere enhances photosynthesis, reduce transpiration, and increase water use efficiency (WUE). Increased biomass is one of the effects with simulations from experiments predicting a 5–20% increase in crop yields at 550 ppm of CO 2 . Rates of leaf photosynthesis were shown to increase by 30–50% in C3 plants, and 10–25% in C4 under doubled CO 2 levels. The existence of a feedback mechanism results

3480-583: The basic structural material in plant cell walls, or metabolized 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 maximize their exposure to sunlight. Once sugar has been synthesized, it needs to be transported to areas of active growth such as

3567-452: 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

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3654-406: 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

3741-512: The day sucrose plays a larger role in regulating stomatal opening. Zeaxanthin in guard cells acts as a blue light photoreceptor which mediates the stomatal opening. The effect of blue light on guard cells is reversed by green light, which isomerizes zeaxanthin. Stomatal density and aperture (length of stomata) varies under a number of environmental factors such as atmospheric CO 2 concentration, light intensity, air temperature and photoperiod (daytime duration). Decreasing stomatal density

3828-404: The diffusion of water back out of the cell by osmosis . This makes the cell plasmolysed , which results in the closing of the stomatal pores. Guard cells have more chloroplasts than the other epidermal cells from which guard cells are derived. Their function is controversial. The degree of stomatal resistance can be determined by measuring leaf gas exchange of a leaf. The transpiration rate

3915-426: The dominant allele , but in the ‘wild type’ recessive allele showed a large increase, both in response to rising CO 2 levels in the atmosphere. These studies imply the plants response to changing CO 2 levels is largely controlled by genetics. The CO 2 fertiliser effect has been greatly overestimated during Free-Air Carbon dioxide Enrichment (FACE) experiments where results show increased CO 2 levels in

4002-595: The early Devonian lycopsid Baragwanathia , first evolved as enations, extensions of the stem. 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

4089-553: 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 . Leaves which have a petiole (leaf stalk) are said to be petiolate . Sessile (epetiolate) leaves have no petiole and

4176-657: 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. 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

4263-569: The family Crassulaceae, which includes the species in which the CAM process was first discovered) open their stomata at night (when water evaporates more slowly from leaves for a given degree of stomatal opening), use PEPcase to fix carbon dioxide and store the products in large vacuoles. The following day, they close their stomata and release the carbon dioxide fixed the previous night into the presence of RuBisCO. This saturates RuBisCO with carbon dioxide, allowing minimal photorespiration. This approach, however,

4350-406: The fraction is 5/13. These arrangements are periodic. The denominator of the rotation fraction indicates the number of leaves in one period, while the numerator indicates the number of complete turns or gyres made in one period. For example: Most divergence angles are related to the sequence of Fibonacci numbers F n . This sequence begins 1, 1, 2, 3, 5, 8, 13; each term is the sum of

4437-645: The genes which encode these factors may alter the development of stomata in the epidermis. For example, a mutation in one gene causes more stomata that are clustered together, hence is called Too Many Mouths ( TMM ). Whereas, disruption of the SPCH (SPeecCHless) gene prevents stomatal development all together.  Inhibition of stomatal production can occur by the activation of EPF1, which activates TMM/ERL, which together activate YODA. YODA inhibits SPCH, causing SPCH activity to decrease, preventing asymmetrical cell division that initiates stomata formation. Stomatal development

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4524-866: The ground, they are referred to as prostrate . Perennial plants whose leaves are shed annually are said to have deciduous leaves, while leaves that remain through winter are evergreens . Leaves attached to stems by stalks (known as petioles ) are called petiolate, and if attached directly to the stem with no petiole they are called sessile. Dicot leaves have blades with pinnate venation (where major veins diverge from one large mid-vein and have smaller connecting networks between them). Less commonly, dicot leaf blades may have palmate venation (several large veins diverging from petiole to leaf edges). Finally, some exhibit parallel venation. Monocot leaves in temperate climates usually have narrow blades, and usually parallel venation converging at leaf tips or edges. Some also have pinnate venation. The arrangement of leaves on

4611-400: The guard cells, whose ends are held firmly in place by surrounding epidermal cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which gas can diffuse. When the roots begin to sense a water shortage in the soil, abscisic acid (ABA) is released. ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises

