CO2 and stomatal conductance

Chapter 4.04 – Effects of Carbon Dioxide Enrichment on Plants

Taub D. R., Wang X. (2013)

Southwestern University, Georgetown, TX, USA / Indiana University-Purdue University Indianapolis, Indianapolis, IN, USA

in Reference Module in Earth Systems and Environmental Sciences – Climate Vulnerability – Understanding and Addressing Threats to Essential Resources 4: 35-50 – https://doi.org/10.1016/B978-0-12-384703-4.00404-4 –

https://www.sciencedirect.com/science/article/pii/B9780123847034004044

Abstract

Increasing concentrations of atmospheric CO₂ over the next decades can be expected to have important implications for plants. Many plant species are likely to exhibit higher rates of photosynthesis and growth, decreased water consumption and changes in tissue chemistry, including decreased concentrations of protein and minerals. The most likely threats to ecosystem services as a result of increased atmospheric CO₂ are (1) decreased crop and forage food values due to decreased concentrations of protein and minerals and (2) undesirable changes in plant community composition, including increases in populations of some major weed species.

4.04.2.1.2 Transpiration and Water Use

Atmospheric concentrations of CO₂ can also affect plant water use. Carbon dioxide concentrations play a central role in controlling the aperture of stomata, small pores on the surfaces of plants, particularly on the underside of leaves. Open stomata provide a pathway for CO₂ to diffuse into leaves for photosynthesis, but also a pathway for water to diffuse out from leaves. Stomatal aperture (closely related to a physiological measure known as stomatal conductance) is therefore regulated to balance the goals of maintaining CO₂ assimilation and minimizing water loss. At higher concentrations of atmospheric CO₂, plants are able to achieve high rates of photosynthesis with relatively low stomatal conductance. In FACE experiments, elevated CO₂ decreases stomatal conductance by an average of 22% (Ainsworth and Rogers 2007).

Effects of elevated CO₂ on water use by whole plants or plant communities will depend not only on effects on stomatal conductance and leaf-level transpiration, but also on other potential effects of elevated CO₂, such as changes in plant size and morphology. For example, increased growth of plants may lead to increased numbers of leaves, offsetting decreased transpiration per leaf (Warren et al. 2011). Nonetheless, the overall effect of elevated CO₂ appears to be a decrease in total plant and ecosystem water usage, by approximately 5–20% in FACE experiments (Kimball 2010; Leakey et al. 2009). This can lead to alterations of ecosystem hydrology, including an increase in soil moisture and perhaps runoff (Leakey et al. 2009; Norby and Zak 2011).

Stomata and stomatal aperture

CHAPTER 8 – SHOOT MORPHOLOGY AND LEAF ANATOMY IN RELATION TO PHOTOSYNTHESIS

Bolhar-Nordenkampf H. R. (1985)

in Techniques in Bioproductivity and Photosynthesis (Second Edition) – https://doi.org/10.1016/B978-0-08-031999-5.50018-2 –

https://www.sciencedirect.com/science/article/pii/B9780080319995500182

Publisher Summary

This chapter discusses shoot morphology and leaf anatomy in relation to photosynthesis. The youngest leaves of a non-shaded plant usually grow in full sunlight. Some weeks later, the same leaves, now totally expanded and mature, may be shaded to a certain extent by the newly developed younger leaves. This would mean that the light intensity available for photosynthesis could be reduced to less than 10% of full sunlight and, therefore, cause very low net leaf photosynthesis. Therefore, plants may show a distinct leaf arrangement along the stem, which guarantees optimal use of the irradiated light and makes full sunlight available for the largest amount of leaf area. In a growing canopy, these differences in available light are even more striking. The other microclimatic factors show comparable gradients because of the height, density, and geometry of the canopy. Some plants have leaves that adapt well to these changes in the microclimate, whereas others lack this ability.

8.3 Experiments

8.3.1 Leaf anatomy

Transverse sections of the leaf blade cut by hand, using a razor blade, are examined in a light microscope. The layers of the palisade parenchyma cells are counted and the relation of the thickness of the palisade parenchyma to the spongy parenchyma can be estimated. Look for sclerenchyma cells and examine the leaf-air system in the different parts of the mesophyll. Do this by drawing a part of the cross-section.

