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A leaf (plural leaves) is the principal lateral appendage of the
vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''phutá'' 'plants'), form a large group of plants ( 300,000 ...
stem Stem or STEM may refer to: Biology * Plant stem, the aboveground structures that have vascular tissue and that support leaves and flowers ** Stipe (botany), a stalk that supports some other structure ** Stipe (mycology), the stem supporting the c ...

stem
, usually borne above ground and specialized for
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
. The leaves, stem, flower and fruit together form the
shoot In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancie ...

shoot
system. Leaves are collectively referred to as foliage, as in "autumn foliage". In most leaves, the primary
photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthetic
tissue, the
palisade mesophyll Palisade cells are plant cell ''The Plant Cell'' is a monthly peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( peers). It functions as a form of self-regulati ...
, is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of ''
Eucalyptus ''Eucalyptus'' () is a of over seven hundred species of s, shrubs or in the , Myrtaceae. Along with several other genera in the , including ', they are commonly known as s. Plants in the genus ''Eucalyptus'' have bark that is either smooth, ...

Eucalyptus
'', palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves are flattened and have distinct upper (') and lower (') surfaces that differ in color, hairiness, the number of
stomata File:LeafUndersideWithStomata.jpg, The underside of a leaf. In this species (''Tradescantia zebrina'') the guard cells of the stomata are green because they contain chlorophyll while the epidermal cells are chlorophyll-free and contain red pigme ...

stomata
(pores that intake and output gases), the amount and structure of
epicuticular wax Epicuticular wax is a coating of wax , a typical wax ester. Image:Beeswax foundation.jpg, Commercial honeycomb foundation, made by pressing beeswax between patterned metal rollers. Waxes are a diverse class of organic compounds that are lipophi ...
and other features. Leaves are mostly green in color due to the presence of a compound called
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf") ...

chlorophyll
that 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 '' 'Panascè', a bicolor (yellow-green) common fig cultivar. This Italian cultivar is a ''Chimera (plant), chimera''. Variegation is the appearance of differently coloured zones in the leaves and sometimes the Plant stem, stems and fruit of plant ...
. Leaves can have many different shapes, sizes, and textures. The broad, flat leaves with complex
venation Venation may refer to: * Venation (botany), the arrangement of veins in leaves * Wing venation, the arrangement of veins in insect wings See also

* {{Disambiguation Biology disambiguation pages ...
of
flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greec ...

flowering plant
s are known as ''megaphylls'' and the species that bear them, the majority, as broad-leaved or megaphyllous plants, which also includes acrogymnosperms and
ferns A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia art ...

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 In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the An ...

bulb
scales, are not above ground. In many aquatic species, the leaves are submerged in water.
Succulent ''. In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the ...
plants often have thick juicy leaves, but some leaves are without major photosynthetic function and may be dead at maturity, as in some
cataphyll In plant morphology Phytomorphology is the study of the physical form and external structure of plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, ...
s 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
phylloclade Phylloclades and cladodes are flattened, photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be releas ...
s 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 the phyllids of
mosses Mosses are small, non-vascular flower A flower, sometimes known as a bloom or blossom Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other plant ...

mosses
and
liverworts The Marchantiophyta () are a division of non-vascular plant, non-vascular embryophyte, land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plan ...

liverworts
.


General characteristics

Leaves are the most important organs of most vascular plants. Green plants are
autotrophic An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bo ...
, meaning that they do not obtain food from other living things but instead create their own food by
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
. They capture the energy in
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
and use it to make simple
sugars Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides o ...
, such as
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
and
sucrose Sucrose is a type of sugar Sugar is the generic name for , soluble s, many of which are used in food. Simple sugars, also called s, include , , and . Compound sugars, also called s or double sugars, are molecules made of two monosacchari ...

sucrose
, from
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
and water. The sugars are then stored as
starch Starch or amylum is a polymeric A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance ...
, further processed by
chemical synthesis As a topic of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they unde ...
into more complex organic molecules such as
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s or
cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important stru ...

cellulose
, the basic structural material in plant cell walls, or
metabolized Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling and self-sustaining processes, ...

metabolized
by
cellular respiration upright=2.5, Typical eukaryotic cell Cellular respiration is a set of metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities ...

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 Xylem is one of the two types of transport tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Ame ...

xylem
and obtain carbon dioxide from the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
by diffusion through openings called
stomata File:LeafUndersideWithStomata.jpg, The underside of a leaf. In this species (''Tradescantia zebrina'') the guard cells of the stomata are green because they contain chlorophyll while the epidermal cells are chlorophyll-free and contain red pigme ...

stomata
in the outer covering layer of the leaf (
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
), 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 the
plant shoots In botany Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes know ...
and
root In vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''phutá'' 'plants'), form a large grou ...

root
s. Vascular plants transport sucrose in a special tissue called the
phloem Phloem (, ) is the living tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa d ...

