Monographs Details: Leucomium
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants smallish to medium-sized, in typically pale-green, sometimes silvery, often dense, extensive, flat mats. Stems creeping, fragile, freely but irregularly branched, often densely so, complanate-foliate; in cross-section all cells thin-walled, the outer row with cells somewhat smaller than inner ones, central strand none; rhizoids tufted on stems, smooth to lightly roughened; pseudoparaphyllia none; axillary hairs 2-celled, with a short brown basal cell and an elongate hyaline distal cell. Leaves sometimes contorted when dry, often ± falcate, densely foliate, lateral and dorsal leaves somewhat differentiated, if so then dorsal leaves broader and more abruptly tapering, lateral (or all) leaves oblong-lanceolate, triangular-lanceolate or ovate-lanceolate to ovate, gradually or abruptly short- to long-acuminate, ± symmetric; margins not bordered, entire, plane or narrowly recurved when dry; costa none or rarely short and double; cells long-rhomboidal to linear-hexagonal, lax, smooth, thin-walled, becoming somewhat shorter across the insertion; alar cells not differentiated. Asexual propagula none. Autoicous or synoicous, rarely dioicous. Perichaetia small to medium-sized, mostly inconspicuous, in leaf axils on stems; leaves pale, ± erect, similar in shape and areolation to vegetative leaves. Setae elongate, smooth throughout or often obscurely roughened at base of capsule; capsules inclined to horizontal, cylindric, symmetric, with a well-defined roughened stomatose neck; exothecial cells short-rectangular, collenchymatous; annulus of several rows of thin-walled cells, usually falling with the operculum; operculum long-rostrate from a conic base; columella broadly cylindric, extending to midurn; peristome double, attached at mouth, exostome teeth narrowly triangular, narrowly bordered, on the front surface with a narrow to broad pale median furrow with a zig-zag line down it, sometimes without a furrow but with a zig-zag median line, the plates densely cross-striolate below, coarsely papillose at the extreme apex, scarcely trabeculate at back, smooth; endostome smooth or papillose, with a fairly high basal membrane, segments keeled, perforate, with baffle-like crosswalls, ca. as long as the teeth, cilia mostly single, short or rudimentary. Spores spherical, finely papillose. Calyptrae cucullate, naked or with a few hairs, smooth or roughened above.
Discussion:Discussion. Leucomium has lanceolate to ovate leaves with entire margins and lax areolation. The stems are fragile due to the lack of a thick-walled outer layer. The capsules are inclined to horizontal with collenchymatous exothecial cells and furrowed exostome teeth. The operculum is long-rostrate and the calyptra is cucullate.
Leucomium differs from the closely related Rhynchostegiopsis primarily in the entire rather than serrate leaf margins. Leucomium may be confused with Vesiculada, but in Vesiculada the stems are usually more regularly branched and not fragile, the leaves are usually broader and with shorter cells, the capsule is asymmetric without a roughened neck, and the endostomial segments lack baffle-like crosswalls.
I had initially considered recognizing two species of Leucomium from the West Indies, primarily based on whether the lateral and dorsal leaves are differentiated from one another or not. However, detailed examination revealed that the two are apparently just forms of one another. Sometimes older portions of a stem show heterophylly but younger portions, presumably in an altered environment, do not. Therefore, only one species is maintained here. When Allen (1987) revised the genus he came to a similar conclusion, finding a Brazilian name as the oldest epithet.