Try to find the stomata and describe their position and arrangement: raised, sunken, in crypts, protected by trichomes or wax; hypostomatic, hyperstomatic or amphistomatic. To count the stomata per unit of leaf area you have to use a good incident light microscope. You can count the stomata on artificial replicas if the leaves have a blade without many trichomes and if the stomata are not deeply sunken. To produce the replicas you can use any transparent paint or varnish. Put a drop of the varnish on the surface of the leaf and pull off the dry transparent film with forceps after a few minutes. Now you can count the stomatal impressions on this replica with the help of a calibrated grid in the eyepiece of the microscope. Take replicas of different parts of the leaf because stomatal frequency varies greatly on one leaf. Less than 60 stomata per mm² and more than 600 stomata per mm² indicate that the plant grows in an extreme habitat (xeromorphic, hygromorphic respectively). Do not measure stomatal widths by the technique of replicas or epidermal strips. It is better to use an incident light microscope with a mirror (reflecting) objective for such measurements. Alternatively stomatal aperture may be determined by indirect methods as detailed in Chapter 5.

8.3.2 Stomatal width

An estimate of the relative stomatal aperture can be obtained by the infiltration method using xylol, alcohol (ethanol) and paraffin oil. Put a drop of each solution on the upper and lower surface of the leaf and judge the degree of infiltration by the size of the area darkened by the liquid entering the internal air spaces. The results may give good response curves of the changes of the climatic factors over one day of one plant only, but you cannot compare the response curves of different plants.

8.3.3 Demonstration of O₂ evolution from whole plants

Fill a large glass container with a 0.01% indigo carmine (indigo disulphonate) solution. While stirring constantly, carefully add single drops of a 10% sodium dithionite solution until the blue indigo carmine is reduced to the yellow form. Place a whole plant or a branch into this solution and seal the container, excluding air. After several minutes of illumination, if there is no excess sodium dithionite, blue areas will appear around all green parts of the plant. The reason for this is that the small amounts of O₂ produced by photosynthesis inside the leaf are enough to reoxidize the yellow indigo carmine back to the blue form.

8.3.4 In situ demonstration of PSII activity

The presence of PS II activity (reducing power) can be demonstrated by using the reduction of tetranitro-blue-tetrazolium chloride (TNBT). Cross-sections of the leaf blade, which need not be very thin, are cut by hand and infiltrated under vacuum with the staining solution (see below). After 5 to 20 minutes of illumination, under a microscope for instance, the areas in the leaf transverse section containing chloroplasts with high PS II activity will be of a dark-blue colour. This occurs first in all wounded cells, then in the mesophyll cells of all plants and finally in the granai bundle-sheath plastids of the C₄ grasses of the NAD-ME and PCK groups. The agranal bundle-sheath plastids of the NADP-ME species turn blue only after several hours of illumination. Misleading results may occur if infiltration is poor because of residual air in the intercellular spaces. Note that the bundle-sheath cells which are longitudinally elongated are more likely to be injured during sectioning than the more rounded mesophyll cells. Thus C₄ plants, especially dicotyledons, may show some coloured bundle-sheath chloroplasts even after a short period of illumination².

Stock solutions:(A)

0.1% TNBT (1.0 mg ml⁻¹). (Do not make up more than 10 ml at one time: store the dye powder and solution in a refrigerator.)(B)

0.1 M phosphate buffer, pH 6.0.(C)

0.3 M sucrose.

Staining solution:

1 part A + 3 parts B + 1 part C.

8.3.5 Differentiation between C₃ and C₄ plants by detection of starch in situ

Cross-sections of the blades of leaves of various types are stained by immersion in iodine solution. Starch will have been formed in the chloroplasts if the leaf has been illuminated for a long time and if net leaf photosynthesis has been proportionally high as compared to translocation of photosynthate. In such leaves chloroplasts will be stained dark blue by iodine. This occurs in chloroplasts of the mesophyll cells of all C₃ plants. In C₄ plants such starch accumulation occurs predominantly in the “Kranz” bundle-sheath cells. The detection of starch in this way is more distinct if chlorophyll is extracted from the leaves, using hot alcohol, before staining.