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 Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
carrying out
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
are close to 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
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s, 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 Pendent is an adjective that describes the condition of hanging, either literally, or figuratively, as in undecided or incomplete. The word is to be distinguished from the spelling "pendant", which is the noun A noun () is a word In ling ...
leaves, such as in many
willow Willows, also called sallows and osiers, from the genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classifica ...

willow
s and
eucalypt Eucalypt is a descriptive name for woody plants with capsule fruiting bodies belonging to seven closely related genera (of the tribe Eucalypteae) found across Australasia: ''Eucalyptus'', ''Corymbia'', ''Angophora'', ''Stockwellia'', ''Allosyncarp ...

eucalypt
s. The flat, or laminar, shape also maximizes
thermal Example of a thermal column between the ground and a cumulus A thermal column (or thermal) is a column of rising air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represe ...

thermal
contact with the surrounding
air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentration ). Number ...

air
, promoting cooling. Functionally, in addition to carrying out photosynthesis, the leaf is the principal site of
transpiration in a tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as a tomato plant. The species originated in western South America South America is a continent entirely in the Western Hemisphere ...

transpiration
, providing the energy required to draw the transpiration stream up from the roots, and
guttation'' Guttation is the exudation of drops of xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water Water is an Inorganic compound, inorgani ...
. Many gymnosperms 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 The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the H ...
ancestors. Some leaf forms are adapted to modulate the amount of light they absorb to avoid or mitigate excessive heat,
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

ultraviolet
damage, or desiccation, or to sacrifice light-absorption efficiency in favor of protection from herbivory. For
xerophyteA xerophyte (from Greek ξηρός ''xeros'' dry, φυτόν ''phuton'' plant) is a species of plant Plants are mainly multicellular organisms, predominantly photosynthetic Photosynthesis is a process used by plants and other organisms ...

xerophyte
s the major constraint is not light
flux of \mathbf(\mathbf) with the unit normal vector \mathbf(\mathbf) ''(blue arrows)'' at the point \mathbf multiplied by the area dS. The sum of \mathbf\cdot\mathbf dS for each patch on the surface is the flux through the surface Flux describes ...
or
intensity
intensity
, but drought. Some window plants such as ''
Fenestraria ''Fenestraria'' (known as babies' toes or window plant) is a (possibly monotypic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical pro ...
'' species and some ''
Haworthia ''Haworthia'' is a large genus of small succulent plants endemic to Southern Africa Southern Africa is the southernmost region of the African continent A continent is one of several large landmasses. Generally identified by conve ...
'' species such as ''Haworthia tesselata'' and '' Haworthia truncata'' are examples of xerophytes. and ''
Bulbine ''Bulbine'' is a genus of plant Plants are mainly multicellular organisms, predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy ...
mesembryanthemoides''. Leaves also function to store chemical
energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...

energy
and water (especially in
succulents ''. In botany Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes ...

succulents
) and may become specialized organs serving other functions, such as tendrils of
pea The pea is most commonly the small spherical seed A seed is an embryonic plant enclosed in a protective outer covering. The formation of the seed is part of the process of reproduction Reproduction (or procreation or breeding) is ...

pea
s and other legumes, the protective spines of cacti and the insect traps in carnivorous plants such as ''
Nepenthes ''Nepenthes'' () is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viru ...

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 plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greec ...

flowering plant
s. The internal organization of most kinds of leaves has evolved to maximize exposure of the photosynthetic
organelles In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
, the
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s, to
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
and to increase the absorption of
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
while at the same time controlling water loss. Their surfaces are waterproofed by the
plant cuticle Image:Kale2.jpg, 250px, Water beads on the waxy cuticle of kale leaves A plant cuticle is a protecting film covering the Epidermis (botany), epidermis of leaf, leaves, young shoots and other aerial plant organs without periderm. It consists of lip ...
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 File:LeafUndersideWithStomata.jpg, The underside of a leaf. In this species (''Tradescantia zebrina'') the guard cells of the stomata are green because they contain chlorophyll while the epidermal cells are chlorophyll-free and contain red pigme ...

stomata
which open or close to regulate the rate exchange of carbon dioxide,
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
, and
water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor pressure 280px, The ''pistol test tube'' experiment. The tube contains alcohol and is closed with a piece of cork. By heating th ...
into and out of the internal intercellular space system. Stomatal opening is controlled by the
turgor pressure Turgor pressure is the force within the cell that pushes the plasma membrane cell membrane vs. Prokaryotes A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In b ...
in a pair of
guard cell Guard cells are specialized plant cell ''The Plant Cell'' is a monthly peer-reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( peers). It functions as a form of self ...
s 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 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 the primary organs responsible for
transpiration in a tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as a tomato plant. The species originated in western South America South America is a continent entirely in the Western Hemisphere ...

