What Is the Function of Stomata Answers Com

Stomatal Anatomy and Stomatal Resistance

M.B. Kirkham , in Principles of Soil and Plant Water Relations (Second Edition), 2014

24.1 Definition of Stomata and Their Distribution

The stomata are apertures in the epidermis, each bounded by two guard cells. In Greek, stoma means "mouth", and the term is often used with reference to the stomatal pore only. Esau (1965, p. 158) uses the term stoma to include the guard cells and the pore between them, and we will use her definition. The plural of stoma is stomata. There is no such word as "stomates".

Stomata occur in vascular plants. Vascular plants include the lower vascular plants such as horsetails (Equisetum), ferns (class Filicinae), gymnosperms, and angiosperms. As noted before, the angiosperms are the flowering plants and this group consists of the two large classes: Monocotyledoneae (monocotyledons) and Dicotyledoneae (dicotyledons) (Fernald, 1950).

By changing their shape, the guard cells control the size of the stomatal aperture. The aperture leads into a substomatal intercellular space, the substomatal chamber, which is continuous with the intercellular spaces in the mesophyll. In many plants, two or more cells adjacent to the guard cells appear to be associated functionally with them and are morphologically distinct from the other epidermal cells. Such cells are called subsidiary, or accessory, cells (Esau, 1965, p. 158).

The stomata are most common on green aerial parts of plants, particularly the leaves. They can also occur on stems, but less commonly than on leaves. The aerial parts of some chlorophyll-free land plants (Monotropa, Neottia) and roots have no stomata as a rule, but rhizomes have such structures (Esau, 1965, p. 158). Stomata occur on some submerged aquatic plants and not on others. The variously colored petals of flowers often have stomata, sometimes nonfunctional. Fruits also can have stomata. Stomata are found on stamens and gynoecia.

Stomata can be distributed in the following ways on the two sides of a leaf:

•: An amphistomatous leaf has stomata on both surfaces. Most plants have such a distribution.
•: A hypostomatous leaf has stomata only on the lower surface. Many tree species are characterized by having hypostomatous leaves, such as horse chestnut (Aesculus hippocastanum) and basswood (Tilia europaea) (Meidner and Mansfield, 1968; see their Table 1.1). The leaf of poplar (Populus sp.) is an exception. It has stomata on both surfaces and a petiole that allows the leaf to turn readily in the wind. These adaptations may allow its fast growth rate. The fast growth rate of poplar is one reason it is widely used in phytoremediation (use of plants to remove pollutants from soil).
•: An epistomatous leaf has stomata only on the upper surface of the leaf. Some floating plants are epistomatous.
•: A heterostomatous leaf has stomata that occur with more than twice the frequency on the abaxial surface than on the adaxial surface. An isostomatous leaf has stomata that occur with approximately equal frequencies on both surfaces.

The stomatal ratio is the ratio of stomatal frequency on the adaxial surface to that on the abaxial surface.

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Phylum Nemata

George O. PoinarJr., in Thorp and Covich's Freshwater Invertebrates (Fourth Edition), 2016

Nemata: Mononchida: Families

1: Stoma narrow, nearly all embedded in pharynx ……………………………………………… 2
1': Stoma large and wide, only 1/4 or less embedded in pharynx (Fig. 9.3 B) ………………………………… 4
2(1): Stoma moderately sclerotized, lacking a tooth ……………………………………………… 3
2': Stoma strongly sclerotized with a large tooth on the ventrolateral side ……………………………… Mononchulidae [p. 178 ]
3(2): Stoma a long cylinder, female prodelphic, tail elongate ………………………………………………………………………………………… Cryptonchidae, one genus; Cryptonchus
3': Stoma open anteriorly; female didelphic; tail short, rounded ……………………………………………………………………………………… Bathyodontidae, one genus; Bathyodontus
4(1): Stoma broad, flattened at base, pharyngeal base with 3 tuberculi …………… Anatonchidae [p. 178 ]
4': Stoma tapering at base; pharyngeal base lacking tuberculi ……………………… Mononchidae [p. 178 ]

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TREE PHYSIOLOGY | Physiology and Silviculture

C. Beadle , R. Sands , in Encyclopedia of Forest Sciences, 2004

Stomatal conductance

Stomata are powerful regulators of gas exchange and linear relationships between rate of photosynthesis and stomatal conductance are often observed in trees. This relationship may become nonlinear at high conductance as the rate limitation may no longer be diffusion of CO ₂ into the leaf but reside in the activity of photosynthetic processes in the mesophyll. As with rates of photosynthesis, there is a considerable variation in maximum stomatal conductance between species where low conductance is often associated with low photosynthetic capacity and vice versa.