After 2 days in the dark the chloroplasts in the leaf are generally destarched. Such a leaf can be used to produce high resolution starch prints or pictures. photographic negative with high contrast is mounted on the upper surface of the leaf exposed to light. Starch will be formed only in those areas which are actually reached by light rays. The starch formation will be proportional to incident light intensity. Thus if the leaf is cut off, killed and extracted in hot water, followed by hot alcohol (in a water bath), and stained with iodine, the negative will be reproduced as a positive image. The high resolution image obtained indicates that starch is in general only formed in those cells which are actually illuminated and that translocation of photosynthate from an illuminated chloroplast, to a non-illuminated chloroplast, even when they both lie in the same cell, does not readily occur¹¹.

Solution for iodine stain (Lugol). First dissolve 2.0 g potassium iodide (KI) in 5 ml water, then add 1.0 g iodine and make up to 300 ml with water.

8.3.6 Demonstration of phloem translocation in detached maize leaves

Corn plants, 80 cm high, are transferred to a darkroom for 48 h. After this dark period no starch ought to be detectable with the iodine/KI test. Under dim light 30 cm long segments are cut from the leaf blade. The midrib is removed and the margins are stripped away. The strips are trimmed under water to a final length of 25 cm. Both ends are dipped in Hoagland’s nutrient solution diluted to 1/20th full strength. The central part of the leaf strip is placed in a chamber with several microlitres of 20% KOH. By pressing hot forceps on the tissue, the phloem is interrupted in a 1 cm wide segment of the leaf strip where it enters this CO₂-free chamber on both sides.

The strip is then exposed to sunlight for 4 hours (or to artificial light for 7 hours). It is recommended that eight replicates are set up for this experiment. The iodine/KI test for starch accumulation should be performed 1, 2 and 4 hours (2, 5 and 7 hours) after the start of the experiment.

The result should show starch formation initially in the lateral free parts of the strip. Towards the end of the experiment the starch should also appear in the central, enclosed part, with the exception of the segment where the phloem is disconnected from the parts of the strip outside the chamber. The starch formation in the central part of the maize leaf strip must be caused by active phloem transport from both sides⁷.

Stomata play a pivotal role in controlling the balance between water loss and carbon gain

CHAPTER 5 – WATER RELATIONS

Beadle C. L., Ludlow M. M., Honeysett J. L., (1985)

in Techniques in Bioproductivity and Photosynthesis (Second Edition) – Pergamon International Library of Science, Technology, Engineering and Social Studies: 50-61 – https://doi.org/10.1016/B978-0-08-031999-5.50015-7 –

https://www.sciencedirect.com/science/article/pii/B9780080319995500157

Publisher Summary

This chapter discusses water relations in plants. To absorb CO₂ for photosynthesis, plants expose wet surfaces to a dry atmosphere and, in consequence, suffer evaporative water loss. The resultant cooling often accounts for considerable heat dissipation by leaves and is probably essential for maintaining equable temperatures for photosynthesis. Plants have, therefore, evolved leaves with an epidermis composed of a relatively impermeable cuticle and turgor-operated valves—stomata. The epidermis not only reduces rates of CO₂ and water vapor exchange but also provides a means of controlling assimilation and transpiration through the size of the stomatal pores. Thus, stomata play a pivotal role in controlling the balance between water loss and carbon gain. For a given leaf, plant, or variety, the length and depth of stomata do not vary among stomata in mature tissues. Most of the changes in aperture are associated with changes in width. In practice, it is not possible to make a direct microscopic observation of stomata and, at the same time, preserve natural conditions.

5.1.3 Methods for stomatal aperture

Stomatal aperture is usually measured by direct microscopic observation or by the extent or rate of infiltration of organic solvents¹⁵.

Direct Observation:

For a given leaf, plant or variety, the length and depth of stomata do not vary among stomata in mature tissues. Instead, most of the changes in aperture are associated with changes in width. In practice it is not possible to make a direct microscopic observation of stomata and at the same time preserve natural conditions. The change in conditions could alter the stomatal aperture. However, it is possible to make a stomatal impression before stomata have time to react by applying a quick-drying substance to the leaf surface^34,15,24. The size of the stomatal aperture can be measured under a microscope either from the impression (where the stomatal pore is represented as a raised area in what is equivalent to a photographic negative) or a positive which is made by painting the negative with a substance such as cosmetic nail varnish (stomatal pores appear as holes in the positive). Stomatal aperture can be converted into an equivalent diffusive resistance (or conductance)^19,20.

