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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 Ancient Greek wo ...
, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of
gas exchange Gas exchange is the physical process by which gases move passively by Diffusion#Diffusion vs. bulk flow, diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liqui ...
. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the stomatal opening. 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 Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...
, which is used in respiration, and
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
, which is used in
photosynthesis 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 released to fuel the organism's activities. Some of this chemica ...
, passes through stomata by gaseous
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
.
Water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 Joule, J/(Kilogram, kg·K) , - , Heat of vaporization , 2.27 Megajoule, MJ/kg , - , Heat capacity , 1.864 Kilojoule, kJ/(kg·K) Water vapor, water vapour or ...
diffuses through the stomata into the atmosphere in a process called
transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaf, leaves, Plant stem, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is ...
. Stomata are present in the
sporophyte A sporophyte () is the diploid multicellular stage in the biological life cycle, life cycle of a plant or alga which produces asexual Spore, spores. This stage alternates with a multicellular haploid gametophyte phase. Life cycle The sporophyt ...
generation of all
land plant The Embryophyta (), or land plants, are the most familiar group of green plant Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all li ...
groups except
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 pla ...
. In vascular plants the number, size and distribution of stomata varies widely.
Dicotyledon The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, t ...
s usually have more stomata on the lower surface of the leaves than the upper surface.
Monocotyledon Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. Th ...
s such as
onion An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus ''Allium''. The shallot is a variety (botany), botanical var ...
, oat and
maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American English, North American and Australian English), is a cereal grain first domesticated by indigenous peoples of Mexico, indigenous ...
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 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.

# Function

## CO2 gain and water loss

Carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
, a key reactant in
photosynthesis 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 released to fuel the organism's activities. Some of this chemica ...
, 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 the
leaf A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...
are saturated with
water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 Joule, J/(Kilogram, kg·K) , - , Heat of vaporization , 2.27 Megajoule, MJ/kg , - , Heat capacity , 1.864 Kilojoule, kJ/(kg·K) Water vapor, water vapour or ...
, which exits the leaf through the stomata in a process known as
transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaf, leaves, Plant stem, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is ...
. Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapour.

## Alternative approaches

Ordinarily, carbon dioxide is fixed to ribulose 1,5-bisphosphate (RuBP) by the enzyme
RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molec ...
in mesophyll cells exposed directly to the air spaces inside the leaf. This exacerbates 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 Photorespiration (also known as the oxidative photosynthetic carbon cycle or C2 cycle) refers to a process in plant physiology, plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. The ...
. 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 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.

## CAM plants

A group of mostly desert plants called "CAM" plants ( Crassulacean acid metabolism, after 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 PEPcarboxylase 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, is severely limited by the capacity to store fixed carbon in the vacuoles, so it is preferable only when water is severely limited.

## Opening and closing

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), a proton pump drives
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s (H+) from the guard cells. This means that the cells'
electrical potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work (physics), work energy needed to move a unit of electric charge from a reference point to the sp ...
becomes increasingly negative. The negative potential opens potassium voltage-gated channels and so an uptake of
potassium Potassium is the chemical element with the Symbol (chemistry), symbol K (from New Latin, Neo-Latin ''wikt:kalium#Latin, kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little fo ...
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 the organic ion
malate Malic acid is an organic compound with the molecular formula . It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms (L ...
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 Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane, selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of ...
. This increases the cell's volume and
turgor pressure Turgor pressure is the force within the cell that pushes the Cell membrane, plasma membrane against the cell wall. It is also called ''hydrostatic pressure'', and is defined as the pressure in a fluid measured at a certain point within itself whe ...
. 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 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 Abscisic acid (ABA) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy Dormancy is a period in an organism, organism's Biological life cycle, life cycle when growth, development, and (in an ...
(ABA) is released. ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of the
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cell (biology), cells (Fluid compartments#Intracellular compartment, intracellular fluid (ICF)). It is separated into compartments by membranes. For ...
of the cells and cause the concentration of free Ca2+ to increase in the cytosol due to influx from outside the cell and release of Ca2+ 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 the diffusion of water back out of the cell by
osmosis Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane, selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of ...
. 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.

