Stomata in Equisetum and Psilotum

Photo credit: AOB

Drawings of Equisetum stomata. (A) E. palustre; fig. 10 from Duval-Jouve (1864). (B) E. fluviatile; fig. 12 from Riebner (1925).

The remarkable stomata of horsetails (Equisetum): patterning, ultrastructure and development

byErin Cullen,

Paula J. Rudall.paula-rudall-cropped


in Ann Bot (2016)doi: 10.1093/aob/mcw094 –

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). –


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.

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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 Results Stomatal 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.

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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.

Screen Shot 2018-04-18 at 20.10.12
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).

Published by

Willem Van Cotthem

Honorary Professor of Botany, University of Ghent (Belgium). Scientific Consultant for Desertification and Sustainable Development.

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