Infiltration by Liquids:

A series of mixtures of two liquids (0–100%) is made, one with a high, the other low, viscosity. The mixtures are then applied to leaf surfaces in sequence from the most to the least viscous. The first mixture to infiltrate the leaf surfaces is an index of the degree of stomatal opening. This index can be correlated for each species with aperture obtained by direct observation or with diffusive resistance¹⁶. The infiltration method is simple and cheap, but of limited accuracy.

A cell-within-a-cell arrangement unknown among stomata elsewhere in the plant kingdom

Stomata of Alethopteris sullivanti: a new stomatal type among seed ferns and vascular plants

Stidd B. M. (1988)

Benton M. Stidd

Amer. J. Bot. 75(6): 790-796 – https://doi.org/10.2307/2443998 – .

https://www.jstor.org/stable/2443998

Abstract

Observations based on new preparations confirm the presence of four cells in parallel alignment in A. sullivanti stomata but not with two flanking subsidiary cells as in paracytic stomata. Rather, each guard cell contains in its interior a smaller inner cell. The stomatal pore is formed by walls of the larger cells and it is not known what role, if any, the interior cells may have played in opening and closing the pore. This cell-within-a-cell arrangement is unknown among stomata elsewhere in the plant kingdom. The inner cells have the appearance of guard cells, especially when the poral walls of the larger cells are removed, and were so designated by Oestry Stidd and Stidd (1976). Cuticle preparations (Reihman and Schabilion, 1985) leave most of the cell structure of the stomatal apparatus intact among other leaf tissues (are not removed with the cuticle) and therefore do not reveal essential features.

Declines in AN were largely due to non-stomatal (diffusional and metabolic) limitations

Assessing stomatal and non-stomatal limitations to carbon assimilation under progressive drought in peanut (Arachis hypogaea L.)

Pilon C., Snider J. L., Sobolev V., Chastain D. R., Sorensen R. B., Meeks C. D., Massa A. N., Walk T., Singh B., Earl H. J. (2018)

Cristiane Pilon ¹, John L Snider ², Victor Sobolev ³, Daryl R Chastain ⁴, Ronald B Sorensen ³, Calvin D Meeks ⁵, Alicia N Massa ³, Travis Walk ³, Bhupinder Singh ⁴, Hugh J Earl ⁶

• ¹Department of Crop and Soil Sciences, University of Georgia, 115 Coastal Way, Tifton, GA, 31794, United States of America. Electronic address: cpilon@uga.edu.
• ²Department of Crop and Soil Sciences, University of Georgia, 115 Coastal Way, Tifton, GA, 31794, United States of America.
• ³USDA, Agricultural Research Service, National Peanut Research Laboratory, 1011 Forrester Drive, Dawson, GA, 39842, United States of America.
• ⁴Delta Research and Extension Center, Mississippi State University, PO Box 197, Stoneville, MS, 38776, United States of America.
• ⁵Fisher Delta Center, University of Missouri, 147 State Highway T, Portageville, MO 63873, United States of America.
• ⁶Department of Plant Agriculture, University of Guelph, 50 Stone Road E., Guelph, Ontario, N1G 2W1, Canada.

J Plant Physiol. 231: 124-134 – doi: 10.1016/j.jplph.2018.09.007 – Epub 2018 Sep 20 – PMID: 30261481 –

https://pubmed.ncbi.nlm.nih.gov/30261481/

Abstract

Drought is known to limit carbon assimilation in plants. However, it has been debated whether photosynthesis is primarily inhibited by stomatal or non-stomatal factors. This research assessed the underlying limitations to photosynthesis in peanuts (Arachis hypogaea L.) grown under progressive drought. Specifically, field-grown peanut plants were exposed to either well-watered or drought-stressed conditions during flowering. Measurements included survey measurements of gas exchange, chlorophyll fluorescence, PSII thermotolerance, pigment content, and rapid A-C_i response (RACiR) assessments. Drought significantly decreased stomatal conductance with consequent declines in photosynthesis (A_N), actual quantum yield of PSII, and electron transport rate (ETR). Pigment contents were variable and depended on stress severity. Stomatal closure on stressed plants resulted in higher leaf temperatures, but F_v/F_m and PSII thermotolerance were only slightly affected by drought. A strong, hyperbolic relationship was observed between stomatal conductance, A_N, and ETR. However, when RACiR analysis was conducted, drought significantly decreased A_N at C_i values comparable to drought-stressed plants, indicating non-stomatal limitations to A_N. The maximum rate of carboxylation and maximum electron transport rate were severely limited by drought, and chloroplast CO₂ concentration (C_C) declined substantially under drought along with a comparable increase in partitioning of electron flow to photorespiration. Thus, while stomatal conductance may be a viable reference indicator of water deficit stress in peanut, we conclude that declines in A_N were largely due to non-stomatal (diffusional and metabolic) limitations. Additionally, this is the first study to apply the rapid A-C_i response method to peanut, with comparable results to traditional A-C_i methods.