## Inferring stomatal behavior from gas exchange

The degree of stomatal resistance can be determined by measuring leaf gas exchange of a leaf. The
transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaf, leaves, Plant stem, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is ...
rate is dependent on the
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
resistance provided by the stomatal pores, and also on the
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...
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 Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic Allotropy, allotrope , breaking down i ...
. Evaporation (''E'') can be calculated as; $E = \left(e_ - e_\right)/Pr$ where ''e''i and ''e''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; $E = \left(e_ - e_\right)g/P$ and solved for ''g''; $g = EP / \left(e_ - e_\right)$ Photosynthetic CO2 assimilation (''A'') can be calculated from $A = \left(C_ - C_\right)g/1.6P$ where ''C''a and ''C''i are the atmospheric and sub-stomatal partial pressures of CO2, respectively. The rate of evaporation from 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 CO2 uptake and thus measure photosynthetic rate.

# Evolution

There 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 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 th ...
was evolving – these two traits together constituted a major advantage for early terrestrial plants.

# Development

There are three major epidermal cell types which all ultimately derive from the outermost (L1) tissue layer of the shoot apical meristem, called protodermal cells:
trichome Trichomes (); ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a plant ...
s, 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 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 Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a biochemical cascade, series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately re ...
components such as ''EPF'' (Epidermal Patterning Factor), ''ERL'' (ERecta Like) and ''YODA'' (a putative MAP kinase kinase kinase). Mutations in any one of 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.  Activation 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, allowing for asymmetrical cell division that initiates stomata formation. Stomatal development is also coordinated by the cellular peptide signal called stomagen, which signals the inhibition 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 Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essenti ...
represses stomatal development by affecting their development at 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 ''Populus'' is a genus of 25–30 species of deciduous flowering plants in the family Salicaceae, native to most of the Northern Hemisphere. English names variously applied to different species include poplar (), aspen, and cottonwood. The we ...
and
willows Willows, also called sallows and osiers, from the genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, vir ...
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 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 ''Acer saccharinum'', commonly known as silver maple, creek maple, silverleaf maple, soft maple, large maple, water maple, swamp maple, or white maple, is a species of maple native to the eastern and central United States and southeastern Canad ...
had small stomata that were more numerous.

