Plagiotheciaceae (Brotherus) M. Fleischer
Robert R. Ireland
Plants somewhat small to robust, in loose to dense mats or tufts. Stems prostrate, mostly complanate but sometimes subjulaceous or erect and julaceous, simple or sparsely and irregularly branched; exterior layer of cortical cells large and thin-walled; rhizoids smooth or minutely papillose, sometimes present just distal to leaf insertion; pseudoparaphyllia lacking. Leaves of stems and branches similar, flaccid, imbricate to distant, sometimes second with apices pointing toward substratum, smooth, erect or spreading, scarcely altered when dry, decurrent, often concave, ovate, ovate-lanceolate, oblong-lanceolate or oblong-ovate, acute, acuminate, rarely piliferous, symmetric or often asymmetric; margins plane or recurved, entire or serrulate to serrate at extreme apex; costa short and double, one branch sometimes reaching to midleaf and costa appearing single, or rarely ecostate; rhizoids at base on abaxial surfaces or adjacent cells, sometimes at leaf tips, smooth; cells smooth, walls usually with few pits at base, rarely pitted to leaf middle; median and distal cells linear, linear-flexuose or sometimes linear-rhomboidal, mostly thin-walled and heavily chlorophyllose, smooth; alar cells differentiated, equally distributed on either side of the costa, extending to decurrencies, the walls ± evenly thickened. Specialized asexual reproduction often present as propagula, cylindrical to fusiform, uniseriate, smooth walled, borne on branched stalks clustered in leaf axils, each propagulum consisting of several cells, or sometimes borne in clusters without stalks on abaxial surface of leaf. Sexual condition autoicous, less commonly dioicous. Seta solitary, smooth, elongate, straight, twisted, curved or rarely circinate. Capsule cernuous or erect, rarely pendulous, straight or arcuate; exothecial cells thin- or thick-walled; annulus present, often persistent; operculum conic to rostrate, shorter than urn; annulus differentiated in 1--3 rows, deciduous, sometimes tardily so; peristome double, exostome teeth on exterior surface cross-striolate proximally, papillose distally, or rarely papillose throughout, projecting on interior surface; endostome with a medium to high basal membrane, the segments broad or narrow, keeled, not or very narrowly perforate, shorter than or almost as long as teeth, the cilia well developed to lacking. Calyptra cucullate, smooth, naked, fugacious. Spores spherical to ovoid, smooth or papillose.
Genus 1, species ca. 80--90 (6 in the flora); primarily in temperate latitudes and at higher elevations in the tropics; worldwide, including Antarctica.
Although I (W. R. Buck and R. R. Ireland 1985) presently take the view of including only one genus, namely Plagiothecium, in the Plagiotheciaceae, several other bryologists have included other genera, especially those in the Hypnaceae, in the family, e.g., E. Nyholm (1965), A. J. E. Smith (1978), S.-H. Lin (1984), L. Hedenäs (1989, 1995) and N. Pedersen and L. Hedenäs (2002). Plagiothecium undulatum (Hedwig) Bruch, Schimper & W. Gümbel has recently been described as a new genus, Buckiella by R. R. Ireland (2001) and placed in the Hypnaceae.
Buck, W. R. and R. R. Ireland. 1985. A reclassification of the Plagiotheciaceae. Nova Hedwigia 41: 89--125. Hedenäs, L. 1995. Higher taxonomic level relationships among diplolepidous pleurocarpous mosses- a cladistic view. J. of Bryology 18: 723--781. Pederson, N. and L. Hedenäs. 2002. Phylogeny of the Plagiotheciaceae based on molecular and morphological evidence. Bryologist 105(3): 310--324.