Monographs Details: Pilotrichidium
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants medium-sized to relatively robust, in dull, stiff, mostly deep-green to golden, often dense, extensive mats. Stems creeping, irregularly to subpinnately branched, obscurely to ± complanate-foliate; in cross-section with a unistratose hyalodermis over small thick-walled cells surrounding larger thin-walled cells, central strand none; rhizoids restricted to branch bases and older stems, tufted, finely roughened; pseudoparaphyllia foliose, small; axillary hairs 2-celled, with a short brown basal cell and an elongate hyaline distal cell. Leaves not or somewhat contorted when dry, densely foliate, lateral and dorsal leaves differentiated from ventral leaves, ventral leaves (against substrate) often differentiated in shape, usually shorter, more ovate and acute, otherwise similar in costa and areolation, lateral and dorsal leaves mostly oblong to oblong-ovate, obtuse and sometimes mucronate to apiculate, to acute, ± asymmetric, shortly rounded to the insertion; margins not bordered, entire except for projecting papillae, plane or narrowly incurved; costa double, strong, subpercurrent to short-excurrent, often the apices ± connivent, not crested or toothed, projecting at apex as single spines; cells short, isodiametric to 4:1, pluripapillose over the lumina or smooth except for marginal papillae, thin- to thick-walled, porose or not, becoming rectangular and smooth toward the insertion; alar cells not differentiated. Asexual propagula rare or absent, when present on costa at leaf base, on reddish stalks, 2-3-celled, hyaline. Dioicous. Perichaetia small, inconspicuous, on primary stems and branch bases; leaves few, pale, erect, linear to ovate; margins not or obscurely bordered, entire, plane to incurved; costa double, strong, projecting as spines at apex; areolation similar to vegetative leaves but always smooth. Setae elongate, long, stout, smooth throughout to lightly roughened above, reddish; capsules erect to pendent, short-cylindric, with a ± roughened neck; exothecial cells short-rectangular, firm- to thick-walled, not collenchymatous; annulus broad, of small thin-walled cells; operculum conic-rostrate; columella broadly cylindric, extending to near mouth; peristome double, attached at the mouth, exostome teeth reddish, narrowly triangular, narrowly bordered, not shouldered, on the front surface with or without a pale median furrow with a zig-zag line down it, the plates cross-striolate below, sometimes with overlying papillae, coarsely papillose above, somewhat trabeculate at back, smooth to finely papillose; endostome with a fairly high basal membrane, smooth to papillose, segments papillose-spiculose, keeled, perforate, with baffle-like crosswalls, cilia none. Spores spherical, smooth to finely roughened, small to medium-sized. Calyptrae slenderly mitrate, falling early, shallowly lobed at base, naked, lightly roughened above, smooth below.
Discussion:Discussion. Pilotrichidium is a small, tropical American genus related to Callicostella. It is characterized by stiff, dioicous plants with a differentiated stem hyalodermis. The leaves are oblong with strong costae projecting at their apices. The margins are entire except for projecting papillae and the leaf cells, depending on environmental parameters, are thin- or thick-walled, and pluripapillose in one species but smooth in the other. The seta is long and smooth (only obscurely roughened at apex) and the capsules are short-cylindric with non-collenchymatous exothecial cells and a broad annulus. The calyptra is slenderly mitrate, almost smooth, and naked.
Pilotrichidium might be confused with Callicostella but differs from it in dull, stiff plants with a stem hyalodermis and annulate capsules. It may be mistaken for Brymela because of the thick-walled and porose leaf cells sometimes present; but it differs in the hyalodermis, much stronger costae, and papillose leaf margins. Pilotrichidium was monographed by Allen and Crosby (1986a). The genus has but two species, both occurring in the West Indies.