4698-443: 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. Each subsequent branching is sequentially numbered, and these are the higher order veins, each branching being associated with a narrower vein diameter. In parallel veined leaves,

4785-523: The leaf veins form, and these have functional implications. Of these, angiosperms have the greatest diversity. 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

4872-551: The leaves of vascular plants . In most cases, they lack vascular tissue, are only a single cell thick, and have no cuticle , stomata, or internal system of intercellular spaces. (The phyllids of the moss family Polytrichaceae are notable exceptions.) The phyllids of bryophytes are only present on the gametophytes , while in contrast the leaves of vascular plants are only present on the sporophytes . These can further develop into either vegetative or reproductive structures. Simple, vascularized leaves ( microphylls ), such as those of

4959-450: The lower surface are hypostomatous , and leaves with stomata only on the upper surface are epistomatous or hyperstomatous . Size varies across species, with end-to-end lengths ranging from 10 to 80 μm and width ranging from a few to 50 μm. Carbon dioxide , a key reactant in photosynthesis , is present in the atmosphere at a concentration of about 400 ppm. Most plants require the stomata to be open during daytime. The air spaces in

5046-493: The lower surface of the leaves than the upper surface. Monocotyledons such as onion , oat and maize may have about the same number of stomata on both leaf surfaces. In plants with floating leaves, stomata may be found only on the upper epidermis and submerged leaves may lack stomata entirely. Most tree species have stomata only on the lower leaf surface. Leaves with stomata on both the upper and lower leaf surfaces are called amphistomatous leaves; leaves with stomata only on

5133-552: The number of stomata (pores that intake and output gases), the amount and structure of epicuticular wax and other features. Leaves are mostly green in color due to the presence of a compound called chlorophyll which is essential for photosynthesis as it absorbs light energy from the sun . A leaf with lighter-colored or white patches or edges is called a variegated leaf . Leaves can have many different shapes, sizes, textures and colors. The broad, flat leaves with complex venation of flowering plants are known as megaphylls and

5220-431: The organic ion malate is produced in guard cells. This increase in solute concentration lowers the water potential inside the cell, which results in the diffusion of water into the cell through osmosis . This increases the cell's volume and turgor pressure . Then, because of rings of cellulose microfibrils that prevent the width of the guard cells from swelling, and thus only allow the extra turgor pressure to elongate

5307-425: The outermost (L1) tissue layer of the shoot apical meristem , called protodermal cells: trichomes , pavement cells and guard cells, all of which are arranged in a non-random fashion. An asymmetrical cell division occurs in protodermal cells resulting in one large cell that is fated to become a pavement cell and a smaller cell called a meristemoid that will eventually differentiate into the guard cells that surround

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5394-477: The pH of the cytosol of the cells and cause the concentration of free Ca to increase in the cytosol due to influx from outside the cell and release of Ca from internal stores such as the endoplasmic reticulum and vacuoles. This causes the chloride (Cl ) and organic ions to exit the cells. Second, this stops the uptake of any further K into the cells and, subsequently, the loss of K . The loss of these solutes causes an increase in water potential , which results in

5481-445: The past 400,000 years experienced below 280 ppm CO 2 . From this figure, it is highly probable that genotypes of today’s plants have diverged from their pre-industrial relatives. The gene HIC (high carbon dioxide) encodes a negative regulator for the development of stomata in plants. Research into the HIC gene using Arabidopsis thaliana found no increase of stomatal development in

5568-404: 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

5655-412: 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

5742-498: The presence of some, if not all, pathogens. However, pathogenic bacteria applied to Arabidopsis plant leaves can release the chemical coronatine , which induce the stomata to reopen. Photosynthesis , plant water transport ( xylem ) and gas exchange are regulated by stomatal function which is important in the functioning of plants. Stomata are responsive to light with blue light being almost 10 times as effective as red light in causing stomatal response. Research suggests this