Stomata open and close as a result of increases and decreases in turgor, respectively, of guard cells that surround the stomatal pore. However these changes in turgor are driven by active mechanisms that involve the transport of ions, in particular potassium. The regulation of stomata is complex but in general they open in response to light, have a parabolic response to temperature and close in response to atmospheric (vapor pressure) and leaf (soil) water deficits (Figure 2). The sensitivity of stomata to these variables varies between species, and as with photosynthesis, there is adaptation of stomatal conductance to the ambient environment. Trees are tall crops and the leaves are closely coupled to the atmosphere (see TREE PHYSIOLOGY | Canopy Processes). As a result, stomatal conductance of trees is often observed to be quite sensitive to vapor pressure deficit.

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Physiological Ecology of Forest Production

Joe Landsberg , Peter Sands , in Terrestrial Ecology, 2011

b Stomatal Conductance

Stomata provide direct pathways between leaves and the air: they are the active interface between plants and their atmospheric environment. Within the sub-stomatal cavities wet cells are exposed to the air and allow the capture of CO ₂, but this wet surface inevitably results in the loss of water vapour through the stomata. Stomatal resistance is a measure of the resistance to diffusion of CO₂ or water vapour molecules from the stomatal walls to the opening of the stomatal pores. Concomitantly, stomatal conductance g _S (m s^{− 1}) is a measure of the rate of diffusion along this pathway. This fundamentally important variable is discussed in detail in Chapter 3.

Once water vapour has diffused through the stomata it then has to cross the leaf boundary layer with conductance g _b, so water vapour traverses two resistances in series between its source on wet walls of the stomatal cavities and the free air. The total conductance g _V for water vapour is given by

(2.28) $g_{V} = \frac{g_{S} g_{b}}{g_{S} + g_{b}}$

from Eq. (2.25) since g _S and g _b are in series.

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Water-Use Efficiency Under Changing Climatic Conditions

Amitav Bhattacharya , in Changing Climate and Resource Use Efficiency in Plants, 2019

3.13.2 Stomatal Conductance

Stomata are important portals for gas and water exchange in plants and have a strong influence on characteristics associated with photosynthesis and transpiration. Stomata vary in size and density among different species and among cultivated species within species. Moreover, stomatal characteristics are greatly influenced by environmental conditions. Woodward and Kelly (1995) showed that a rise in atmospheric CO₂ concentration and temperature results in a decrease in stomatal density. Under conditions of short-term water stress, plants increase their water-use efficiency by reducing stomatal aperture and thereby transpiration rate; however, under conditions of prolonged water deficit, plants frequently also produce leaves with reduced maximum stomatal conductance (Li et al., 2017), resulting from altered stomatal density and/or size (Franks et al., 2015; Doheny-Adams et al., 2012). Stomata control the flow of water vapor and CO₂ into and out of the leaf (Kim et al., 2010), and thus stomatal function plays an important role in determining both the rate of net CO₂ fixation and water-use efficiency (Antunes et al., 2012) during photosynthesis (Condon et al., 2004). In C3 and C4 plants, stomata open during the day as the guard cells that form these pores accumulate solutes and consequently expand as they take up water by osmosis (Lawson, 2009). Stomatal conductance mediates the exchanges of water vapor and carbon dioxide between leaves and the atmosphere. Sensitivity of sorghum stomatal conductance to soil water availability and vapor pressure deficit varies between genotypes. Sorghum closes stomata, rolls leaves, and has a narrow leaf angle in response to water and heat stress, effectively reducing transpiration and exposure area to solar radiation. Under intermittent water stress, partial closure of stomata is used to sustain reduced photosynthetic activity, which ultimately results in high and stable water-use efficiency in sorghum compared to other drought-susceptible cereals (Takele and Farrant, 2013).

