Monographs Details: Brachythecium
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants slender to rather robust, in ± lustrous, often soft, green to golden or whitish green, often dense, extensive mats. Stems creeping, decumbent or ascending, mostly irregularly but freely branched, sometimes subpinnate, the branches straight or curved; in cross-section with small thick-walled cells surrounding larger thinner-walled cells, central strand present; paraphyllia none; pseudoparaphyllia foliose; axillary hairs with 1-2 short hyaline to brown basal cells and 1 to several elongate hyaline distal cells. Stem and branch leaves somewhat differentiated with stem leaves mostly longer, broader, and longer-acuminate with a stronger costa, branch leaves crowded, erect-spreading to spreading, little altered when moist, lanceolate to ovate, mostly acuminate, ± concave, often biplicate, not to distinctly decurrent; margins entire to serrulate above or throughout, usually plane; costa single, ending just above midleaf to subpercurrent, mostly not projecting as a spine at apex; cells mostly linear, rarely shorter, smooth, thin- to thick-walled, porose or not, often becoming shorter in the extreme apex, becoming more rectangular toward the insertion; alar cells often gradually differentiated, rarely abruptly so, but usually well-marked, quadrate to short-rectangular to the margins, rarely inflated. Asexual propagula none. Autoicous or dioicous. Perichaetia inconspicuous or rarely prominent; leaves erect or spreading from an erect base, usually oblong-lanceolate, slenderly long-acuminate; margins subentire to serrulate, plane; costa short and single or absent; cells linear, smooth, firm-to thick-walled, ± porose, becoming rectangular and lax toward the insertion; alar cells not differentiated. Setae elongate, smooth or roughened above or throughout, reddish, mostly straight, often twisted; capsules inclined to horizontal or less often erect, typically broadly short-cylindric to oblong-ovoid and asymmetric, less often cylindric and ± arcuate, usually dark-brown; exothecial cells quadrate to short-rectangular, ± thick-walled, not collenchymatous, stomata round-pored; annulus mostly differentiated, small; operculum typically high conic-apiculate, occasionally short-rostrate, rarely oblique; exostome teeth reddish brown, triangular, shouldered, bordered, on the front surface cross-striolate below, coarsely papillose above, trabeculate at back; endostome with a high basal membrane, segments keeled, narrowly perforate to gaping, ca. as long as the teeth, cilia in groups of 1-3, nodose to appendiculate, shorter than the segments, rarely rudimentary to absent (in erect-capsuled species). Spores spherical, smooth to finely papillose. Calyptrae cucullate, naked, smooth.
Discussion:Brachythecium Schimp. in Bruch, Schimp. & W. Gümbel, Bryol. Eur. 6(fasc. 52-54, Monogr. 1): 5. 1853; Hypnum sect. Brachythecium (Schimp.) Sull. in A. Gray, Manual, ed. 2, 675. 1856; Hypnum subgen. Brachythecium (Schimp.) Hobk., Syn. Brit. Mosses 148. 1873.
Chamberlainia Grout, Moss Fl. N. Amer. 3: 27. 1928.
Cryptoneuron Thér. & P. de la Varde in Thér., Rev. Bryol. Lichénol. 14(Trav. Bryol. déd. Husnot 2): 22. 1944.
Discussion. Brachythecium is characterized by typically biplicate leaves with the costa ending above midleaf. The alar cells are quadrate to short-rectangular and extend to the margins (rather than with 1-2 rows of narrow, marginal cells). The setae are often roughened; the capsules are short and dark-brown with a conic-apiculate operculum.
On a global scale, Brachythecium is a notoriously difficult genus with species separated on minor morphological variation and sexual differences. Fortunately, it is not so bad in the West Indies. We have one very common species, B. ruderale, with the others much rarer and mostly restricted to high-elevation habitats. McFarland (1988) monographed Brachythecium for the Americas south of the United States, and his keys and illustrations are exceedingly helpful.