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.
![University of California [UC]](http://ucjeps.berkeley.edu/common/images/logo_uc_80.png){kind=logo}