transpiration
and
guttation'' Guttation is the exudation of drops of xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water Water is an Inorganic compound, inorgani ...
(beads of fluid forming at leaf margins). Leaves can also store
food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fungus, fungal origin, and contains essential nutrients, such as carbohydrates, fats, protein (nutrient), proteins, vi ...

food
and
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
, and are modified accordingly to meet these functions, for example in the leaves of succulent plants and in
bulb In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the An ...

bulb
scales. The concentration of photosynthetic structures in leaves requires that they be richer in
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
,
minerals In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proces ...

minerals
, and
sugars Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides o ...
than, say, woody stem tissues. Accordingly, leaves are prominent in the
diet Diet may refer to: Food * Diet (nutrition) In nutrition, diet is the sum of food consumed by a person or other organism. The word diet often implies the use of specific intake of nutrition for #Health, health or #Weight management, weight-mana ...
of many
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s. 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
phytolith Phytoliths (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxim ...
s,
lignin Lignin is a class of complex organic polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its s ...

lignin
s,
tannin Tannins (or tannoids) are a class of , ic s that bind to and s and various other organic compounds including s and s. The term ''tannin'' (from ''tanner'', from ''tannāre'', from ''tannum'', ) refers to the use of oak and other bark ...
s and
poison In biology, poisons are Chemical substance, substances that can cause death, injury or harm to organs, Tissue (biology), tissues, Cell (biology), cells, and DNA usually by chemical reactions or other activity (chemistry), activity on the molecul ...

poison
s.
Deciduous In the fields of horticulture Horticulture is the art of cultivating plants in gardens to produce food and medicinal ingredients, or for comfort and ornamental purposes. Horticulturists are agriculturists who grow flowers, fruits and nuts, ...

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 ''Welwitschia'' is a monotypic taxon, monotypic gymnosperm genus, comprising solely the distinctive ''Welwitschia mirabilis'', Endemism, endemic to the Namib desert within Namibia and Angola. The plant is commonly known simply as welwitschia in En ...

Welwitschia
'' retains its two main leaves throughout a lifetime that may exceed a thousand years. The leaf-like organs of
bryophyte Bryophytes are an informal group consisting of three divisions Division or divider may refer to: Mathematics *Division (mathematics) Division is one of the four basic operations of arithmetic, the ways that numbers are combined to make new n ...

bryophyte
s (e.g.,
moss Mosses are small, non-vascular flower A flower, sometimes known as a bloom or blossom Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other plant ...

moss
es and
liverworts The Marchantiophyta () are a division of non-vascular plant, non-vascular embryophyte, land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plan ...
), known as phyllids, differ morphologically from the leaves of
vascular plants Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from the Greek ''trācheia''), form a large group of plants ( 300,000 accepted known species) that are defined as land plants with lignified tissue ...
in that they lack vascular tissue, are usually only a single cell thick, and have no
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homology (biology), homologous, differing in the ...
stomata or internal system of intercellular spaces. The leaves of bryophytes are only present on the
gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all ...
s, while in contrast the leaves of vascular plants are only present on the sporophytes, and are associated with
buds In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the A ...

buds
(immature shoot systems in the leaf
axils A leaf (plural leaves) is the principal lateral appendage of the vascular plant stem, usually borne above ground and specialized for photosynthesis Photosynthesis is a process used by plants and other organisms to convert light en ...
). These can further develop into either vegetative or reproductive structures. Simple, vascularized leaves (
microphylls In plant anatomy and evolution a microphyll (or lycophyll) is a type of plant leaf with one single, unbranched leaf vein. Plants with microphyll leaves occur early in the fossil record, and few such plants exist today. In the classical concept of ...
), such as those of the early
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic The Paleozoic (or Palaeozoic) Era ( ; from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the H ...
lycopsid ''
Baragwanathia ''Baragwanathia'' is a genus of extinct Lycopodiopsida, lycopsid plants of Late Silurian to Early Devonian age (), fossils of which have been found in Australia, Canada, China and Czechia. The name derives from William Baragwanath who discovered t ...
'', 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 the gymnosperms and angiosperms. Euphylls are also referred to as Microphyll, macrophylls or megaphylls (large leaves).


A structurally complete leaf of an angiosperm consists of a petiole (botany), 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 is called a stipe (botany), stipe in
ferns A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia art ...

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 Stem or STEM may refer to: Biology * Plant stem, the aboveground structures that have vascular tissue and that support leaves and flowers ** Stipe (botany), a stalk that supports some other structure ** Stipe (mycology), the stem supporting the c ...

stem
above the node, where the latter is attached. Leaf sheathes typically occur in grasses and Apiaceae (umbellifers). Between the sheath and the lamina, there may be a pseudopetiole, a petiole like structure. Pseudopetioles occur in some monocotyledons including bananas, Arecaceae, 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 tremendous variety shown in leaf structure (anatomy) from species to species is presented in detail below under #Morphology, 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 (Glossary of botanical terms#A, 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 Polymorphism (biology), polymorphic). The longest leaves are those of the Raffia palm, ''R. regalis'' which may be up to long and wide. The terminology associated with the description of leaf morphology is presented, in illustrated form, at :wikibooks:Botany/Leaves (forms), Wikibooks. Where leaves are basal, and lie on the ground, they are referred to as Glossary of plant morphology#prostrate, prostrate.