# Types

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 is based on the size, shape and arrangement of the subsidiary cells that surround the two guard cells. They distinguish for dicots: * (meaning ''star-celled'') stomata have guard cells that are surrounded by at least five radiating cells forming a star-like circle. This is a rare type that can for instance be found in the family
Ebenaceae The Ebenaceae are a family (biology), family of flowering plants belonging to order (biology), order Ericales. The family includes ebony and persimmon among about 768 species of trees and shrubs. It is distributed across the tropics, tropical and ...
. * (meaning ''unequal celled'') stomata have guard cells between two larger subsidiary cells and one distinctly smaller one. This type of stomata can be found in more than thirty dicot families, including
Brassicaceae Brassicaceae () or (the older) Cruciferae () is a medium-sized and economically important Family (biology), family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family. Most are herbaceous plants, while some ar ...
,
Solanaceae The Solanaceae , or nightshades, are a family (biology), family of flowering plants that ranges from Annual plant, annual and perennial herbs to vines, lianas, epiphytes, shrubs, and trees, and includes a number of agricultural crops, medicinal ...
, and
Crassulaceae The Crassulaceae (from Latin ''crassus'', thick), also known as the stonecrop family or the orpine family, are a diverse family of dicotyledon flowering plants characterized by succulent leaves and a unique form of photosynthesis, known as C ...
. It is sometimes called ''cruciferous type''. * (meaning ''irregular celled'') stomata have guard cells that are surrounded by cells that have the same size, shape and arrangement as the rest of the epidermis cells. This type of stomata can be found in more than hundred dicot families such as Apocynaceae, Boraginaceae,
Chenopodiaceae Amaranthaceae is a family of flowering plants commonly known as the amaranth family, in reference to its type (biology), type genus ''Amaranthus''. It includes the former goosefoot family Chenopodiaceae and contains about 165 genera and 2,040 spe ...
, and
Cucurbitaceae The Cucurbitaceae, also called cucurbits or the gourd family, are a plant family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or ...
. It is sometimes called ''ranunculaceous type''. * (meaning ''cross-celled'') stomata have guard cells surrounded by two subsidiary cells, that each encircle one end of the opening and contact each other opposite to the middle of the opening. This type of stomata can be found in more than ten dicot families such as Caryophyllaceae and
Acanthaceae Acanthaceae is a Family (biology), family (the acanthus family) of dicotyledonous flowering plants containing almost 250 genera and about 2500 species. Most are Tropics, tropical Herbaceous plant, herbs, shrubs, or twining vines; some are epiphyt ...
. It is sometimes called ''caryophyllaceous type''. * stomata are bordered by just one subsidiary cell that differs from the surrounding epidermis cells, its length parallel to the stoma opening. This type occurs for instance in the Molluginaceae and
Aizoaceae The Aizoaceae, or fig-marigold family, is a large Family (biology), family of dicotyledonous flowering plants containing 135 genus, genera and about 1800 species. They are commonly known as ice plants or carpet weeds. They are often called vygies ...
. * (meaning ''parallel celled'') stomata have one or more subsidiary cells parallel to the opening between the guard cells. These subsidiary cells may reach beyond the guard cells or not. This type of stomata can be found in more than hundred dicot families such as
Rubiaceae The Rubiaceae are a family (biology), family of flowering plants, commonly known as the coffee, madder, or bedstraw family. It consists of terrestrial trees, shrubs, lianas, or herbs that are recognizable by simple, opposite leaves with Petiole ...
,
Convolvulaceae Convolvulaceae (), commonly called the bindweed, bindweeds or morning glory, morning glories, is a Family (biology), family of about 60 genera and more than 1,650 species. These species are primarily herbaceous vines, but also include trees, sh ...
and
Fabaceae The Fabaceae or Leguminosae,International Code of Nomenc ...
. It is sometimes called ''rubiaceous type''. In
monocots Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. Th ...
, several different types of stomata occur such as: * gramineous or graminoid (meaning ''grass-like'') stomata have two guard cells surrounded by two lens-shaped subsidiary cells. The guard cells are narrower in the middle and bulbous on each end. This middle section is strongly thickened. The axis of the subsidiary cells are parallel stoma opening. This type can be found in monocot families including
Poaceae Poaceae () or Gramineae () is a large and nearly ubiquitous Family (biology), family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species culti ...
and
Cyperaceae The Cyperaceae are a family of graminoid (grass-like), monocotyledonous flowering plants known as sedges. The Family (biology), family is large, with some 5,500 known species described in about 90 genera, the largest being the "true sedges" g ...
. * (meaning ''six-celled'') stomata have six subsidiary cells around both guard cells, one at either end of the opening of the stoma, one adjoining each guard cell, and one between that last subsidiary cell and the standard epidermis cells. This type can be found in some monocot families. * (meaning ''four-celled'') stomata have four subsidiary cells, one on either end of the opening, and one next to each guard cell. This type occurs in many monocot families, but also can be found in some dicots, such as ''
Tilia ''Tilia'' is a genus of about 30 species of trees or bushes, native throughout most of the temperateness, temperate Northern Hemisphere. The tree is known as linden for the European species, and basswood for North American species. In Britain a ...
fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The Polypodiophyta, polypodiophytes include all living pteridop ...
s, four different types are distinguished: * stomata have two guard cells in one layer with only ordinary epidermis cells, but with two subsidiary cells on the outer surface of the epidermis, arranged parallel to the guard cells, with a pore between them, overlying the stoma opening. * stomata have two guard cells that are entirely encircled by one continuous subsidiary cell (like a donut). * stomata have two guard cells that are entirely encircled by one subsidiary cell that has not merged its ends (like a sausage). * stomata have two guard cells that are largely encircled by one subsidiary cell, but also contact ordinary epidermis cells (like a U or horseshoe).