In 15 cultivars of soybean grown under controlled conditions, mesophyll conductance (g _m) and water-use efficiency were measured under standardized environmental conditions (Bunce, 2016). It was reported that leaf water-use efficiency varied by a factor of 2.6 among the cultivars, and g _m varied by a factor of 8.6. However, there was no significant correlation (r = −0.047) between g _m and leaf water use efficiency. Leaf water-use efficiency was linearly related to the substomatal CO₂ concentration (Figs. 3.3 and 3.4).

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The Atmosphere - History

D.L. Royer , in Treatise on Geochemistry (Second Edition), 2014

6.11.3.1 CO₂: Stomata

Stomata are the microscopic pores on leaf surfaces that facilitate gas exchange with the atmosphere, namely, CO ₂, O₂, and H₂O. Approximately 200% and 16% of the total content of atmospheric water vapor and CO₂ are cycled through stomata each year (Hetherington and Woodward, 2003). As such, stomata are finely tuned to the atmosphere. Woodward (1986) demonstrated that stomatal density typically increases with elevation. This response, replicated in experiments (Woodward and Bazzaz, 1988), is largely driven by lower pCO₂ and the plant's requirement to maintain photosynthetic rates but with a transpirational cost (Royer et al., 2001a; Woodward, 1987). Because both atmospheric pressure and atmospheric CO₂ mass affect pCO₂, if pressure is controlled for CO₂, concentration (ppm) can be reconstructed. Typically, stomatal index (100 × stomatal density/[stomatal density + epidermal cell density]), not stomatal density, is used for CO₂ reconstruction because stomatal index is influenced by fewer environmental factors (Royer, 2001). For example, stomatal density is more sensitive to water potential gradients within leaves and canopies (including sun and shade leaves) because water stress affects epidermal cell size (and thus stomatal density) but not stomatal initiation rates (and thus stomatal index) (Kürschner, 1997; Royer, 2001; Sun et al., 2003). Light intensity affects both stomatal density and stomatal index (Lake et al., 2001); however, in natural forest systems, the impact on stomatal index appears minor (Royer, 2001; Sun et al., 2003).

A strength of the stomatal approach for quantifying paleo-CO₂ is that the genetic (Casson and Gray, 2008; Gray et al., 2000), functional (Kleidon, 2007; Konrad et al., 2008; Wynn, 2003), and signaling (Lake et al., 2001, 2002) pathways that underpin the inverse relationship are fairly well understood. Stomata-based CO₂ reconstructions also compare favorably to coeval estimates from Pleistocene and Holocene ice cores (McElwain et al., 2002; Rundgren and Beerling, 1999, 2003; Rundgren et al., 2005).

The principal limitation of the proxy is that the stomata lose sensitivity at high CO₂ faster than the other proxies ( Figure 3 ). Above ~ 700 ppm, the upper error limits for most stomata-based CO₂ estimates are unbounded. Another constraint on using the stomata to reconstruct CO₂ is that atmospheric pressure needs to be considered. Thus, fossil studies must account for paleoelevation; in most cases, if the paleoelevation of a site is < 1000 m, then the impact of atmospheric pressure is minor (Beerling and Royer, 2002). The final constraint is that the stomatal responses to CO₂ are commonly species-specific (Beerling, 2005; Haworth et al., 2010; Jordan, 2011; Royer et al., 2001a). This means that measurements from fossil species should be calibrated to the same extant species. An alternative approach is to compare closely related (but not identical) species; the ratio of the two stomatal measurements is then directly related to CO₂ (McElwain and Chaloner, 1995). The errors associated with this stomatal ratio approach are considerably smaller (green dashed lines in Figure 3 ). But because a stomatal–CO₂ relationship is assumed, not calculated, the CO₂ estimates are semiquantitative, including the associated errors.

A superior alternative is to reconstruct CO₂ from stomatal dimensions and gas exchange considerations alone (Grein et al., 2011). This approach retains sensitivity even at high CO₂ (e.g., standard deviation for an estimate of 2000 ppm is ~ 200 ppm) and does not require an extant calibration between CO₂ and stomatal index, opening up much more of the paleobotanical record (i.e., species that are extinct today). However, as with the traditional approaches, it requires some uniformitarian assumptions, specifically about photosynthetic and conductance parameters.