Stomatal conductance as associated to vapor pressure deficit

Seasonal trends in leaf photosynthesis and stomatal conductance of drought stressed and nonstressed pearl millet as associated to vapor pressure deficit

Tewolde H., Dobrenz A. K., Voigt R. L., (1993)

Texas A&M University Agricultural Research and Extention Center, 1619 Garner Field Rd., 78801, Uvalde, TX, USA.

Photosynth Res. 38(1): 41-49 – doi: 10.1007/BF00015060 – PMID: 24317829 –

https://pubmed.ncbi.nlm.nih.gov/24317829/

Abstract

Single leaf photosynthesis (Pn) and stomatal conductance (Cg) of drought stressed and nonstressed pearl millet [Pennisetum americanum (L.) Leeke] were measured across growth stages to determine if a pattern exists in Pn and Cg during the growing season and to evaluate the influence of air vapor pressure deficit (VPDa) on the seasonal variations of Pn and Cg. Leaf photosynthesis and Cg were measured independently on pearl millet plants grown at the driest (drought stressed) and wettest (nonstressed) ends of a line-source irrigation gradient system. Well defined and predictable variations in both Pn and Cg were found across two growing seasons. Leaf photosynthesis of the nonstressed plants declined from a maximumof 25.8 μmol m(-2) s(-1) at the flag leaf emergence (48 days after planting, DAP) to a minimum of 14.5 μmol m(-2) s(-1) at physiological maturity. Stomatal conductance of the nonstressed plants peaked at the flowering and early grain fill stages and declined as plants approached maturity. In contrast, Pn and Cg of the stressed plants declined from a maximum at flag leaf emergence to a minimum at flowering and increased as plants approached maturity. High VPDa during the flowering and grain fill stages induced stomatal closure and decreased Pn in the stressed plants. High mid-season VPDa did not induce stomatal closure and did not reduce leaf photosynthesis in nonstressed plants. The lack of sensitivity of Pn to VPDa in the nonstressed treatment suggests large air VPD such as that prevalent in southern Arizona does not limit the growth of irrigated pearl millet by limiting CO2 assimilation.

The relative sensitivity of stomata to changes in VPD was closely related to the weighted stomatal density or ‘crowding index’

Stomatal response to air humidity and its relation to stomatal density in a wide range of warm climate species

El-Sharkawy M. A., Cock J. H., Hernandez A. D. P. (1985)

M A El-Sharkawy ¹, J H Cock, A Del Pilar Hernandez,

• Centro Internactional De Agricultura Tropical, A.A. 6713, CALI, Colombia.

• PMID: 24443083

===

Photosynth. Res. 7: 137–149 – doi: 10.1007/BF00037004 –

https://pubmed.ncbi.nlm.nih.gov/24443083/

Abstract

The gas exchange of 19 widely different warm climate species was observed at different leaf to air vapour pressure deficits (VPD). In all species stomata tended to close as VPD increased resulting in a decrease in net photosynthesis. The absolute reduction in leaf conductance per unit increase in VPD was greatest in those species which had a large leaf conductance at low VPDs. This would be expected even if stomata of all species were equally sensitive. However the percentage reduction in net photosynthesis (used as a measure of the relative sensitivity of stomata of the different species) was also closely related to the maximal conductance at low VPD. Similarily the relative sensitivity of stomata to changes in VPD was closely related to the weighted stomatal density or ‘crowding index’.The hypothesis is presented that stomatal closure at different VPDs is related to peristomatal evaporation coupled with a high resistance between the epidermis and the mesophyll and low resistance between the stomatal apparatus and the epidermal cells. This hypothesis is consistent with the greater relative sensitivity of stomata on leaves with a high crowding index.The results and the hypothesis are discussed in the light of selection, for optimal productivity under differing conditions of relative humidity and soil water availablility, by observation of stomatal density and distribution on the two sides of the leaf.