Basic leaf types

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 Petiole (botany), petioles) are called petiolate, and if attached directly to the stem with no petiole they are called sessile. * Ferns have fronds. * Pinophyta, Conifer leaves are typically needle- or awl-shaped or scale-like, they are usually evergreen, but can sometimes be deciduous. Usually, they have a single vein. * Flowering plant (Angiosperm) leaves: the standard form includes stipules, a petiole, and a glossary of botanical terms#lamina, lamina. * Lycophytes have microphylls and megaphylls, microphylls. * Monocotyledon#Leaves, Sheath leaves are the type found in most Poaceae, grasses and many other monocots. * Other specialized leaves include those of ''
Nepenthes ''Nepenthes'' () is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viru ...

Nepenthes
'', a pitcher plant. Dicot leaves have blades with pinnate vegetation (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 (botany), 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.


Arrangement on the stem

The arrangement of leaves on the stem is known as phyllotaxis. A large variety of phyllotactic patterns occur in nature: ;Alternate: One leaf, branch, or flower part attaches at each point or node on the stem, and leaves alternate direction, to a greater or lesser degree, along the stem. ;Basal: Arising from the base of the stem. ;Cauline: Arising from the aerial stem. ;Opposite: Two leaves, branches, or flower parts attach at each point or node on the stem. Leaf attachments are paired at each node. ;Decussate: An opposite arrangement in which each successive pair is rotated 90° from the previous. ;Whorled, or verticillate: Three or more leaves, branches, or flower parts 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.). Opposite leaves may appear whorled near the tip of the stem. Pseudoverticillate describes an arrangement only appearing whorled, but not actually so. ;Rosulate: Leaves form a rosette (botany), rosette. ;Rows: The term, ''distichous'', literally means ''two rows''. Leaves in this arrangement may be alternate or opposite in their attachment. The term, ''2-ranked'', is equivalent. The terms, ''tristichous'' and ''tetrastichous'', are sometimes encountered. For example, the "leaves" (actually microphylls) of most species of ''Selaginella'' are tetrastichous, but not decussate. 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 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 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: * 180° (or ): two leaves in one circle (alternate leaves) * 120° (or ): three leaves in one circle * 144° (or ): five leaves in two gyres * 135° (or ): eight leaves in three gyres. Most divergence angles are related to the sequence of Fibonacci numbers . This sequence begins 1, 1, 2, 3, 5, 8, 13; each term is the sum of the previous two. Rotation fractions are often quotients 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 . When a circle is divided into two arcs whose lengths are in the ratio , the angle formed by the smaller arc is the golden angle, which is . Because of this, many divergence angles are approximately . 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 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 the sequence 180°, 90°, 180°, 270°.


Divisions of the 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 (botany), 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. ;Palmately compound: Leaves have the leaflets radiating from the end of the petiole, like fingers of the palm of a hand; for example, ''Cannabis'' (hemp) and ''Aesculus'' (buckeyes). ;Pinnately compound: Leaves have the leaflets arranged along the main or mid-vein. ;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 group of pinnules on each secondary vein forms a ''pinna''; for example, ''Albizia'' (silk tree). ;Trifoliate (or trifoliolate): A pinnate leaf with just three leaflets; for example, ''clover, Trifolium'' (clover), ''Laburnum'' (laburnum), and some species of ''Toxicodendron'' (for instance, Toxicodendron radicans, poison ivy). ;Pinnatifid: Pinnately dissected to the central vein, but with the leaflets not entirely separate; for example, ''Polypodium'', some ''Sorbus'' (whitebeams). In pinnately veined leaves the central vein is known as the ''midrib''.


Characteristics of the petiole

Leaves which have a petiole (botany), petiole (leaf stalk) are said to be ''petiolate''. Sessility (botany), 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". ;Free, lateral: As in ''Hibiscus''. ;Adnate: Fused to the petiole base, as in ''Rose, Rosa''. ;Ochreate: Provided with ochrea, or sheath-formed stipules, as in Polygonaceae; e.g., rhubarb. ;Encircling the petiole base:


Veins

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 (botany), 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 a variety of patterns (venation) and form cylindrical bundles, usually lying in the median plane of the #Mesophyll, mesophyll, between the two layers of #Epidermis, epidermis. This pattern is often specific to taxa, and of which angiosperms possess two main types, Parallel (geometry), parallel and #Venation, 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 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, 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 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 wikt:areola, 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 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 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 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 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.