5829-434: 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

5916-410: The previous two. Rotation fractions are often quotients F n / F n + 2 of a Fibonacci number by the number two terms later in the sequence. This is the case for the fractions 1/2, 1/3, 2/5, 3/8, and 5/13. The ratio between successive Fibonacci numbers tends to the golden ratio φ = (1 + √5)/2 . When a circle is divided into two arcs whose lengths are in the ratio 1:φ , the angle formed by

6003-416: The primary photosynthetic tissue is the palisade mesophyll and is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus , palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves are flattened and have distinct upper ( adaxial ) and lower ( abaxial ) surfaces that differ in color, hairiness,

6090-465: 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

6177-429: 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. In contrast, leaves with reticulate venation have

6264-416: 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. The number of vein endings is very variable, as is whether second order veins end at the margin, or link back to other veins. There are many elaborate variations on the patterns that

6351-429: The protective spines of cacti and the insect traps in carnivorous plants such as Nepenthes and Sarracenia . 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) and from which flowers are constructed in flowering plants . The internal organization of most kinds of leaves has evolved to maximize exposure of

6438-714: The rate exchange of carbon dioxide (CO 2 ), oxygen (O 2 ) and water vapor 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. 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

6525-498: The receptor level like the ERL and TMM receptors. However, a low concentration of auxin allows for equal division of a guard mother cell and increases the chance of producing guard cells. Most angiosperm trees have stomata only on their lower leaf surface. Poplars and willows have them on both surfaces. When leaves develop stomata on both leaf surfaces, the stomata on the lower surface tend to be larger and more numerous, but there can be

6612-531: 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 25 m (82 ft) long and 3 m (9.8 ft) wide. The terminology associated with the description of leaf morphology is presented, in illustrated form, at Wikibooks . Where leaves are basal, and lie on

6699-446: 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. These minor veins act as the sites of exchange between the mesophyll and the plant's vascular system. Thus, minor veins collect

6786-455: The sequence 180°, 90°, 180°, 270°. 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,

6873-454: The smaller arc is the golden angle , which is 1/φ × 360° ≈ 137.5° . Because of this, many divergence angles are approximately 137.5° . In plants where a pair of opposite leaves grows from each node, the leaves form a double helix. If the nodes do not rotate (a rotation fraction of zero and a divergence angle of 0°), the two helices become a pair of parallel lines, creating a distichous arrangement as in maple or olive trees. More common in

6960-953: The species that bear them, the majority, as broad-leaved or megaphyllous plants, which also include acrogymnosperms and ferns . In the lycopods , with different evolutionary origins, the leaves are simple (with only a single vein) and are known as microphylls . 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. Some structures of non-vascular plants look and function much like leaves. Examples include

7047-421: The stem is known as phyllotaxis . A large variety of phyllotactic patterns occur in nature: In the simplest mathematical models of phyllotaxis , the apex of the stem is represented as a circle. Each new node is formed at the apex, and it is rotated by a constant angle from the previous node. This angle is called the divergence angle . The number of leaves that grow from a node depends on the plant species. When

7134-547: 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

7221-417: The stomatal crypts are very pronounced. However, dry climates are not the only places where they can be found. The following plants are examples of species with stomatal crypts or antechambers: Nerium oleander , conifers, Hakea and Drimys winteri which is a species of plant found in the cloud forest . Stomata are holes in the leaf by which pathogens can enter unchallenged. However, stomata can sense

7308-511: 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 conifers have thin needle-like or scale-like leaves that can be advantageous in cold climates with frequent snow and frost. These are interpreted as reduced from megaphyllous leaves of their Devonian ancestors. Some leaf forms are adapted to modulate

7395-455: The transpiration problem for two reasons: first, RuBisCo has a relatively low affinity for carbon dioxide, and second, it fixes oxygen to RuBP, wasting energy and carbon in a process called photorespiration . For both of these reasons, RuBisCo needs high carbon dioxide concentrations, which means wide stomatal apertures and, as a consequence, high water loss. Narrower stomatal apertures can be used in conjunction with an intermediary molecule with

7482-632: The transportation system. 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 eucalypts . The flat, or laminar, shape also maximizes thermal contact with

7569-462: The vascular structure of the organ, extending into the leaf via the petiole and providing 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. Within the lamina of the leaf, while some vascular plants possess only a single vein, in most this vasculature generally divides (ramifies) according to

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