Monographs Details: Pilosium
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, in lustrous, yellow-green, mostly dense, ± flat, often extensive mats. Stems creeping, irregularly branched, complanate-foliate; in cross-section with a hyalodermis, subtended by small firm-walled cells surrounding large thin-walled cells, central strand absent; pseudoparaphyllia filamentous; axillary hairs with a single short brown basal cell and 3-4 elongate hyaline or colored distal cells. Stem and branch leaves similar but lateral and dorsal leaves differentiated, crowded, ± stiff, wide-spreading, somewhat curled when dry, curled downward when moist, complanate, dorsal leaves oblong-ovate, symmetric, lateral leaves oblong-lanceolate, cultriform, obtuse to short-acute, flat or concave, not plicate, not decurrent; margins entire to serrulate above, entire below, plane; costa single in lateral leaves, weak, rarely extending 1/3 the leaf length, absent in dorsal leaves; cells linear, smooth, thin- to firm-walled, porose or not, heavily chlorophyllose; alar cells differentiated in lateral leaves, numerous, large, oblong, colored, porose, extending across adaxial surface of costa, dorsal leaves with few differentiated cells, small, quadrate to subquadrate. Asexual propagula none. Autoicous, often fertile; perigonia scattered along stems, bracts ovate, ecostate. Perichaetia along stems, often in pairs; leaves lanceolate to ovate, acuminate; margins incised at shoulders; costa none; cells linear, smooth, thick-walled. Setae elongate, smooth, orange-red, straight; capsules cernuous, symmetric, not arcuate, short-cylindric, not plicate, constricted below mouth when dry; exothecial cells quadrate to short-rectangular, collenchymatous; annulus not differentiated; operculum short-rostrate; exostome teeth narrowly triangular, shouldered, bordered, on the front surface with a zig-zag median line, cross-striolate below, papillose above, trabeculate at back; endostome papillose, with a high basal membrane, segments keeled, ± perforate, ca. as long as the teeth, cilia single, shorter than the segments. Spores spherical, papillose. Calyptrae cucullate, naked, smooth.
Discussion:Pilosium (Müll. Hal.) M. Fleisch., Musci Buitenzorg 4: 1158. 1923; Stereophyllum sect. Pilosium Müll. Hal., Flora 83: 340. 1897.
Discussion. Pilosium is characterized by strongly complanate plants with lateral and dorsal leaves strongly differentiated. The lateral leaves are cultriform, usually costate, and have numerous alar cells. The dorsal leaves are symmetric and ecostate and have few differentiated alar cells.
Pilosium is in its own subfamily, the Pilosioideae Ireland & W. R. Buck. However, its placement in the Stereophyllaceae is uncertain and since there is no other obvious relationship, its inclusion in the family is one of convenience. However, the alar cells of the lateral leaves are unequally distributed and extend across the costa, providing some positive evidence of a possible relationship.