Morphology changes within a single plant

;Homoblasty: Characteristic in which a plant has small changes in leaf size, shape, and growth habit between juvenile and adult stages, in contrast to; ;Heteroblasty: Characteristic in which a plant has marked changes in leaf size, shape, and growth habit between juvenile and adult stages.


Anatomy


Medium-scale features

Leaves are normally extensively vascularized and typically have networks of vascular bundles containing
xylem Xylem is one of the two types of transport tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Ame ...

xylem
, which supplies water for
photosynthesis Photosynthesis is a process used by plants and other organisms to into that, through , can later be released to fuel the organism's activities. Some of this chemical energy is stored in molecules, such as s and es, which are synthesized fro ...

photosynthesis
, and
phloem Phloem (, ) is the living tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa d ...

phloem
, which transports the sugars produced by photosynthesis. Many leaves are covered in trichomes (small hairs) which have diverse structures and functions.


Small-scale features

The major tissue systems present are * The epidermis (botany), epidermis, which covers the upper and lower surfaces * The leaf#Mesophyll, mesophyll tissue inside the leaf, which is rich in chloroplasts (also called chlorenchyma) * The arrangement of veins (the vascular tissue) These three tissue systems typically form 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.


Major leaf tissues

File:Bifacial leaf cross section.jpg, Cross-section of a leaf File:Leaf epidermis 2.jpg, Epidermal cells File:Leaf spongy mesophyll.jpg, Spongy mesophyll cells


Epidermis

The epidermis (botany), epidermis is the outer layer of
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
covering the leaf. It is covered with a waxy
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homology (biology), homologous, differing in the ...
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. The epidermis serves several functions: protection against water loss by way of
transpiration in a tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as a tomato plant. The species originated in western South America South America is a continent entirely in the Western Hemisphere ...

transpiration
, regulation of gas exchange and secretion of secondary metabolite, 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 File:LeafUndersideWithStomata.jpg, The underside of a leaf. In this species (''Tradescantia zebrina'') the guard cells of the stomata are green because they contain chlorophyll while the epidermal cells are chlorophyll-free and contain red pigme ...

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

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 (biology), 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 vertically elongated cells, one to two cells thick, directly beneath the adaxial epidermis, with intercellular air spaces between them. Its cells contain many more chloroplasts than the spongy layer. 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 (chemistry), absorption of carbon dioxide. 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 branched and not so tightly packed, so that there are large intercellular air spaces between them. The pores or ''stomata'' of the epidermis open into substomatal chambers, which are connected to the intercellular air spaces between the spongy and palisade mesophyll cell, so that oxygen, carbon dioxide and water vapor can diffuse into and out of the leaf and access the mesophyll cells during respiration, photosynthesis and transpiration. Leaves are normally green, due to
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf") ...

chlorophyll
in
chloroplast A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplast
s in the mesophyll cells. Plants that lack chlorophyll cannot photosynthesis, photosynthesize.


Vascular tissue

The 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 (arrangement of the veins), venation. In angiosperms the venation is typically parallel in monocotyledons and forms an interconnecting network in dicotyledon, broad-leaved plants. They were once thought to be typical examples of pattern formation through ramification (botany), ramification, but they may instead exemplify a pattern formed in a stress tensor field. A vein is made up of a vascular bundle. At the core of each bundle are clusters of two distinct types of conducting cells: ; Xylem: Cells that bring water and minerals from the roots into the leaf. ; Phloem: Cells that usually move sap, with dissolved
sucrose Sucrose is a type of sugar Sugar is the generic name for , soluble s, many of which are used in food. Simple sugars, also called s, include , , and . Compound sugars, also called s or double sugars, are molecules made of two monosacchari ...

sucrose
(
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
to sucrose) produced by photosynthesis in the leaf, out of the leaf. 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 development

According to Agnes Arber's partial-shoot theory of the leaf, leaves are partial shoots, being derived from leaf primordia of the shoot apex. Early in development they are dorsiventrally flattened with both dorsal and ventral surfaces. Compound leaves are closer to shoots than simple leaves. Developmental studies have shown that compound leaves, like shoots, may branch in three dimensions. 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."


Ecology


Biomechanics

Plants 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 optimize their exposure to the sun, generally more or less horizontally. However, horizontal alignment maximizes 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. Both leaf blade and petiole structure influence the leaf's response to forces such as wind, allowing a degree of repositioning to minimize drag (physics), 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 reducing 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 (mechanics), torsion, enabling elastic deforming without damage. 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). 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 favors efficiency in photosynthesis and water conservation, involving further trade offs. On the other hand, I-beam leaves such as ''Banksia marginata'' involve specialized 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. Long narrow leaves bend more easily than ovate leaf blades of the same area. Monocots typically have such linear leaves that maximize surface area while minimising self-shading. In these a high proportion of longitudinal main veins provide additional support.


