Atrichum parallelum Mitten, J. Linn. Soc., Bot. 8: 48, plate 8. 1864; A. leiophyllum Kindberg

Plants pale- to dark olive green, in loose tufts. Stems 2-6(-8) cm, simple or branching by inno-vations, bracteate proximally, gradually larger distally. Leaves 5-6 mm, crisped and contorted when dry, when moist laxly spreading, plane, ovate-lanceolate to elliptic-oblong, abruptly acuminate at the apex, ending in a short spine, the margins coarsely and sharply serrate to the middle or below; costa percurrent or shortly excurrent in the apex, with a few low dentate abaxial lamellae or lamelliform teeth near the apex; abaxial surface of lamina smooth or with a few scattered teeth; adaxial lamellae 4-6, straight, not undulate, 3-5(-8) cells high, entire in profile; median basal cells short-rectangular, 3:1, rather thin-walled, finely striate-papillose; median cells of lamina 20-30 µm, subquadrate to hexagonal, thin- to firm-walled with minute trigones, smooth; 1-3 rows of cells along the margins evenly thicker-walled, often colored, the marginal serrations ending in a short tooth cell, with 2-4 supporting cells; perichaetial leaves 6.5-7 mm, subsheathing at base, spreading. Seta to 5 cm, 1(-2) per perichaetium, reddish, wiry. Capsule 4-7 mm, light yellowish brown, greenish near base, reddish at the mouth, cylindric, slightly curved, somewhat larger toward the base, strongly contracted below the mouth when dry, the surface rugose; exothecial cells irregularly short-rectangular to hexagonal, rather thin-walled, smooth, much smaller and thicker-walled below the mouth; stomata superficial, numerous in proximal ¹/4 of capsule; peristome teeth 32, obtuse to truncate at the apex. Spores 12-16 µm.

Wet rocks and soil, shaded banks, wet cliffs beside waterfalls, cutbanks in ravines, over boulders in creeks, at higher elevations in late snow areas, drainage channels on steep slopes, beside snowmelt streams; low to high elevations (1000-2000 m); B.C., Yukon; Alaska, Wash.; Asia (Japan, Russia in Kamchatka).

Oligotrichum parallelum is an oceanic, arctic-montane species occurring northward in interior Alaska to 67°N. When optimally developed O. parallelum is a handsome plant, differing from other regional Oligotrichum species in its larger size, leaves crisped when dry, laxly spreading when moist, with coarsely, and often doubly serrate margins, and lower, straight lamellae. The leaf margins may occasionally show a hint of a border of somewhat elongated cells. Typically, however, the marginal cells are ± isodiametric, thicker-walled, and yellowish.