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Stomatal Control and Water Transport in the Xylem

Peter Franks , Timothy J. Brodribb , in Vascular Transport in Plants, 2005

Evolution of Stomatal Function

Stomata, or at least stoma-like structures, are evident in living and fossilized representatives of some of the earliest known forms of land plants. These include the sporangia of some hornworts and mosses, as well as in fossils of the earliest known vascular plants, such as Cooksonia and Zosterophyllum from around 400 Myr ago (Edwards, 1993). Therefore, it seems that stomata have played a role since the very earliest attempts at land colonization by plants.

One of the earliest land plants, the thallose liverworts of the order Marchantiales, has anatomical features that resemble, in gross form, the leaves of higher plants. Proctor (1981) described them as the nearest analog to flowering-plant leaves among the bryophytes. Their special features include an epidermis covered in a waxy cuticle and a convoluted and highly porous internal layer of chlorenchyma cells. Most significant to the discussion here, however, are the pores in the epidermis through which CO₂ uptake and water loss occur (Fig 4.1A). These pores are formed by a ring of cells, rather than two kidney-shaped guard cells, and do not open and close with the dynamic range of movement exhibited by "true" stomata. For these reasons the pore-bearing Marchantialean liverworts are usually described together with all other liverworts as lacking stomata. However, given that these pores allow photosynthetic gas exchange between inner thallus and atmosphere, across what is an otherwise relatively impermeable cuticle, they are stomata in the very broadest sense. It is tempting to think that early, prearchetypal stomata could have resembled pores like these.

A major advantage for plants that photosynthesize in a gaseous rather than aqueous environment is that CO₂ diffusivity is about 10,000 times higher in air than in water. A problem for many of the earliest forms of land plants is that in a sense they never really left their aquatic environment. Those that retain an ectohydric structure (water conduction via surface capillary structures) are burdened with a relatively thick water film between atmosphere and sites of photosynthesis, creating a low diffusive conductance to photosynthetic gas exchange. Furthermore, these plants cannot regulate the rate of evaporative water loss, which is entirely dependent on atmospheric conditions and laminar boundary layer conductance. However, Marchantialean liverworts represent the beginning of a radically new water management paradigm for plants. Their development of a water-impermeable cuticle on the surface of the thallus necessitated two important changes: (1) pores through which photosynthetic gas exchange could take place, and (2) endohydry (water conduction via internal pathways). With this, humid air chambers can be maintained beneath the cuticle in which the absence of excessive extracellular water increases diffusive conductance to sites of photosynthesis. Green and Snelgar (1982) demonstrated how this structure improved photosynthetic productivity in thalli of pore-bearing versus non-pore-bearing liverworts. In principle, this scheme of endohydric water balance management has changed little over the course of terrestrial plant diversification: More elaborate vascular systems have placed chlorenchyma at ever greater distances from source water, and better stomatal regulation has minimized water deficits in drier atmospheres, but cuticle, stomata, and endohydry are the key components underlying the success of plants on land.

Although the pores in liverwort thalli show some ability to reduce their aperture in response to unfavorable moisture status (Walker and Pennington, 1939; Proctor, 1981), their ability to restrict evaporative water loss is limited. It was the advent of the kidney-shaped (reniform) guard cell pair (Fig. 4.1B-D) that provided the mechanical means to open and close the stomatal pore and, potentially, to regulate gas exchange across the epidermis. This, together with the design of substomatal chambers for optimal CO₂ diffusion into the leaf (Pickard, 1982), is what sustains the high-photosynthetic-capacity homoiohydric tracheophytes that dominate the landscape today. However, the appearance of reniform guard cells is likely to have been only the beginning of a sequence of adaptations in the stomatal apparatus that transformed the gas exchange characteristics of plants during their colonization of land. The easiest of these to visualize are the mechanical characteristics of stomatal movement, which show a progression from very limited aperture range in moss sporophytes and pteridophytes, to a much larger range of movement in angiosperms (Ziegler, 1987). The selection pressures that drove this progression are not well understood, but they may be linked to accompanying evolutionary developments in plant water-conducting systems.