Interactions with other organisms

Although 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
tannin Tannins (or tannoids) are a class of , ic s that bind to and s and various other organic compounds including s and s. The term ''tannin'' (from ''tanner'', from ''tannāre'', from ''tannum'', ) refers to the use of oak and other bark ...
s, 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 crypsis, cryptic adaptations by which they use leaves in avoiding predators. For example, the caterpillars of Tortricidae, some leaf-roller moths will create a small home in the leaf by folding it over themselves. Some Pamphiliidae, 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 mimicry, 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 loss

Leaves in temperate, boreal ecosystem, 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 autumn leaf color, change color, and turn yellow, bright-orange (colour), orange, or red, as various accessory pigments (carotenoids and xanthophylls) are revealed when the tree responds to cold and reduced
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

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. 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.


Evolutionary adaptation

In the course of evolution, leaves have adapted to different environment (biophysical), environments in the following ways: * Epicuticular wax, Waxy micro- and nanostructures on the surface reduce wetting by rain and adhesion of contamination (''See Lotus effect''). * Divided and compound leaves reduce wind resistance and promote cooling. * Hairs on the leaf surface trap humidity in dry climates and create a boundary layer reducing water loss. * Waxy plant cuticles reduce water loss. * Large surface area provides a large area for capture of sunlight. * In harmful levels of sunlight, specialized leaves, opaque or partly buried, admit light through a translucent leaf window for photosynthesis at inner leaf surfaces (e.g. ''
Fenestraria ''Fenestraria'' (known as babies' toes or window plant) is a (possibly monotypic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical pro ...
''). * Kranz leaf anatomy in plants who perform C4 carbon fixation * Succulent leaves store water and organic acids for use in CAM photosynthesis. * Aromatic oils, poisons or pheromones produced by leaf borne glands deter herbivores (e.g. eucalypts). * Inclusions of crystalline minerals deter herbivores (e.g. silica
phytolith Phytoliths (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxim ...
s in grasses, raphides in Araceae). * Petals attract pollinators. * Spine (botany), Spines protect the plants from herbivores (e.g. cacti). * Stinging plant, Stinging hairs to protect against herbivory, e.g. in ''Urtica dioica'' and ''Dendrocnide moroides'' (Urticaceae). * Special leaves on carnivorous plants are adapted for trapping food, mainly invertebrate prey, though some species trap small vertebrates as well (see carnivorous plants). * Bulbs store food and water (e.g. onions). * Tendrils allow the plant to climb (e.g. peas). * Bracts and pseudanthium, pseudanthia (false flowers) replace normal flower structures when the true flowers are greatly reduced (e.g. spurges and spathes in the Araceae.


Terminology


Shape


Edge (margin)


Apex (tip)


Base

;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 notch towards the stalk. ;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

The leaf surface is also host to a large variety of microorganisms; in this context it is referred to as the phyllosphere.


Hairiness

"Hairs" on plants are properly called trichomes. Leaves can show several degrees of hairiness. The meaning of several of the following terms can overlap. ;Arachnoid, or 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. ;Glabrescent: Losing hairs with age. ;Glabrous: No hairs of any kind present. ;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, or lanose: With woolly hairs. ;Pilose: With soft, clearly separated hairs. ;Puberulent, or puberulous: With fine, minute hairs. ;Pubescent: With soft, short and erect hairs. ;Scabrous, or 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, or stelliform: With star-shaped hairs. ;Strigose: With appressed, sharp, straight and stiff hairs. ;Tomentose: Densely pubescent with matted, soft white woolly hairs. ;Tomentulose: Minutely or only slightly tomentose. ;Villous: With long and soft hairs, usually curved. ;Woolly: With long, soft and tortuous or matted hairs.


Timing

;Hysteranthous: Developing after the flowers ;Synanthous: Developing at the same time as the flowers Kew Glossary: Synanthous
/ref>


Venation


Classification

A number of different classification systems of the patterns of leaf veins (venation or veination) have been described, starting with Ettingshausen (1861), together with many different descriptive terms, and the terminology has been described as "formidable". 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): (''see also'': Simpson Figure 9.12, p. 468)


