Orthotrichum tenellum, Bryol. Univ. 1: 786. 1827. -- Orthotrichum cylindrocarpum Lesq., Trans. Am. Phil. Soc. n. ser. 13: 6. 1864. -- Orthotrichum tenellum var. cylindrocarpum (Lesq.) Koch, Leafl. West. Bot. 6: 23. 1950. -- Orthotrichum coulteri Mitt., J. Linn. Soc. Bot. 8: 125. 1864. -- Orthotrichum tenellum var. coulteri (Mitt.) Grout, N. Am. Fl. 15A: 19. 1946.

   Plants erect in small green to rather dark green tufts, to 1 cm high. Leaves spreading when moist but erect with incurved apices when dry, ligulate to oblong lanceolate with acute to blunt apices, to 2.5 mm long, 3–4: 1. Median laminal cells isodiametric with rounded lumens, thick walled with strong corner thickening, to 12 µm wide, with 2–3 papillae on both surfaces. Basal cells rectangular, 2–5: 1, with straight lateral walls. Margins recurved from base to above the middle, entire or with a few serrulations at the extreme apex. Costa ending within about ten cells of apex. Costa cross-section homogeneous or with bands of poorly differentiated substereids surrounding a single layer of thinner walled and larger internal cells. Axillary hairs to 8 cells long but with the centrally inserted ones shorter , to 250 µm, with one basal brown cell, not offset from leaf insertion. Rhizoids mostly near base of plant, smooth, red-brown, to 15 µm in diameter at base, repeatedly branched. Stem cross-section without a central strand, with somewhat pachydermous and hyaline inner corticals and with 1–2 layers of strongly pachydermous, red brown outer cortical cells. Multicellular gemmae occasionally present, oblong elliptic, uniseriate, to 90 µm long, about 2: 1, to 6 cells long, borne on the abaxial surface of leaves, especially near the bases.

   Autoicous with perigonia in leaf axils near the perichaetia or on the apices of separate short branches. Perichaetial bracts similar to the vegetative leaves. Capsule emergent to somewhat exserted on a yellow to yellow brown, smooth and erect seta of about 1.5–2 mm in length. Urn orange-brown when fresh, to 2 mm long, 2–3: 1, erect, strongly sulcate with eight ribs, not or inconspicuously strangulate. Operculum short rostrate. Annulus well-defined, revoluble. Exothecial cells at capsule mouth strongly reddened, to 12 µm broad, isodiametric with rounded lumens in up to 4 rows. Exothecial cells at middle of urn quadrate to rectangular, 1–5: 1, to 20 µm broad, very thick walled on ridges of capsule with lumen:wall ratio 0.25–1: 1, thinner walled in valleys of the sulcate capsule. Stomata restricted to neck of capsule, cryptoporous, nearly obscured by subsidiary cells. Exostome teeth ? when dry, fused in pairs and appearing to be only 8 in number, light brown, to 300 µm long, finely and densely papillose. Endostome segments 8 or 16, nearly as long as exostome, lightly papillose, arising from 2 irregular rows of cells. Calyptra campanulate, with rather sparse, upwardly directed papillose hairs especially near its apex. Spores green, to 16 µm, rather coarsely papillose.

   In North America O. tenellum is nearly restricted to the Coast Range and Central Valley of California. It is, however, widely distributed throughout Europe, and is found also in North Africa and Macaronesia. O. tenellum is one of the more common and widespread of Orthotrichum at low elevations in California. It is almost exclusively epiphytic and is found in closely spaced tufts on the sunny bases of trees in the humid coastal forests. In the very hot and dry forests of the Sacramento valley, O. tenellum is scattered in such small tufts as to be inconspicuous. It may generally be recognized by its small size, epiphytic habitat and by its strongly 8-ribbed capsules with erect to reflexed peristome teeth and with almost no strangulation. When dry, it may be recognized with virtual certainty by its inflexed, often somewhat reddened leaf apices.

Catalog Notes: Excluded by Malcolm et al. 2009.

Vouchers: Alameda Co.: Brushy Peak northeast of Livermore, Norris 87190; Fresno Co.: Patterson Creek along Kings River, Sierra National Forest, Norris 50993; Lake Co.: north end of Bachelor Valley near Hell's Peak, Toren 975 (SFSU); Orange Co.: Trabuco Canyon about 3 miles east of O'Neill Park near Cleveland National Forest boundary, Shevock 3988; Placer Co.: Clover Valley Creek at Sierra College Road, Norris 81949; San Francisco Co.: Panhandle, Golden Gate Park, Shevock 18930; Santa Cruz Co.: San Lorenzo Park, City of Santa Cruz, Kellman 1832 (CAS); Sonoma Co.: South Ridge Trail above Lake Sonoma at Skaggs Spring Road, Norris 86911.

Literature: Harpel 1980a; Harthill et al. 1979; Kellman 2003; Koch 1950a, 1951e; Koch and Ikenberry 1954; Long 1978; McCleary 1972; Mishler 1978; Shevock and Toren 2001; Steere 1954; Steere et al. 1954; Thomson and Ketchledge 1958; Toren 1977; Whittemore and Sommers 1999; Yurky 1990, 1995. As Orthotrichum coulteri Lesquereux 1868. As Orthotrichum cylindrocarpum Bradshaw 1926; Kingman 1912; Lesquereux 1868; Lesquereux and James 1884; Millspaugh and Nuttall 1923; Sayre 1940; Steere 1954; Steere et al. 1954; Watson 1880. As Orthotrichum tenellum var. coulteri McCleary 1972.

Illustration References: Sharp et al. 1994; Smith 1978; Vitt 1973.

Elevation by latitude plot for Orthotrichum tenellum
   in California