Hypnum circinale Musci Exot. 2: 107. 1819. -- Hypnum sequoieti C. Muell., Flora 58: 91. 1875. -- Hypnum pseudo-recurvans Kindb, Eur. & N. Amer. Bryin. 140. 1897.

   Plants in dense mats, green to almost golden green, irregularly but rather densely branched with branches and stems little different. Leafy axes similar wet or dry, to 5 cm long. Leaves to 2.5 mm, 3–4: 1, falcate secund to almost circinate, lanceolate to ovate-lanceolate, gradually contracted from the non-auriculate base to the very long acumen. Margins plane, narrowly decurrent, serrate near apex. Median cells smooth, 4–7 µm wide, linear, up to 20: 1, thin walled, not pitted. Two or three cells of extreme alar region inflated and isodiametric, to 20 µm wide, accompanied by up to 6 quadrate but not inflated supra-alar cells with moderately thick but unpitted walls. Costa inconspicuous, short and double. Pseudoparaphyllia deltoid to orbicular, strongly overarching the bud. Axillary hairs 5-celled, to 250 µm with a very long basal brown cell, not offset from leaf insertion. Stem cross-section without a central strand, with a hyaloderm markedly enlarged and thin walled compared to the adjacent several layers of small and brownish pachydermous cells, and with inner cortical cells enlarged with thin and hyaline walls.

   Dioicous with male plants similar to females. Perichaetia in leaf axils, much shorter than the subtending leaves. Seta to 2.5 cm long, erect, pale yellow to yelloww. Capsule light brown, inclined to almost erect, arcuate, often strongly so, neither sulcate nor strangulate. Urn to 3 mm, about 3–4: 1. Exothecial cells near mouth of capsule isodiametric with thick walls and rounded lumens, to 15 µm wide. Exothecial cells at mid urn short rectangular, 1.5–3: 1, in regular rows, not collenchymatous, to 22 µm broad. Stomata phaneroporous, restricted to neck. Annulus present, not revoluble. Operculum conic, much shorter than urn. Exostome to 300 µm long, horizontally striate below, papillose above, light-brown. Endostome segments smooth to lightly papillose, perforate, about as long as exostome with basal membrane about 1/2 of total length. Cilia 2–3, nodose. Spores smooth to very lightly papillose, to 15 µm.

Vouchers: Butte Co.: Brush Creek, Plumas National Forest, Dillingham 856 (CAS); Del Norte Co.: Highway 199 along Smith River about 9 miles east of junction with Highway 101, Norris 85019; Lake Co.: Summit Lake east of Mt. Sanhedrin, Mendocino National Forest, Toren & Dearing 7202 (CAS); Marin Co.: Muir Woods National Monument, Hermann 17496 (CAS); Mendocino Co.: Fox Creek north of Branscomb, The Nature Conservancy Branscomb Reserve, Norris 47117; Plumas Co.: below Cresta Power Station, Norris 52645; Santa Cruz Co.: Sempervirens Creek near Camp 7, Big Basin Redwood State Park, Koch 2030a (UC) and Opal Creek, Kellman 1015 (CAS); Siskiyou Co.: between Wilderness Falls and Doe Flat, Klamath National Forest, Norris 67806.

Literature: Bourell 1981; Holmberg 1969; Howe 1897; Jamieson 1969; Kellman 2003; Koch 1950a, 1951e; Koch and Ikenberry 1954; Lawton 1971; Lesquereux 1868; Smith 1970; Spjut 1971; Thomson and Ketchledge 1958; Toren 1977; Watson 1880; Yurky 1990, 1995.

Illustration References: Malcolm et al. 2009 p. 284; Lawton 1971.

Elevation by latitude plot for Hypnum circinale
   in California