= Hickey system

= ;1. Pinnate (feather-veined, reticulate, pinnate-netted, penniribbed, penninerved, or penniveined): The veins arise pinnately (feather like) from a single primary vein (mid-vein) and subdivide into secondary veinlets, known as higher order veins. These, in turn, form a complicated network. This type of venation is typical for (but by no means limited to) "dicotyledons" (non monocotyledon angiosperms). E.g., ''Ostrya''. There are three subtypes of pinnate venation: These in turn have a number of further subtypes such as eucamptodromous, where secondary veins curve near the margin without joining adjacent secondary veins. ;2. Parallelodromous (parallel-veined, parallel-ribbed, parallel-nerved, penniparallel, striate): Two or more primary veins originating beside each other at the leaf base, and running Parallel (geometry), parallel to each other to the apex and then converging there. Commissural veins (small veins) connect the major parallel veins. Typical for most monocotyledons, such as Poaceae, grasses. The additional terms marginal (primary veins reach the margin), and reticulate (primary veins do not reach the margin) are also used. ;3. Campylodromous (' - curve): Several primary veins or branches originating at or close to a single point and running in recurved arches, then converging at apex. E.g. ''Maianthemum'' . ;4. Acrodromous: Two or more primary or well developed secondary veins in convergent arches towards apex, without basal recurvature as in Campylodromous. May be basal or suprabasal depending on origin, and perfect or imperfect depending on whether they reach to 2/3 of the way to the apex. E.g., ''Miconia'' (basal type), ''Endlicheria'' (suprabasal type). ;5. Actinodromous: Three or more primary veins diverging radially from a single point. E.g., ''Menispermaceae, Arcangelisia'' (basal type), ''Givotia'' (suprabasal type). ;6. Palinactodromous: Primary veins with one or more points of secondary dichotomous branching beyond the primary divergence, either closely or more distantly spaced. E.g., ''Platanus''. 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. Melville also had six divisions, based on the order in which veins develop. ; Arbuscular (arbuscularis): Branching repeatedly by regular dichotomy to give rise to a three dimensional bush-like structure consisting of linear segment (2 subclasses) ; Flabellate (flabellatus): Primary veins straight or only slightly curved, diverging from the base in a fan-like manner (4 subclasses) ; Palmate (palmatus): Curved primary veins (3 subclasses) ; Pinnate (pinnatus): Single primary vein, the midrib, along which straight or arching secondary veins are arranged at more or less regular intervals (6 subclasses) ; Collimate (collimatus): Numerous longitudinally parallel primary veins arising from a transverse meristem (5 subclasses) ; Conglutinate (conglutinatus): Derived from fused pinnate leaflets (3 subclasses) 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; ; Brochidodromous: Closed form in which the secondaries are joined in a series of prominent arches, as in ''Hildegardia (plant), Hildegardia''. ; Craspedodromous: Open form with secondaries terminating at the margin, in toothed leaves, as in ''Celtis''. ; Eucamptodromous: Intermediate form with upturned secondaries that gradually diminish apically but inside the margin, and connected by intermediate tertiary veins rather than loops between secondaries, as in ''Cornus''. ; Cladodromous: Secondaries freely branching toward the margin, as in ''Rhus''. terms which had been used as subtypes in the original Hickey system. Further descriptions included the higher order, or minor veins and the patterns of areoles (''see'' Leaf Architecture Working Group, Figures 28–29). ;Flabellate: Several to many equal fine basal veins diverging radially at low angles and branching apically. E.g. ''Paranomus''. 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. At its simplest the primary vein types can be considered in three or four groups depending on the plant divisions being considered; * pinnate * palmate * parallel 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). ;Palmate, Palmate-netted, palmate-veined, fan-veined: Several main veins of approximately equal size divergence, diverge from a common point near the leaf base where the petiole attaches, and radiate toward the edge of the leaf. Palmately veined leaves are often lobed or divided with lobes radiating from the common point. They may vary in the number of primary veins (3 or more), but always radiate from a common point. e.g. most maple, ''Acer'' (maples).


= Other systems

= Alternatively, Simpson uses: ; Uninervous: Central midrib with no lateral veins (microphyllous), seen in the non-seed bearing tracheophytes, such as horsetails ; Dichotomous: Veins successively branching into equally sized veins from a common point, forming a Y junction, fanning out. Amongst temperate woody plants, ''Ginkgo biloba'' is the only species exhibiting dichotomous venation. Also some fern, pteridophytes (ferns). ; Parallel: Primary and secondary veins roughly parallel to each other, running the length of the leaf, often connected by short perpendicular links, rather than form networks. In some species, the parallel veins join at the base and apex, such as needle-type evergreens and grasses. Characteristic of monocotyledons, but exceptions include ''Arisaema'', and as below, under netted. ; Netted (reticulate, pinnate): A prominent midvein with secondary veins branching off along both sides of it. The name derives from the ultimate veinlets which form an interconnecting net like pattern or network. (The primary and secondary venation may be referred to as pinnate, while the net like finer veins are referred to as netted or reticulate); most non-monocot angiosperms, exceptions including ''Calophyllum''. Some monocots have reticulate venation, including ''Colocasia'', ''Dioscorea'' and ''Smilax''. However, these simplified systems allow for further division into multiple subtypes. Simpson, (and others) divides parallel and netted (and some use only these two terms for Angiosperms) on the basis of the number of primary veins (costa) as follows; ; Parallel: ; Netted (Reticulate): These complex systems are not used much in morphological descriptions of taxa, but have usefulness in plant identification, although criticized as being unduly burdened with jargon. An older, even simpler system, used in some flora uses only two categories, open and closed. * Open: Higher order veins have free endings among the cells and are more characteristic of non-monocotyledon angiosperms. They are more likely to be associated with leaf shapes that are toothed, lobed or compound. They may be subdivided as; ** Pinnate (feather-veined) leaves, with a main central vein or rib (midrib), from which the remainder of the vein system arises ** Palmate, in which three or more main ribs rise together at the base of the leaf, and diverge upward. ** Dichotomous, as in ferns, where the veins fork repeatedly * Closed: Higher order veins are connected in loops without ending freely among the cells. These tend to be in leaves with smooth outlines, and are characteristic of monocotyledons. ** They may be subdivided into whether the veins run parallel, as in grasses, or have other patterns.


