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Schofield, W.B. 1968. Bryophytes of British Columbia I. Mosses of Particular Interest. J. Hattori Bot. Lab. 31: 205–226.

Trachybryum is the only species of a genus endemic to the west coast of North America. Brotherus (1925) recognized Trachybryum as a separate section of the genus Camptothecium along with C. pseudolutescens (Hampe) Jaeger, a poorly documented species of New Granada. Schofield (1968) elevated the single North American species (T. megaptilum) as a segregate from the pleurocarpous genus Homalothecium with which it shares the features of plicate leaves and a strong costa. He quotes Robinson (1962) who claimed that Homalothecium megaptilum is "at least as divergent element in Homalothecium as any of those which have been given generic status." In contrast, Ignatov & Huttunen (2002) claim that Trachybryum "appears well-nested within Homalothecium in all their (cladistic) analyses." The most obvious differential character is the plumose growth pattern from an upwardly directed central stem axis. It might be noted in this regard that Homalothecium nevadense usually has the central stem axis upwardly directed but that axis is short and appears to be upwardly directed primarily because of the dense growth habit of the colonies. In microscopic view, Trachybryum is segregated from Homalothecium on the basis of the elongate (not quadrate) cells of the alar region. The leaves are broadly ovate rather than variously lanceolate as in Homalothecium, and the n b cells are very strongly porose.

Trachybryum megaptilum is one of several species of moss that have been called "pseudautoicous" on the basis of the presence of dwarf males in the axils of leaves very near the perichaetia. Such dwarf males arise from separate spores which fall into those leaf axils and grow into structures consisting of nothing more than a very short stem without vegetative leaves but with perigonial leaves surrounding a group of antheridia and paraphyses.

Trachybryum megaptilum (Sullivant) W.B.Schofield
   
   J. Hattori Bat. Lab. 31: 222. 1968. -- Camptothecium megaptilum Sullivant, Icones Musc. Suppl. 102. 1874. -- Homalothecium megaptilum (Sullivant) H. Robinson, Bryologist 65: 100. 1962.
    Camptothecium megaptilum var. fosteri Grout, Bryologist 31: 46, 1928.

Plants loosely scattered or in dense mats, glossy whitish-green to light yellow-green. Stems to 12 cm high, erect to decumbent from an inconspicuous prostrate, heavily eroded stolon. Stem leaves to 5 mm, 2–2.5: 1, symmetric, ovate-lanceolate, acuminate, often with the apex twisted, loosely imbricate when dry with at least the apices somewhat divergent when dry. Branch leaves to 3 mm, becoming gradually smaller toward the apex, relatively shorter and less acuminate than stem leaves. Leaf margins recurved from base to near the apex, mostly only a few cells decurrent, entire to serrulate below the middle, but serrate to dentate near the apex. Median cells smooth and prosenchymatous, 4–5 µm wide, more than 10: 1, rather thin-walled, not at all pitted. Alar cells rectangular, 3–5: 1, thick-walled (lumen/wall ratio 2–4: 1), abruptly differentiated from adjacent laminal cells, extending about 4 cells up margin but extending inwardly to the costa, becoming more and more pitted near the costa. Apical cells elongate, often strongly dentate with elongate teeth. Costa extending about 3/4, usually ending in one or several abaxial spines. Pseudoparaphyllia of a Brachytheciaceae form, triangular-lanceolate, serrulate, strongly overarching the bud. Axillary hairs 4–5 celled, to 70 µm with 2 basal brown cells, not offset from leaf insertion. Erect stem with stereome brown, 3–6 cells thick, with internal cells leptodermous and hyaline, with central strand inconspicuous. Rhizoids brown to red-brown, 10–12 µm at insertion, smooth and monopodial with branch rhizoids markedly reduced in diameter and length compared with the main axis, sparingly produced, mainly on prostrate base of main stem axis.
    Dioicous with male plants reduced to small, bud-like plants in axils of leaves of female plants. Perichaetia on main axis with proximal portion of bracts erect and sheathing but abruptly squarrose near the middle. Perichaetial bracts with nearly entire margins, to 4 mm long, about 3: 1, inconspicuously if at all costate, ovate-lanceolate and long acuminate. Cells of perichaetial bracts long vermicular, to 15: 1, thick-walled and somewhat porose with lumen: wall ratio about 1: 1. Seta to 2 cm long, papillose throughout. Capsule exserted, to 2 mm long, oblong-ovate, asymmetric, about 2: 1, brown, strongly inclined to pendulous, somewhat strangulate when dry. Exothecial cells in vertical rows, to 20 µm wide, isodiametric with rounded lumens and with thick (lumen/wall ratio 2–3: 1) red-brown walls. Stomata restricted to elevated pustules on capsule base. Operculum conic, about 1/6–1/8 as long as urn. Exostome teeth to 700 µm long, closely trabeculate throughout, finely papillose above, red-brown. Endostome segments low papillose, open on the narrow keel, about as long as exostome with basal membrane constituting about 1/2 of total endostome length. Cilia 2–3, often appendiculate. Spores smooth to finely papillose, 10–12 µm. Calyptra cucullate, glabrous.

In the field, the size and pattern of growth of Trachybryum may be reminiscent of plants in the vicinity of Rhytidiadelphus. However, the double costae of Rhytidiadelphus and Rhytidiopsis allow quick microscopic elimination of those genera; and the highly rugose leaves of Rhytidium strongly contrast with the deeply plicate leaves of Trachybryum.

Elevation by latitude plot for Trachybryum megaptilum
   in California

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