The science of the stomata of plants: a continuously growing list of references, abstracts and illustrations, helping researchers to data on publications.
As the most basal monilophytes, eusporangiate ferns can provide key insights into the origins of plant physiological adaptations. The genus Equisetum, the most morphologically and physiologically unusual genus of eusporangiate ferns, has a stomatal apparatus that is unique among all plants.
Patterns of stomatal diffusive conductance (gw) were measured in the giant horsetail, Equisetum giganteum L. in southern South America. Maximum gw values (<200 mmol·m−2·s−1) were low in comparison with typical angiosperm leaves, but were in the range measured in other pteridophytes. The range of measured gw was similar in contrasting environments of the Atacama Desert and northwestern Argentina.
Stems in shade had a significantly lower gw than those in light. Developing stems had a higher average gw than mature stems. Stomatal conductance was higher for upper stem internodes than for middle internodes. Late-morning gwwas primarily related to stem diameter, stem surface temperature, and interactions among these factors and vapor pressure deficit (VPD), light, elevation, and groundwater salinity.
Equisetum giganteum likely has a passive system of stomatal regulation depending on overall stem turgor and red light. The stomatal conductance of patterns of this species exhibited a diurnal pattern typical of other pteridophytes, despite its unusual structure.
Equisetum myriochaetum (A, B, LM; C, DIC; E, F, SEM). (A) Thin paradermal section of a mature stoma showing radiating ribs on subsidiary cells. (B) Thick paradermal section of a mature stoma with radiating ribs. Both guard cells and superadjacent subsidiary cells are visible. (C) Oblique view of a mature stoma showing radiating ribs. (D) Transverse section of a mature sunken stoma, showing silica on the surface of subsidiary cells. (E) Macerated stoma showing radiating ribs. gc, guard cell; gcn, guard cell nucleus; rr, radiating ribs; sc, subsidiary cell; sc, silica. Scale bars: 10 μm in (A−D), 5 μm in (E). – http://aob.oxfordjournals.org/content/early/2016/06/03/aob.mcw094/F3.small.gif
Abstract
Background and Aims The stomata of Equisetum – the sole extant representative of an ancient group of land plants – are unique with respect to both structure and development, yet little is known about details of ultrastructure and patterning, and existing accounts of key developmental stages are conflicting.
Methods We used light and electron microscopy to examine mature stomata and stomatal development in Equisetum myriochaetum, and compared them with other land plants, including another putative fern relative, Psilotum. We reviewed published reports of stomatal development to provide a comprehensive discussion of stomata in more distantly related taxa.
Equisetum myriochaetum (TEM). (A, B) Transverse sections of mature stomata. (C) Paradermal view of a mature stomatal complex with radiating ribs; parts of both guard cells and superadjacent subsidiary cells are visible in this plane of the section, which lies below the outer ledges that delimit the pore. (D) Detail of interlocking outer cuticular ledges on subsidiary cells, and thinner ledges on guard cells below. (E) Transverse section of a young stoma. (F) Transverse section of a mesophyll cell below the stoma. chl, chloroplast; gc, guard cell; icl, inner cuticular ledge (on guard cells); ocl, outer cuticular ledge (on subsidiary cells); rr, radiating ribs; sc, subsidiary cell. Scale bars: 10 μm in (A–C), 2 μm in (D−F).
Key ResultsStomatal development in Equisetum is basipetal and sequential in strict linear cell files, in contrast with Psilotum, in which stomatal development occurs acropetally. In Equisetum, cell asymmetry occurs in the axial stomatal cell file, resulting in a meristemoidal mother cell that subsequently undergoes two successive asymmetric mitoses. Each stomatal cell complex is formed from a single precursor meristemoid, and consists of four cells: two guard cells and two mesogene subsidiary cells. Late periclinal divisions occur in the developing intervening cells.
Equisetum myriochaetum, stomatal development (A, C−F, LM; B, SEM; all images oriented with plant apex uppermost). (A) Composite image showing the series of developmental stages along a single axial stomatal cell file. (B) Series of developmental stages in surface view, increasingly sunken towards the apex. (C) Longitudinal section of a stem with fully differentiated stomata arrowed; less well-developed stomata are closer to the internode. (D) Undifferentiated cells in a stomatal cell file, close to the internode; meristemoids are slightly larger than intervening cells. (E) Later stages of development, showing initial asymmetric cell division and the resulting pair of cells. (F) Later stages of development, showing the second asymmetric cell division and resulting triad. (G) Differentiated stomatal complex. gc, guard cell; gmc, guard mother cell; ic, intervening cell; m, meristemoid; sc, subsidiary cell, st, stoma. Scale bars = 20 μm in (A), 100 μm in (B), (C), 7.5 μm in (D), (F), (G).
Conclusions In addition to the unique mature structure, several highly unusual developmental features include a well-defined series of asymmetric and symmetric mitoses in Equisetum, which differs markedly from Psilotum and other land plants. The results contribute to our understanding of the diverse patterns of stomatal development in land plants, including contrasting pathways to paracytic stomata. They add to a considerable catalogue of highly unusual traits of horsetails – one of the most evolutionarily isolated land-plant taxa.