Other descriptive terms

There are also many other descriptive terms, often with very specialized usage and confined to specific taxonomic groups. 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. Terms that describe vein prominence include bullate, channelled, flat, guttered, impressed, prominent and recessed (''Fig''. 6.1 Hawthorne & Lawrence 2013). Veins may show different types of prominence in different areas of the leaf. For instance ''Pimenta racemosa'' has a channelled midrib on the upper surface, but this is prominent on the lower surface. Describing vein prominence: ;Bullate: Surface of leaf raised in a series of domes between the veins on the upper surface, and therefore also with marked depressions. e.g. ''Rytigynia, Rytigynia pauciflora'', ''Vitis vinifera'' ;Channelled (canalicululate): Veins sunken below the surface, resulting in a rounded channel. Sometimes confused with "guttered" because the channels may function as gutters for rain to run off and allow drying, as in many Melastomataceae. e.g. (''see'') ''Pimenta racemosa'' (Myrtaceae), ''Clidemia hirta'' (Melastomataceae). ;Guttered: Veins partly prominent, the crest above the leaf lamina surface, but with channels running along each side, like gutters ;Impressed: Vein forming raised line or ridge which lies below the plane of the surface which bears it, as if pressed into it, and are often exposed on the lower surface. Tissue near the veins often appears to pucker, giving them a sunken or embossed appearance ;Obscure: Veins not visible, or not at all clear; if unspecified, then not visible with the naked eye. e.g. ''Berberis gagnepainii''. In this ''Berberis'', the veins are only obscure on the undersurface. ;Prominent: Vein raised above surrounding surface so to be easily felt when stroked with finger. e.g. (''see'') ''Pimenta racemosa'', ''Spathiphyllum, Spathiphyllum cannifolium'' ;Recessed: Vein is sunk below the surface, more prominent than surrounding tissues but more sunken in channel than with impressed veins. e.g. ''Viburnum plicatum''. Describing other features: ;wikt:-plinerved, Plinervy (plinerved): More than one main vein (nerve) at the base. Lateral secondary veins branching from a point above the base of the leaf. Usually expressed as a suffix, as in 3-plinerved or triplinerved leaf. In a 3-plinerved (triplinerved) leaf three main veins branch above the base of the lamina (two secondary veins and the main vein) and run essentially parallel subsequently, as in ''Ceanothus'' and in ''Celtis occidentalis, Celtis''. Similarly, a quintuplinerve (five-veined) leaf has four secondary veins and a main vein. A pattern with 3-7 veins is especially conspicuous in Melastomataceae. The term has also been used in Vaccinieae. The term has been used as synonymous with acrodromous, palmate-acrodromous or suprabasal acrodromous, and is thought to be too broadly defined. ;Scalariform: Veins arranged like the rungs of a ladder, particularly higher order veins ;Submarginal: Veins running close to leaf margin ;Trinerved: 2 major basal nerves besides the midrib


Diagrams of venation patterns


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.


See also

* Glossary of leaf morphology * Glossary of plant morphology#Leaves, Glossary of plant morphology:Leaves * Crown (botany) * Evolution of leaves, Evolutionary history of leaves * Plant evolutionary developmental biology#Evolution of leaves, Evolutionary development of leaves * Leaf Area Index * Leaf protein concentrate * Leaf sensor – a device that measures the moisture level in plant leaves * Leaf shape * Vernation – sprouting of leaves, also the arrangement of leaves in the bud * slek leaf as a musical instrument


References


Bibliography


Books and chapters

* * * * * * * * * * * * * * , in * * * * * * *
1st ed.
* * (''see'' The Encyclopedia Americana) * * * * * *


Articles and theses

* * * * * * * * * * * * * * , in Volume XVII * * * * * * * * * * * * * * *


Websites

* * * * * * ** , in * * * * * ;Glossaries * ** , in * * *


External links

* {{Authority control Leaves, Plant anatomy Plant morphology Plant physiology Photosynthesis