Plants: in dense or open turfs, red, pink, green, or yellow-green. Stems: 0.5–3 cm, fertile stems comose, innovations evenly foliate. Leaves: shiny yellow-green to red or pink, crowded, strongly contorted to shrunken when dry, ovate-lanceolate, flat, 0.5–3 mm, often gradually enlarged toward stem apex; base usually green, not decurrent; margins revolute proximally, limbidium strong, in 2 or 3 rows; apex acute to acuminate; costa excurrent to long-excurrent, awn smooth or denticulate; proximal laminal cells long-rectangular, 3–5:1; medial and distal cells 15–25 µm wide, 2–3:1, walls thin to moderately thick. : Specialized asexual reproduction absent. Sexual: condition polyoicous, variable, often populations either synoicous or only archegoniate. Seta: brown or red-brown, 1–3 cm, slender, straight to somewhat flexuose. Capsule: brown, shape highly variable, elongate-pyriform to clavate, symmetric to somewhat curved, 3–5 mm, mouth orange-brown to red; hypophysis slender; operculum short-conic, weakly apiculate; peristome variable, reduced; exostome teeth yellow or brown, hyaline distally, lamellae straight, sometimes with small pores near base along mid line; endostome weakly adherent to exostome, basal membrane low to 1/2 exostome height, segments with slitlike perforations, cilia absent or rudimentary. Spores: with size often variable in same capsule, 24–30(–32) µm, yellow. Phenology: Capsules mature Jul–Aug.

Moist soil banks, wet soil. low to high elevations (0-4500 m). Greenland, Alta., B.C., Man., Nfld. and Labr., Que., Alaska, Calif., Mont., Nev., Oreg., Wash., Wyo., Eurasia.

Ptychostomum arcticum is one of the most variable and confusing species in the genus. Much of this variation has been described as a series of very closely related species, including P. bryoides and P. purpurascens, differing in minor details of the leaves and capsule. D. T. Holyoak (2004) synonymized several of these segregates with P. arcticum, which is here tentatively accepted. The segregates that have been reported from the flora area are distinguished below based on the work of E. Nyholm (1986+, fasc. 3). Their distribution and ecology remain poorly understood, but P. purpurascens is reported from arctic North America, while P. bryoides is reported from Greenland. Capsules are needed for identification. Ptychostomum purpurascens is immediately distinguished by the distal laminal cells 22–35 µm wide, capsules elongate-pyriform and symmetric, exothecial cells transversely rectangular, in 2–4 rows, and exostome teeth with small holes present along the mid line at base. Ptychostomum arcticum and P. bryoides contrast by distal laminal cells 17–24 µm wide; capsules clavate to ovate, symmetric or curved, exothecial cells quadrate, in 2–6 rows, and exostome teeth lacking small holes.