Stomata of Psilotum (A, B, E, F, G, H, P. nudum; C, D, P. intermedium). (A, B) P. nudum, transverse section of a mature stem with detail of a stoma in (B) (C) P. intermedium, transverse section of a mature stoma. (D) P. intermedium, LM stem surface. (E) P. nudum, SEM stem surface. (F) P. nudum, paradermal section of the epidermis with guard mother cells and a recently divided stoma. (G) P. nudum, TEM paradermal section of a mature stoma. (G) P. nudum, TEM transverse section of a mature stoma (slightly off-centre, since most stomata are not quite parallel with the axis). chl, chloroplast; gc, guard cell; m, meristemoid; st, stoma. Scale bars = 50 μm in (A), 10 μm in (B), (C), (G), (H); 100 μm in (D), (E); 25 μm in (F).
Light and scanning electron microscopic studies and guard cell isolation techniques have confirmed the well-known ridges of the Equisetumstomatal apparatus as belonging to the subsidiary cells.
Hitherto unknown features of the subsidiary cells such as the presence of a concentrated H2SO4-resistant region on the ridges and an interlocking mechanism for the closure of the aperture of the subsidiaries are described. These are presented as further evidence for the differences between the two subgenera of Equisetum.
Boiling in dilute NaOH is shown to be a simple but effective means for the isolation of the guard cells in Equisetum as well as in several other plants with sunken stomata.
Silicification of the outer layer of the epidermis makes cuticular isolation a difficult process by usual methods (treatments with enzymes or cellulose hydrolyzing reagents).
Treatment with concentrated sulfuric acid followed by hydrofluoric acid results in the isolation of the cuticular membranes in Equisetum spp. and in similarly silicified grasses. Involvement of potassium ions in stomatal movements is indicated for two ferns and suggested for two species of Equisetum.
Dayanandan (1977) observed that Equisetum species “possess perhaps the most structurally complex stomata in the entire plant kingdom” (p. 175). The stomata of Equisetum are so unique that “a single well-preserved stomatal apparatus is all that is needed to identify the genus Equisetum (even the two subgenera) from among all other living plants” (Dayanandan, 1977). The uniqueness of Equisetum stomata is the result of two characteristics (Dayanandan, 1977):
1.) The two subsidiary cells overlie the guard cells completely, whereas in other plants the guard cells are the superficial cells.
2.) “The inner tangential wall of each subsidiary cell develops 7 to 24 ridge-like thickenings, a feature not found in any other genus.” (Dayanandan, 1977)
These unique features are nicely shown in illustrations from the plates accompanying Milde (1867) of:
1. a stoma of E. giganteum (subgenus Hippochaete)
2. a stoma of E. bogotense (subgenus Equisetum) note: The subsidiary cells are illustrated transparently so that the kidney shaped guard cells can be seen beneath.
See also:
• Dayanandan P. and Kaufman P. B. 1973. Stomata in Equisetum. Canadian Journal of Botany 51:1555-1564.
• Dayanandan P. 1977. Stomata in Equisetum: A structural and functional study. Doctoral Dissertation (Botany). University of Michigan.
• Hauke R. L. 1957. The stomatal apparatus of Equisetum . Bulletin of the Torrey Botanical Club 84:178-181.
• Mehra P.N., and Soni S.L. 1983. Stomatal patterns in pteridophytes – an evolutionary approach. Proceedings of the Indian National Science Academy, Part B, Biological Sciences 49(2):155-203,
SEM micrograph of the cell surface structures of the common Horsetail (E. arvensis). In the middle of the figure, a stomata is shown. The stomatal cells and the surrounding cells are convex and have a micropattern of some small enhanced spots on the cells. The latter are formed by subcuticular inserts of bioactive silicon oxide crystals, which provide a defence mechanism. The surface consists of a three-dimensional wax layer producing a hierarchical structure (Koch et al. 2008).
Stomata in Equisetum
by Dayanandan P., Kaufman P. B. (1973)
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in Canadian Journal of Botany 51(9): 1555-1564, DOI: 10.1139/b73-198
ABSTRACT
Light and scanning electron microscopic studies and guard cell isolation techniques have confirmed the well-known ridges of the Equisetum stomatal apparatus as belonging to the subsidiary cells. Hitherto unknown features of the subsidiary cells such as the presence of a concentrated H2SO4-resistant region on the ridges and an interlocking mechanism for the closure of the aperture of the subsidiaries are described. These are presented as further evidence for the differences between the two subgenera of Equisetum. Boiling in dilute NaOH is shown to be a simple but effective means for the isolation of the guard cells in Equisetum as well as in several other plants with sunken stomata. Silicification of the outer layer of the epidermis makes cuticular isolation a difficult process by usual methods (treatments with enzymes or cellulose hydrolyzing reagents). Treatment with concentrated sulfuric acid followed by hydrofluoric acid results in the isolation of the cuticular membranes in Equisetum spp. and in similarly silicified grasses. Involvement of potassium ions in stomatal movements is indicated for two ferns and suggested for two species of Equisetum.
Fairchild Tropical Botanic Garden
Arturo Prat University, Iquique 
National University of Tucuman, San Miguel de Tucumán
PLANT STOMATA ENCYCLOPEDIA 
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