Terry T. McIntosh, Steven G. Newmaster

Plants (0.5-)2-4(-8) cm, in open to compact tufts or mats. Stems red, reddish brown, or brown, rarely yellowish brown, erect, simple or branched distally, not dendroid; rhizoids brown, macronemata mainly proximal, micronemata absent. Leaves green to dark green, sometimes yellowish, often crisped and contorted, sometimes undulate, sometimes spirally twisted when dry, patent, erect-spreading, or spreading, usually ± flat when moist, elliptic, obovate, oblong-ovate, ovate, ovate-elliptic, ovate- or elliptic-lanceolate, or obovate-spatulate, 1.5-6.5 mm, proximal leaves much smaller; base short- to long-decurrent; margins plane, green, reddish brown, or brown, 1- or 2-stratose, sometimes multistratose with stereid band, toothed distally or to below mid leaf, teeth usually paired and often sharp, sometimes single, small, and blunt, rarely rounded or indistinct; apex acute, obtuse, rarely rounded, or acuminate, apiculate or sometimes cuspidate, cusp often toothed; costa percurrent or excurrent, rarely subpercurrent or ending well before apex, distal abaxial surface smooth or toothed; medial laminal cells usually elongate or ± isodiametric, sometimes oblong, (10-)17-50 µm, sometimes in diagonal or longitudinal rows, collenchymatous or not, walls usually not pitted (pitted in M. arizonicum); marginal cells differentiated, linear, short-linear, or rhomboidal, in 1-4(-5) rows. Specialized asexual reproduction absent. Sexual condition synoicous or dioicous. Seta single, rarely double or triple, usually yellowish and becoming red from base with age, 1-5 cm, straight or flexuose. Capsule horizontal to pendent, yellow, yellow-brown, or brown, oblong-cylindric, 2-7 mm; operculum conic-mammillate or conic-rostrate; exostome brown, greenish yellow, yellowish brown, reddish brown, or purplish; endostome yellowish or brown. Spores 15-40 µm.

Species 19 (9 in the flora): North America, Mexico, Central America (Guatemala), South America, Eurasia, Africa; circumboreal.

In most cases, Mnium is distinguished from other genera of Mniaceae by the presence of paired teeth along the leaf margins. However, M. blyttii and M. stellare usually have single, indistinct marginal teeth, and leaves may lack teeth altogether. Trachycystis flagellaris also has paired marginal teeth but is readily distinguished by mammillose laminal cells. Depauperate plants of M. arizonicum and M. thomsonii, but also, in a few cases, M. marginatum, have leaves with only a few single, small teeth. Although Mnium is characterized by a wide range in morphological variation related to environmental conditions, plant age, and sexuality (T. J. Koponen 1974), most vigorous collections with well-developed leaves should be readily identified.

Blue postmortal color (mnioindigon) is often used to separate Mnium blyttii and M. stellare from other species of Mnium. Although often observed in younger collections when leaves are soaked only in water, it is most readily observed in older specimens if plants are dipped sequentially in alcohol (70-90%), 2% KOH, and water.

Lower Taxa

• Mnium arizonicum J. J. Amann
• Mnium blyttii Bruch & Schimper
• Mnium hornum Hedwig
• Mnium lycopodioides Schwagrichen
• Mnium marginatum (Dickson ex Withering) P. Beauvois
• Mnium spinosum (Voit) Schwagrichen
• Mnium spinulosum Bruch & Schimper
• Mnium stellare Hedwig
• Mnium thomsonii Schimper