For the same stomatal pore depth, leaf conductance to water vapor is a function of both the mean stomatal pore width and the stomatal density (stomata per unit leaf area). Thus plants with similarly high leaf diffusive conductances may differ considerably in the dynamic range of movement of their guard cells (i.e., plants with limited range of guard cell movement may achieve high leaf diffusive conductance by having high stomatal density). Modifications to mechanical properties of the guard cells that enable a greater range of movement without increased energetic cost should yield a selective advantage for improved flexibility in the regulation of leaf gas exchange. Such flexibility would allow the plant to maintain desirable (safe) hydrodynamic conditions in the vascular system over a broader range of environmental conditions.

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Abscisic Acid Signal Perception and Transduction

Lalit M. Srivastava , in Plant Growth and Development: Hormones and Environment, 2002

3.1. Mechanics of Stomatal Opening and Closing

Stomata open because of a rise in osmotic pressure (OP) of guard cell vacuoles, which is due to an influx of K ⁺ and anions such as Cl^- from the neighboring epidermal cells (Fig. 23-2). Organic solutes, such as sucrose and malate, also contribute to the rise in osmotic pressure. Accumulation of these solutes in the vacuole results in an influx of water and a consequent volume change in the guard cells. The wall architecture of guard cells is such that ballooning out of the thin outer wall pulls with it the thicker inner wall (bordering the pore), thus opening the pore (see Fig. 16-8). During stomatal closure, K⁺ and anions (and other solutes) move out of the cell or to intracellular compartments, which results in loss of water and closure of the pore.

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Stem Anatomy and Pressure–Volume Curves

M.B. Kirkham , in Principles of Soil and Plant Water Relations (Second Edition), 2014

17.1.4 Stomata, Cortex, Pith, and Vascular Bundles in Primary Xylem

Stomata can be present on stems, but constitute a less prominent epidermal component in the stem than in the leaf ( Esau, 1977, p. 259). The stem epidermis commonly consists of one layer of cells and has a cuticle and cutinized walls. It is a living tissue capable of mitotic activity, an important characteristic in view of the stresses to which the tissue is subjected during the primary and secondary increase in thickness of the stem. The epidermal cells respond to these stresses by enlargement and divisions (Esau, 1977, p. 259).

The cortex of stems contains parenchyma, usually with chloroplasts. Intercellular spaces are prominent, but sometimes are largely restricted to the median part of the cortex. In many aquatic angiosperms, the cortex develops as an aerenchyma with a system of large intercellular spaces (Esau, 1977, p. 259). The peripheral part of the cortex frequently contains collenchyma (Figure 17.1). Collenchyma is a supporting tissue composed of more or less elongated living cells with unevenly thickened, nonlignified primary walls. It is in regions of primary growth in stems and leaves. In some plants, notably grasses, sclerenchyma rather than collenchyma develops as the primary supporting tissue in the outer region of the stem. Sclerenchyma is a tissue composed of sclerenchyma cells. A sclerenchyma cell is a cell variable in form and size and having more or less thick, often lignified, secondary walls. It is a supporting cell and may or may not be devoid of a protoplast at maturity.

As noted when we studied root anatomy (Chapter 15), the innermost layer of the cortex (endodermis) of roots of vascular plants has the casparian strip. Stems commonly lack a morphologically differentiated endodermis. In young stems, the innermost layer or layers may contain abundant starch and thus be recognized as a starch sheath (Figure 17.1). Some dicotyledons, however, do develop casparian strips in the innermost cortical layer of the stem, and many lower vascular plants have a clearly differentiated stem endodermis (Esau, 1977, p. 259).

The pith of stems is commonly composed of parenchyma, which may contain chloroplasts. In many stems, the central part of the pith is destroyed during growth. Frequently, this destruction occurs only in the internodes, whereas the nodes retain their pith. The pith has prominent intercellular spaces, at least in the central part. The peripheral part may be distinct from the inner part in having compactly arranged small cells and greater longevity (Esau, 1977, p. 261).

The discrete individual strands of the primary vascular system of seed plants are commonly referred to as vascular bundles. The phloem and xylem show variations in their relative position in vascular bundles. The prevalent arrangement is collateral, in which the phloem occurs on one side (abaxial, or directed away from the axis) of the xylem (Figures 17.1 and 17.2). That is, the phloem is closest to the outside of the stem, even in monocots with scattered vascular bundles (Figure 17.2). The xylem in the corn plant shown in Figure 17.2 makes "monkey faces" (two eyes and one large mouth) and is directed toward the center of the stem (away from the epidermis). In some dicotyledons (e.g., Cucurbitaceae, the squash family, and Solanaceae, the nightshade family, which includes potato), one part of the phloem occurs on the outer side and another on the inner side of the xylem. This arrangement is called bicollateral, and the two parts of the phloem are referred to as the external (abaxial) and the internal (adaxial) phloem (Esau, 1977, p. 261). Adaxial means directed toward the axis.

