Ann. Bot. Fenn. 5: 145, figs. 14, 15, 20, 31, 32, 35, 36, 41, 43, 47, 60, 69, 78, 81, 98, 100. 1968.
Plants (1–)2–5(–10) cm, dense or open mats. Stems green or yellow-green, usually brownish with age; stems of two types, fertile stems erect, branching distally or not, dendroid or not, sterile stems plagiotropic or arching, rarely erect, to 20 cm; rhizoids brown, macronemata mainly proximal, occasionally along underside of sterile stems, micronemata present. Leaves green or yellow-green, rarely black with age, variously crisped or contorted when dry, erect-spreading, usually flat, occasionally irregularly wavy or transversely undulate when moist, elliptic, obovate, oblong, oblong-lingulate, oblong-elliptic, or rarely orbicular or diamond-shaped, 1–14 mm; base decurrent or not; margins plane, green or yellow-green, 1-stratose, toothed, often to near base, sometimes only distally, rarely entire, teeth single, sharp or blunt, of 1–2(–4) cells, rarely hooked; apex acute, acuminate, obtuse, rounded, truncate, retuse, or emarginate, mucronate, apiculate, or cuspidate, cusp toothed or not, rarely bent to one side; costa percurrent, excurrent, or rarely subpercurrent, distal abaxial surface smooth; medial laminal cells short-elongate, elongate, or ± isodiametric, (15–)30–70(–85) µm, sometimes in diagonal or longitudinal rows, collenchymatous or not, walls pitted or not; marginal cells differentiated, linear or rhomboidal, in 2–4(–5) rows. Specialized asexual reproduction usually absent (by stolonlike stems in P. undulatum). Sexual condition synoicous or dioicous. Seta single or multiple, yellow, yellow-green, brown, sometimes reddish or greenish, rarely orange, dark red, or blackish with age, 1.2–5 cm, straight to flexuose. Capsule horizontal to pendent, yellow or yellowish brown, cylindric, oblong, oblong-cylindric, obovoid, or ovoid, 1.5–5 mm; operculum conic-apiculate or rostrate; exostome yellow or brown; endostome yellow to yellowish brown. Spores 18–40 µm.
Species 26 (11 in the flora).
Plagiomnium is characterized by singly serrate, 1-stratose leaf margins, the absence of red stem tissue, and, in most species, the production of long sterile plagiotropic or arching stems. Plagiotropic stems are absent in P. venustum and some fertile patches of a few other species, in particular P. insigne. In many Plagiomnium populations, fertile stems are absent with plagiotropic stems producing vegetative mats over and among litter and other mosses. Morphological variation is common in Plagiomnium and has led to misidentifications in regional herbaria. Many characters that have been used in extant keys, such as sharpness, size, and cell composition of marginal teeth, presence and length of leaf decurrencies, and degree of laminal cell pitting, can vary within and between populations. Extant keys often use sexuality to separate species; unfortunately, many collections are sterile, and this character is of little use. As with Brachythecium, sterile collections of Plagiomnium may be unidentifiable, especially weakly developed plants from wetland habitats. Based on the review of the collections examined for this treatment, more species are likely present within the range of the flora, and a thorough review of North American Plagiomnium is recommended.
Three species, Plagiomnium carolinianum, P. rostratum, and P. undulatum, have rostrate opercula; the remaining species have conic-apiculate opercula. Plagiomnium includes four species endemic to the flora area, P. carolinianum, P. ciliare, P. floridanum, and P. venustum, and one apparently introduced species, P. undulatum.
|1||Leaves usually oblong-lingulate or lingulate, 5-14 mm, transversely undulate when moist||> 2|
|1||Leaves usually elliptic, ovate, or obovate, often less than 6 mm, usually flat, rarely weakly transversely undulate when moist||> 3|
|2||Leaves 5-8(-10) mm; apices retuse or emarginate, rarely truncate or rounded, usually short-mucronate; marginal teeth usually blunt; e North America.||Plagiomnium carolinianum|
|2||Leaves 6-10(-14) mm; apices obtuse or rounded, occasionally acute, usually cuspidate; marginal teeth usually sharp; w North America.||Plagiomnium undulatum|
|3||Leaf margins toothed from about mid leaf to apex, teeth sharp; leaves obovate, sometimes diamond-shaped or elliptic||> 4|
|3||Leaf margins toothed from apex to near base and teeth sharp, or toothed mainly distally and teeth usually blunt; leaves never diamond-shaped, usually elliptic, rarely obovate||> 6|
|4||Medial laminal cells 30-50(-60) µm, not or weakly collenchymatous; setae often multiple.||Plagiomnium drummondii|
|4||Medial laminal cells 20-30(-40) µm, strongly collenchymatous; setae single||> 5|
|5||Leaves obovate, ± diamond-shaped or occasionally elliptic; bases broadly decurrent; medial laminal cells 20-30(-40) µm; sexual condition synoicous.||Plagiomnium cuspidatum|
|5||Leaves elliptic; bases narrowly decurrent; medial laminal cells 20-30 µm; sexual condition dioicous.||Plagiomnium floridanum|
|6||Plagiotropic sterile stems absent; leaves often densely twisted distally around stem when dry; laminal cells strongly collenchymatous, walls not pitted; capsule necks distinct, brown, often wrinkled.||Plagiomnium venustum|
|6||Plagiotropic sterile stems usually present; leaves crisped and contorted, not twisted around stem when dry; laminal cells collenchymatous or not, walls pitted or not; capsule necks not distinct||> 7|
|7||Leaf apices often rounded, retuse, or emarginate; leaves broadly elliptic, ovate, orbicular, or oblong-elliptic; bases not or short-decurrent; marginal teeth blunt, usually of 1 cell (sometimes margins entire)||> 8|
|7||Leaf apices usually acute or obtuse, sometimes rounded, rarely retuse; leaves elliptic, narrowly elliptic, occasionally ovate, oblong, obovate, or oblong-elliptic; bases long-decurrent; marginal teeth sharp or blunt, of 1-3(-4) cells||> 9|
|8||Medial laminal cells usually elongate, (30-)50-65(-85) µm, distinctly smaller near margins, not or weakly collenchymatous, walls pitted, pits sometimes indistinct; sexual condition dioicous.||Plagiomnium ellipticum|
|8||Medial laminal cells short-elongate or ± isodiametric, 22-35(-45) µm, slightly smaller near margins, strongly collenchymatous, walls not pitted; sexual condition synoicous.||Plagiomnium rostratum|
|9||Leaf marginal teeth often blunt, of (1-)2-3(-4) cells; leaf apices rounded, occasionally truncate or obtuse, rarely retuse.||Plagiomnium ciliare|
|9||Leaf marginal teeth usually sharp, of 1-2(-3) cells; leaf apices usually acute, acuminate, or obtuse, sometimes rounded, or rarely retuse||> 10|
|10||Medial laminal cells usually short-elongate or ± isodiametric, somewhat smaller near margins to about 1/2 size; sexual condition dioicous; w North America.||Plagiomnium insigne|
|10||Medial laminal cells usually elongate, short-elongate, or occasionally ± isodiametric, less than 1/2 size near margins; sexual condition synoicous; widespread.||Plagiomnium medium|