Stylites derives nearly all of its photosynthetic carbon through its roots

Stylites, a vascular land plant without stomata absorbs CO2 via its roots

Keeley J. E., Osmond C. B., Raven J. A. (1984)

Jon E. Keeley, C. Barry Osmond, John A. Raven,

Nature 310: 694–695 – https://doi.org/10.1038/310694a0 –

https://www.nature.com/articles/310694a0#citeas

Abstract

Photosynthetic organs of most higher plants normally have access to atmospheric CO₂ through stomatal pores which also serve as variable valves to control the loss of H₂O vapour which accompanies CO₂ uptake¹. The acquisition of stomata is commonly thought to have been a crucial development permitting ‘conquest’ of land and direct access of plants to atmospheric CO₂. Only in desert stem succulents during drought do stomata remain so tightly closed in the light that the photosynthetic tissues are dependent on internal CO₂ generated through the photosynthetic pathway known as crassulacean acid metabolism². Functional stomata are absent in submerged aquatic plants and in non-vascular land plants (for example, mosses) which are normally covered by a water film. Although it is now clearly established that some aquatic plants assimilate large amounts of CO₂ from the sediment via roots^3–5, terrestrial plants are thought to assimilate only insignificant amounts of CO₂ via this path⁶. Here we report on a terrestrial plant, Stylites andicola, which lacks stomata and is unable to exchange gas with the aerial atmosphere. Rather, it derives nearly all of its photosynthetic carbon through its roots. In addition, this species possesses characteristics of crassulacean acid metabolism.

Significant correlations have been obtained between stoma number and seta length, and stoma size and epidermal cell size in mosses

On the stomata of some tropical African mosses

Egunyomi A. (1982)

Lindbergia 8: 121–124 –

https://www.jstor.org/stable/20149430

Abstract

The occurrence of stomata in 29 tropical African moss species representing 12 families is reported. The stomata (22-51 μm × 22-29 μm) are mostly round-pored with two guard-cells each, ranging from 2 to more than 200 per capsule. In Wijkia trichocoleoides (C. Muell.) Crum, Trichosteleum microcalyx Ren. & Card., Stereophyllum radiculosum (Hook.) Mitt. and Stereophyllum virens Card. stomata are raised above the level of epidermis but are sunken in Brachymenium leptophyllum C. Muell.) Jaeg. and Bryum coronatum Schwaegr. Significant correlations have been obtained between stoma number and seta length, and stoma size and epidermal cell size. 

The morphological features of the stomata in ten moss species have not any taxonomical value

Observations on the stomatal complex in ten species of mosses (Pottiaceae, Bryopsida)

Abella L., Alcalde M., Estébanez B., Cortella A., Alfayate C., Ron E., (1999)

• L. ABELLALaboratorio de Etnobotánica y Botánica Aplicada (L.E.B.A.), Facultad de Ciencias Naturales y Museo. Universidad Nacional de La Plata
• M. ALCALDEDpto. Biología Vegetal I, Facultad de Biología, Universidad Complutense
• B. ESTÉBANEZDpto. Biología Vegetal I, Facultad de Biología, Universidad Complutense
• A. CORTELLALaboratorio de Etnobotánica y Botánica Aplicada (L.E.B.A.), Facultad de Ciencias Naturales y Museo. Universidad Nacional de La Plata
• C. ALFAYATEDpto. Biología Vegetal I, Facultad de Biología, Universidad Complutense
• E. RONDpto. Biología Vegetal I, Facultad de Biología, Universidad Complutense

===

J. Hattori Bot. Lab. 86: 179–185 – doi: 10.18968/jhbl.86.0_179 –

https://www.jstage.jst.go.jp/article/jhbl/86/0/86_179/_article

Abstract

The stomatal complex in Bryophyta is morphologically more uniform than in vascular plants. Nevertheless there are many species without data about the morphology, physiology and ontogeny of their stomata. We present here a morphological and histochemical study of the stomatal complex in ten species of Crossidium, Didymodon, Pottia and Tortula (Pottiaceae).The number of stomata per capsule, their size, orientation, location, neighbouring cells, morphological type and the results of the histochemical tests for the guard and adjacent cells for starch, callose, cellulose and pectin are described. The results suggest that the morphological features of the stomata in these ten species have not any taxonomical value.