Monographs Details: Fabronia
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 minute, in green, silky mats or occasionally tufts. Stems creeping or occasionally ascending, not or irregularly branched, fragile, easily broken on dissection; in cross-section with all cells similar in size, thin-walled, central strand none; paraphyllia none; pseudoparaphyllia foliose; axillary hairs 2-celled, with a short basal cell and an elongate apical one, one or both hyaline, difficult to demonstrate. Stem and branch leaves similar, crowded or ± laxly foliate, loosely appressed when dry, spreading when moist, linear-lanceolate to ovate to rhomboidal, gradually or abruptly acuminate, often piliferous, not concave, not decurrent; margins entire or denticulate to ciliate, plane; costa single, slender, usually extending to ca. midleaf, sometimes ending in a small spine; cells short- to long-rhomboidal, smooth, thin-walled, becoming longer and thicker-walled in the acumen; alar cells quadrate in large groups in basal angles, extending well up the margins and to the costa and across it. Asexual propagula none. Autoicous. Perichaetia along stems, conspicuous or not; leaves spreading to sheathing, lanceolate to ovate, acuminate; margins subentire to dentate or ciliate, plane; costa single, ending near midleaf, or absent; cells mostly long-rhomboidal, smooth, thin- to firm-walled; alar cells gradually differentiated, often rectangular, not as distinct as in vegetative leaves. Setae elongate (but short), smooth, yellowish, erect; capsules exserted, erect, symmetric, ovoid to pyriform, appearing rough when dry from bulging cells, often with the mouth flaring on aging; exothecial cells ± isodiametric to short-rectangular, with strongly wavy walls except at mouth where cells ± narrowly oblate with straight walls; annulus not differentiated; operculum umbonate to mammillate; columella cylindric with an expanded subglobose apex, ending above midum but below mouth; peristome single and exostomial or absent, inserted below the mouth, erect to spreading when dry, incurved when moist, exostome teeth 16, usually fused in pairs, reddish brown, triangular and blunt, gradually tapered and not shouldered, densely papillose-striolate throughout, on the front surface with a ± straight median line and large plates, not trabeculate at back. Spores spherical, smooth to coarsely papillose. Calyptrae cucullate, naked, smooth.
Discussion:Discussion. Fabronia is characterized by small, creeping plants, often growing in somewhat xeric habitats. The leaves are singly costate and relatively short-celled, and often the margins are dentate to ciliate. The small capsules have wavy-walled exothecial cells and a single peristome of paired, brown, papillose exostome teeth. The stems are very fragile and often break on dissection. This is due to two different anatomical features: the stems are slender and all the cells are thin-walled, and the end walls of the external layer of cells are lined up, forming natural breakage points.
Fabronia has been burdened with scores of names with little or no morphological meaning. This is particularly true in South America. However, I (Buck, 1983a) reduced the tropical American Fabroniae to three species, one with three varieties; additionally, F. pusilla Raddi is known from the Mexican Baja Peninsula.