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PLANT MACROFOSSIL METHODS AND STUDIES | CO2 Reconstruction from Fossil Leaves

M. Rundgren , in Encyclopedia of Quaternary Science (Second Edition), 2013

Counting

Counting of stomata and epidermal cells is conducted using epifluorescence or transmitted light microscopy. In the first method, fluorescent light reaches the leaf from above through the objective, and excitation highlights cell walls and other structures on its surface. In the second method, normal light from below passes through the isolated and stained cuticle and makes its morphological features visible.

The size of the field area used for counting varies among species and is optimized with regard to cuticular morphology, in particular cell size. A larger field of view results in more representative values but, in practice, field size is limited by the distribution of veins on the leaf surface ( Figure 1 ). Field area size should be selected so that, in general, at least five stomata (including two flanking guard cells) and 50 epidermal cells are visible, and the same size should be used throughout the study. Typically, field areas of 0.03–0.06 mm² and magnifications of 200–600 × are used.

Stomata and epidermal cells can be tallied directly at the microscope using, for example, a hand counter. In this case, a graticule in one of the eyepieces is normally used to define the field area and facilitate counting. Alternatively, if a camera is attached to the microscope and linked to a computer, counting may be made (immediately or at a later time) with the aid of image analysis software. Although slightly more time consuming, this arrangement enhances counting accuracy and allows images of all counted fields to be conveniently archived for later study. Irrespective of counting procedure, stomata and epidermal cells are usually counted within a rectangular or quadratic area, and standardized criteria for how to deal with stomata (including two flanking guard cells) and epidermal cells along the field area margin are needed. An appropriate approach is that described by Poole and Kürschner (1999), where all stomata and epidermal cells crossing two predefined sides of the field area and the corner between them are included in the count, while those crossing the two remaining sides and the three remaining corners are not ( Figure 1 ).

To minimize the effect of intraleaf variability, counts should be made from several field areas distributed over the leaf surface and used to calculate leaf SI and SD mean values. These areas should, however, not be placed along margins, or near the leaf base or tip, where stomatal frequency values often are found to be anomalous (Poole et al., 1996; Tichá, 1982; Figure 1 ). To minimize variability caused by the presence of veins that do not carry stomata, counting should also be restricted to interveinal areas. The specific number of counts needed to produce reliable mean values varies from species to species and should be determined in a pilot study (Poole and Kürschner, 1999). This may be done through successive calculation of the mean as more counts are added. Fluctuation of the mean is typically found to be within acceptable limits after 7–10 counts per leaf. The number of counts needed is partly dependent on stomatal arrangement, with hypostomatous species (having stomata only on the lower leaf surface) normally found to require fewer counts than amphistomatous species (having stomata on both surfaces). To account for interleaf variability, sample SI and SD mean values are calculated based on leaf (or leaf fragment) mean values and used in CO₂ calibration. It should, however, be noted that it is not always possible to determine if small-sized fragments are derived from separate leaves.

The counting procedure described above only applies to broad-leaved angiosperm species. Gymnosperms exhibit a different mode of leaf development and stomatal patterning, which requires alternative approaches. Different parameters have been used to quantify stomatal frequency in conifer needles, including SD and number of stomatal rows. In contrast to the situation in broad-leaved species, stomatal initiation is usually more accurately reflected by SD rather than SI in conifers, but the number of stomata per millimeter of needle length is considered to be the most sensitive parameter (Kouwenberg et al., 2003). The often weak CO₂ responses found in earlier studies of conifer species (Royer, 2001) may partly be due to inappropriate quantification of stomatal frequency.

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What Is the Function of Stomata Answers Com

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Burnett Undiers

What Is the Function of Stomata Answers Com

Stomatal Anatomy and Stomatal Resistance

24.1 Definition of Stomata and Their Distribution