Plants yellow-brown to green, gregarious and somewhat bulbiform. Leafy stems to 4 mm high but with the stem seldom exceeding 1 mm. Leaves concave and keeled, erect and enclosing the capsules but distally contorted when dry. Leaves oblong to oblong-lanceolate, 2–3: 1, scale-like below but grading imperceptibly to the perichaetial leaves which may exceed 3 mm in length. Lamina unistratose throughout. Median and distal cells only gradually differentiated from the basal cells. Median laminal cells quadrate in regular longitudinal rows, to 14 µm broad, 1–1.2: 1, with angular lumens and small corner thickenings. Basal cells rectangular with straight lateral walls, to 20 µm broad, 3–5: 1, smooth, moderately thick-walled, unistratose throughout. Costa narrow from its base to its percurrent or subpercurrent apex. Costa cross-section at mid-leaf terete, about 2–3 times as thick as the adjacent lamina, with a strong dorsal stereid band, and with several large and thin walled cells adaxial to the single layer of guide cells. Margin entire to minutely crenulate, recurved to well above the middle. Axillary hairs hyaline, to 200 µm long, 4–5 celled, with no basal brown cells. Rhizoids hyaline and inconspicuous, smooth and sparingly branched, to 10 µm wide at insertion.
Autoicous with the perigonia on short lateral branches arising from near the base of the female branch. Perichaetial leaves much larger than but only gradually demarcated from the vegetative leaves. Seta mostly less than 1 mm long, cygneous. Capsule globose, to 0.5 mm in diameter, apiculate at apex but without a differentiated operculum. Calyptra cucullate, extending to about mid-urn. Peristome absent. Exothecial cells mostly short rectangular, sometimes quadrate, with curved lateral walls and often with oblique end walls, quadrate to irregularly isodiametric, 20–30 µm wide; in cross-section with the spore sac distinguishable from the exothecium. Spores 20–35 µm, spherical, papillose to warty papillose.
Phascum cuspidatum is so small that it is seldom collected without sporophytes. In that condition, only Acaulon offers a source of confusion. Both these genera have erect leaves which so completely enclose the sporophyte as to hide all parts of it. Numerous differences between the two genera can be noted. The costae of Phascum are terete and several times as thick as the adjacent lamina, and the abaxial stereid band is obvious in a cross-section. In contrast, Acaulon has costae which are not much thicker than the leaf laminae, and stereid cells are usually absent in a section of the median leaf. Both genera have concave leaves with recurved margins, but the recurvature of Phascum often extends to near the leaf apex. Phascum has a strongly excurrent costa which, at least distally, appears to be keeled in the dry leaf. The laminal cells of the two genera differ markedly in that Phascum has predominantly quadrate cells arranged in regular longitudinal rows; those of Acaulon are predominantly rhomboidal, and the linear arrangement of those cells is not obvious. Phascum is typically distinguished from Acaulon by the papillosity of its leaves. That is a good distinction but the uninitiated may misinterpret the oil bodies which appear plenteously in the leaves of both genera. It is best to place an entire plant on a slide under the microscope so that the leaves can be inspected from a lateral perspective thus allowing the papillae also to be viewed laterally.
|Literature||Brandegee 1891; Crum and Anderson 1981; Harthill et al. 1979; Howe 1897; Kellman 2003; Koch 1950a; Koch and Ikenberry 1954; Lawton 1971; Lesquereux 1868; Shevock and Toren 2001; Stark and Whittemore 1992; Steere et al. 1954; Toren 1977; Watson 1880. As Tortula acaulon Zander 1993.|
|Illustrations||Malcolm et al. 2009 p. 137; Crum and Anderson 1981; Flowers 1973; Ireland 1982; Lawton 1971; Sharp et al. 1994; Smith 1978. As Tortula acaulon Ignatov and Ignatova 2003.|
|Bioregions||CaR, CW, DMoj, NW, SN, SW.|
|Vouchers||Alameda Co.: Brushy Peak northeast of Livermore, Norris 87164; Contra Costa Co.: Antioch Dunes National Wildlife Refuge, Norris 97052; Kern Co.: Highway 178, Kern River Canyon above first dam, Sequoia National Forest, Norris 80806; Lake Co.: Indian Valley Lake, Norris 67544; Orange Co.: Mesa Picnic Area, O'Neal Regional Park, Santa Ana Mountains, Norris & Piippo 82168; San Bernardino Co.: Mid-Hills Campground, Providence Mountains, Mojave National Preserve, Norris 81412; San Francisco Co.: Potrero Hill, Toren 7823 (CAS); Shasta Co.: Highway 299 at Oak Run Road, Norris 80343.|