Coroll. Bryol. Eur.,12. 1856 ,
Northern Hemisphere, largely in temperate and subarctic areas.
Species 2 (2 in the flora).
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.
|1||Autoicous; leaf margins mostly plane at base, margins prominently papillose near insertion; capsule strumose, irregularly furrowed when dry; marginal laminal cells strongly papillose by projecting distal cell ends to within 10 cells of the insertion, margins of leaf apex strongly papillose but without teeth; median leaf cells always with horned (sometimes branched) papillae on both surfaces; cells on adaxial surface of the costa quadrate or short-rectangular (1-2:1), papillose, scarcely different from adjacent laminal cells.||Dichodontium olympicum|
|1||Dioicous; leaf margins weakly to strongly recurved in proximal half, margins generally smooth near the insertion; capsule not strumose, smooth when dry; marginal laminal cells weakly papillose by projecting distal cell ends or smooth in proximal half of the leaf, leaf apex variously both papillose and toothed; median leaf cells with horned papillae on both surfaces varying to nearly smooth; cells on adaxial surface of the costa elongate (3-6:1), often smooth, clearly different from adjacent laminal cells.||Dichodontium pellucidum|