Rhytidiadelphus triquetrus, Krypt. Fl. Brandenburg 2: 920. 1906. -- Hypnum triquetrum Hedw., Spec. Musc. 256. 1801. -- Hylocomium triquetrum (Hedw.) B.S.G., Bryol. Eur. fasc. 49–51. 1852,

   Plants to 15 cm long, in pale-green or yellowish-green wefts, irregularly branched. Leaves plicate and strongly squarrose, closely foliate, to 7 mm, 2.5–3: 1, ovate and gradually contracted to the acumen. Line of leaf insertion encircling more than 180 degree of stem circumference. Margins plane or nearly so throughout, not at all decurrent, entire to serrulate at base, serrulate to serrate above. Median cells smooth, 6–8 µm wide, linear, about 4–8: 1, thick-walled with lumen: wall ratio 2–3: 1, strongly pitted. Median basal cells orange to red-brown, broader and shorter than adjacent, more distal cells, thick-walled with lumen: wall ratio about 1: 1. Alar cells somewhat differentiated in a rounded rather pellucid pocket, short-rectangular, 2–3: 1, strongly pitted, to 10 µm broad. Costa double with forks separate from base, symmetrically inserted in center of leaf base, with both forks extending above the middle. Paraphyllia absent. Juvenile leaves orbicular, serrate on margin, closely overlapping bud with first such leaf lateral to bud. Axillary hairs 5–7 celled, to 250 µm with two basal cells brown and short, mostly offset from leaf insertion. Stem cross-section with a small central strand, without a hyaloderm, with a well marked stereome of pachydermous, pigmented cells, and with inner cortical cells larger and mostly rather thick-walled. Rhizoids very rarely produced on old and prostrate stems or on bases of perichaetia, to 15 µm in diameter at insertion, arising from area immediately lateral to alar cells at leaf insertions, red-brown, monopodially branched with branches short, smooth and rather thick-walled.

   Dioicous with male plants similar to females. Perichaetia on old, erect axes, with bracts to 3.5 mm long, clasping at base but abruptly contracted into the squarrose and narrow acumen. Seta to 4 cm long, erect, redddish brown, smooth. Capsule red brown, inclined and arcuate, neither sulcate nor strangulate. Urn to 3 mm, oblong and asymmetric, about 2: 1. Exothecial cells isodiametric to short rectangular, to 25 µm broad, in regular rows, with lumen: wall ratio 4–6: 1. Stomata phaneroporous, small and vertical pored, restricted to neck. Annulus present, not revoluble. Operculum conic, much shorter than urn. Exostome to 500 µm long, almost smooth but with light horizontal striae throughout, red-brown. Endostome segments lightly papillose, perforate along keel, about as long as exostome with basal membrane about 1/2 of total length. Cilia 1–3, lightly papillose, somewhat nodose. Spores green, smooth to very lightly papillose, to 15 µm.

Vouchers: Del Norte Co.: French Hill Road about 3.5 miles above Highway 199 southwest of Gasquet, Six Rivers National Forest, Norris 85071 and along Forest Road 4803, Siskiyou National Forest, Norris 70879; Humboldt Co.: headwaters of Freshwater Creek about 2 miles southeast of Kneeland, Norris 68303; Lake Co.: Forest Road 23N56 north of Low Gap west of Round Mountain just south of Glenn County line, Mendocino National Forest, Toren 8805 (CAS); Marin Co.: Rock Spring, Mt. Tamalpais State Park, Robertson 2038 (CAS); Placer Co.: Canyon Creek near Dutch Flat, MacFadden 9796 (MO); Plumas Co.: 1 mile southwest of Meadow Valley Cemetery and 7 miles west of Quincy, Plumas National Forest, Dillingham 928 (CAS); Siskiyou Co.: about 1 mile west of White Mountain near Cook and Green Pass, Klamath National Forest, Norris 50190, near West Branch Campground on Happy Camp-O'Brien Road, Norris 75772 and Elliott Creek near Seattle Bar, Rogue River National Forest, Shevock & Toren 20088.

Literature: Holmberg 1969; Ireland 1982; Koch 1950a, 1951e; Lawton 1971. As Hylocomium triquetrum var. californicus Renauld and Cardot 1890.

Illustration References: Malcolm et al. 2009 p. 295; Abramov and Volkova 1998; Ignatov et al. 1996; Lawton 1971.

Elevation by latitude plot for Rhytidiadelphus triquetrus
   in California