7. Dichodontium Schimper, Coroll. Bryol. Eur. 12. 1856.
[Greek dicha, in two, and odontos, tooth, alluding to partially divided peristome teeth]
Patricia M. Eckel
Plants forming mats, cushions, or loosely caespitose, dull yellow or green to dark green. Stems 1-5(-8) cm, erect, simple or sparingly branched; sparsely tomentose or radiculose proximally. Leaves straight, not secund, more or less strongly and irregularly contorted or crisped with incurved apices when dry, the limb concave and erect-spreading to squarrose from an erect base when wet, narrowly to broadly oblong- or ovate-lanceolate or ligulate; apex acute to rounded-obtuse; margins involute-tubulose to plane or strongly recurved, rarely 2-stratose, distally irregularly and abruptly serrulate-dentate, with narrow, fragile decurrencies; apex acute to rounded-obtuse; costa narrow or broad, subpercurrent, sharply mammillose or densely covered with horned (columnar) papillae on both surfaces to nearly smooth, not rhizoidiferous, guide cells in 1 row, often individual cells in pairs, stereid bands 2 proximal to midleaf, adaxial band less developed, abaxial band strong, adaxial and abaxial epidermis distinct; all cells not pitted; medial laminal cells rounded-quadrate to short-rectangular, 1(-2):1, sharply to bluntly mammillose on both surfaces or merely convex near apex, or densely covered with horned (columnar) papillae on both surfaces, sometimes 2-stratose near costa, walls evenly thickened; alar cells not differentiated in size or color; basal cells rectangular, 3(-5):1. Specialized asexual reproduction occasional, by multicellular gemmae borne on branching stalks in leaf axils. Sexual condition autoicous or dioicous; perigonial buds below the perichaetium and sessile or terminal on male plants, perigonial leaves ovate, concave, short-acuminate; perichaetial leaves similar to distal cauline leaves. Seta single, yellow at first, becoming red-brown, 4-20 mm, erect. Capsule erect to weakly inclined, exserted, oblong-ovate, straight, arcuate to zygomorphic, with or without struma, smooth or irregularly furrowed-rugose when dry; operculum obliquely long-rostrate, usually as long as urn; basal membrane low, projecting above the mouth; peristome teeth 16, divided to the middle into 2 divisions, dark red, vertically pitted-striolate basally, papillose distally; annulus none. Calyptra cucullate, somewhat roughened at apex, covering capsule to just below the mouth. Spores 12-18(-20) µm, smooth or minutely papillose.
Species 2 (2 in the flora): Northern Hemisphere, largely in temperate and subarctic areas.
Dichodontium had eight recognized taxa world-wide before J.-P. Frahm et al. (1998) reduced the number to three. The number declined to two when D. brasiliense Brotherus was synonymized with Chrystoblastella chilensis (Montagne) Reimers of the Ditrichaceae (R. Ochyra 1999; P. Sollman 1999), reinforcing the possibility that the genus may be artificial. Dichodontium flavescens is here considered to be a synonym of D. pellucidum (see discussion below). The morphologically similar Cynodontium differs from Dichodontium by its autoicous sexual condition (Dichodontium may be autoicous or dioicous) with leaves narrowly lanceolate, apex acute, and keeled, not tubulose. The dry capsule of all species of Cynodontium is furrowed with regular striations but in Dichodontium the dry capsules are either smooth or irregularly furrowed-rugose. The distinctly tall, conical papillae in some species of Cynodontium seem to relate the two genera; the papillae separate both genera from the smooth-celled Dicranella and Rhabdoweisia. Species of Dichodontium are hygrophilic mosses on soil banks, soil over cliffs and in other rocky habitats.
The color of Dichodontium ranges from a dark, pellucid, emerald green in smaller plants, to a more general sordid yellowish to orange-green. The species have stems in section with distinct central strands, enlarged cells in the stem cortex and smaller thick-walled epidermal cells in 1-2 rows, and no hyalodermis. The rhizoids are smooth, the leaves slightly undulate. The leaf section is not well differentiated into distinct layers, especially distal to midleaf where the adaxial stereid band usually vanishes, while the abaxial one disappears farther towards the apex. Both epidermal layers together with the guide cells often form an undifferentiated cylinder distally. This condition may also be seen in Cynodontium.
A. J. E. Smith (2004) included Dicranella palustris in Dichodontium, based on molecular studies that indicate a close relationship with Dichodontium pellucidum, citing the work of M. Stech (1999). The two species have a superficial similarity morphologically: a generally short, oblong-lanceolate leaf (the limb of Dicranella palustris strongly squarrose from an erect base), nearly identical capsule and peristome, the leaf apex rounded-obtuse, a long leaf decurrency, stem central strand present, and heterogeneous costal anatomy in section, as in small leaves of Dichodontium pellucidum. However, in Dicranella palustris the leaf cells are elongate (7-12:1) to the leaf tip with short- to long-rectangular cells only on the leaf margins, the leaves are flaccid, and the cell walls lax, somewhat inflated in section, and smooth. Many other characters are also those associated with the genus Dicranella: dioicous sexuality, capsule shape and peristome divided to the middle, vertically pitted-striolate, and strongly squarrose leaves. Many species in Dicranella have rhizoidal gemmae, as has Dicranella palustris, rather than those located on the stem, as in Dichodontium pellucidum. Morphologically Dicranella schreberiana is difficult to distinguish from Dicranella palustris and further genetic analysis may relate that species with Dichodontium pellucidum as well. Placement of Dicranella palustris in the genus Dicranella is retained here pending further study.
SELECTED REFERENCES
Frahm, J.-P. et al. 1998. Revision der Gattung Dichodontium (Musci, Dicranaceae). Trop. Bryol. 14: 109-118. Tan, B. C. and W. B. Schofield. 1980. On Dichodontium pellucidum and D. olympicum. Canad. J. Bot. 58: 2067-2072.
