Plants medium sized, forming tufts or loose mats. Stems procumbent, creeping or erect, arcuate, stoloniform sterile basal branches with rhizoids, fertile stems often radiculose to tomentose; in cross-section central strand well developed. Leaves spirally arranged when erect, when procumbent appearing complanate-foliate, unistratose, undulate or plain, crisped when dry, obovate to oblong or elliptical, apex acute to obtuse-rounded and mucronate or cuspidate, base narrowly decurrent; margins limbate, singly serrate. Nerve single, ending in apex to percurrent. Laminal cells smooth, upper and median cells hexagonal-rounded to subquadrate, collenchymatous; juxtacostal cells often larger, basal cells rectangular-rounded or oblong; marginal cells linear, forming a border of 2 to several rows. Dioicous or synoicous. Perichaetia terminal. Seta single or clustered, elongate, smooth. Capsule exserted, pendent, obloid to obloid-cylindrical, neck short. Operculum conic-long rostrate, oblique. Peristome double, exostome reddish-orange, teeth 16, finely papillose; endostome basal membrane high, segments 16, keeled, finely papillose, cilia 3 (2-4), nodose. Calyptra cucullate, smooth and naked. Spores spherical, finely papillose..

Habitat. On soil, humus and decaying logs, epiphytic on larger branches, on tree bases, less often on trunks; on rocks or in damp crevices; in humid often shaded sites in montane to high montane forests, occasionally on roadside banks, 1300--3480 m.

Discussion. Past reports of Mnium in tropical Africa are in error (Mnium is distinguished by paired teeth on leaf margins). The three species can be separated as follows: Plagiomnium cuspidatum (Hedw.) T.J.Kop. is known from Uganda and Kenya, and is distinguished from all other species by having teeth that extend from mid-leaf to the apex only (in all other species the teeth extend from the leaf base to the apex). Plagiomnium undulatum (Hedw.) T.J.Kop. is known from high elevations of Ethiopia, and is the only dioicous species in tropical Africa and the only species to display an erect to arcuate habit. Plagiomnium rhynchophorum (Hook.) T.J.Kop. has sterile creeping shoots, but (like P. undulatum) has undulate, narrow elliptic to lingulate leaves. It differs from P. undulatum in smaller leaf cells, a costa that ends at or somewhat below the apex (in P. undulatum the costa is excurrent) and the marginal teeth are small and often blunt. Two varieties are recognised, var. rhynchophorum and var. reidii (for the differences see Koponen, 1981). In tropical Africa the typical variety seems to be restricted to the higher mountains in the west, east and central part of the continent, and to Madagascar and the Mascarenes; variety reidii occurs in southern Africa and the east African mountains.

Literature. Koponen, T. 1980. A synopsis of Mniaceae (Bryophyta). IV. Taxa in Europe, Macaronesia, NW Africa and the Near East. Annales Botanici Fennici 17: 125-162. Koponen, T. 1981. A synopsis of Mniaceae (Bryophyta). V. Taxa in Africa south of the Sahara. Annales Botanici Fennici 18: 105-111. Koponen, T. 1992. Miscellaneous notes on Mniaceae (Bryophyta). XVI. Plagiomnium undulatum (Hedw.) T. Kop. in Ethiopia. Bulletin du Jardin Botanique de l'Etat 62: 397-402 [includes a key to all sub-Saharan taxa].