# Stomatal crypts

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 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, and '' Drimys winteri'' which is a species of plant found in the
cloud forest A cloud forest, also called a water forest, primas forest, or tropical montane cloud forest (TMCF), is a generally tropical or subtropical, evergreen, Montane forest, montane, Tropical and subtropical moist broadleaf forests, moist forest cha ...
.

# Stomata as pathogenic pathways

Stomata are obvious holes in the leaf by which, as was presumed for a while, pathogens can enter unchallenged. However, it has been recently shown that stomata do in fact sense the presence of some, if not all, pathogens. However, with the virulent bacteria applied to ''Arabidopsis'' plant leaves in the experiment, the bacteria released the chemical coronatine, which forced the stomata open again within a few hours.

# Stomata and climate change

## Response of stomata to environmental factors

Drought inhibits stomatal opening, but moderate drought has not had a significant effect on stomatal closure of soya beans. There are different mechanisms of stomatal closure. Low humidity stresses guard cells causing
turgor Turgor pressure is the force within the cell that pushes the Cell membrane, plasma membrane against the cell wall. It is also called ''hydrostatic pressure'', and is defined as the pressure in a fluid measured at a certain point within itself whe ...
loss, termed hydropassive closure. Hydroactive closure is contrasted as the whole leaf effected by drought stress, believed to be most likely triggered by abscisic acid.
Photosynthesis 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 released to fuel the organism's activities. Some of this chemica ...
, plant water transport (
xylem Xylem is one of the two types of transport tissue in vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of embryophyte, land plants ( accepted known species) that have lignin, ...
) 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 is because the light response of stomata to blue light is independent of other leaf components like
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Ancient Greek, Greek words , ("pale green") and , ("leaf"). Chlorophyll al ...
. Guard cell protoplasts swell under blue light provided there is sufficient availability of
potassium Potassium is the chemical element with the Symbol (chemistry), symbol K (from New Latin, Neo-Latin ''wikt:kalium#Latin, kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little fo ...
. 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 the day
sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...
plays a larger role in regulating stomatal opening. Stomatal density and aperture (length of stomata) varies under a number of environmental factors such as atmospheric CO2 concentration, light intensity, air temperature and photoperiod (daytime duration). Decreasing stomatal density is one way plants have responded to the increase in concentration of atmospheric CO2 ( O2sub>atm). Although changes in O2sub>atm response is the least understood mechanistically, this stomatal response has begun to plateau where it is soon expected to impact
transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaf, leaves, Plant stem, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is ...
and
photosynthesis 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 released to fuel the organism's activities. Some of this chemica ...
processes in plants.

## Future adaptations during climate change

It is expected for O2sub>atm to reach 500–1000 ppm by 2100. 96% of the past 400 000 years experienced below 280 ppm CO2 levels. From this figure, it is highly probable that
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleot ...
s of today’s plants diverged from their pre-industrial relative. 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 ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter an ...
'' found no increase of stomatal development in the dominant
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the Locus (genetics), same place on a long DNA molecule, as described in leading textbooks on genetics and evoluti ...
, but in the ‘wild type’ recessive allele showed a large increase, both in response to rising CO2 levels in the atmosphere. These studies imply the plants response to changing CO2 levels is largely controlled by genetics.

## Agricultural implications

The CO2 fertiliser effect has been greatly overestimated during Free-Air Carbon dioxide Enrichment (FACE) experiments where results show increased CO2 levels in the atmosphere enhances photosynthesis, reduce transpiration, and increase water use efficiency (WUE). Increased
biomass Biomass is plant-based material used as a fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work (physics), work. The concept was originally applied ...
is one of the effects with simulations from experiments predicting a 5–20% increase in crop yields at 550 ppm of CO2. Rates of leaf photosynthesis were shown to increase by 30–50% in C3 plants, and 10–25% in C4 under doubled CO2 levels. The existence of a
feedback mechanism Feedback occurs when outputs of a system are routed back as inputs as part of a Signal chain (signal processing chain), chain of Causality, cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. ...
results a
phenotypic plasticity Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompa ...
in response to O2sub>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 the best suited species such as heat and drought resistant crop varieties that could naturally evolve to the change in the face of food security challenges.