Monographs Details: Hylocomium
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 robust, in loose, dull to lustrous, yellow- to brown-green or golden wefts. Stems prostrate with arcuate-ascending innovations, regularly (1-)2-3-pinnate and frondose from an unbranched stipe, in regular tiers from annual growth; in crosssection with small thick-walled cells surrounding larger thinner-walled cells, central strand none; paraphyllia usually abundant, branched, filamentous; pseudoparaphyllia foliose; axillary hairs with a single short brown basal cell and 2-3 elongate hyaline distal cells. Stem and branch leaves differentiated, stem leaves erect to erect-spreading, those of the stipe sometimes appressed, ovate to broadly oblong-ovate, ± abruptly tapered to a mostly long, flexuose acumen, rarely obtuse to acute, concave, ± plicate, not decurrent; margins serrate, often coarsely so, rarely subentire throughout, plane to recurved; costa double, often reaching midleaf, rarely weak to absent; cells linear, irregularly prorulose at upper ends at back, or less often smooth, thin- to firm-walled, subporose, becoming colored, short-rectangular to oblong, thick-walled and porose toward the insertion; alar cells scarcely differentiated. Branch leaves smaller, erect-spreading to spreading, ovate-lanceolate to ovate, usually gradually acute to short-acuminate, concave; margins serrulate above, subentire below, broadly incurved; costa strong and double but usually not to midleaf; areolation similar to stem leaves. Dioicous. Perichaetial leaves erect, sheathing at base, often with spreading apices, mostly oblong-lanceolate, long-acuminate; margins serrulate above, entire below, plane; costa often absent; cells linear, smooth, thick-walled, porose, becoming shorter, broader and colored across the insertion. Setae elongate, smooth, orange to reddish, usually not twisted; capsules horizontal, arcuate, asymmetric, broadly short-cylindric, slightly constricted below the mouth when dry and empty; exothecial cells isodiametric to short-rectangular, firm-walled; annulus differentiated; operculum stoutly and obliquely long-rostrate; exostome teeth on the front surface reticulate below, coarsely papillose above, trabeculate at back; endostome with a high, nearly smooth basal membrane, segments keeled, usually gaping, rarely only perforate, shorter than the teeth, cilia in groups of 2-4, nodulose. Spores spherical, finely papillose. Calyptrae cucullate, naked, smooth.
Discussion:Hylocomium Bruch & Schimp. in Bruch, Schimp. & W. Gümbel, Bryol. Eur. 5(fasc. 49-51, Monogr. 1): 169. 1852; Hypnum sect. Hylocomium (Bruch & Schimp.) Sull. in A. Gray, Manual, ed. 2, 668. 1856; Stereodon sect. Hylocomium (Bruch & Schimp.) Mitt., J. Linn. Soc., Bot. 7: 158. 1863; Hypnum subgen. Hylocomium (Bruch & Schimp.) Hobk., Syn. Brit. Mosses 156. 1873.
Discussion. Hylocomium is treated here in a narrow, restricted sense, exclusive of Loeskeobryum and Hylocomiastrum M. Fleisch. ex Broth. The monospecific genus is characterized by tiered branching, stems lacking a central strand, weakly plicate stem leaves, a strong double costa, and prorulose laminal cells. Our only other confamilial genus, Loeskeobryum, has irregularly pinnate branching, stems with a central strand, and smooth laminal cells. It would not be surprising to find Pleurozium Mitt, at higher altitudes in Jamaica or Hispaniola. It differs from all genera of the Hylocomium alliance most conspicuously by the lack of paraphyllia.
Ireland (1971) listed every North American moss with paraphyllia as lacking pseudoparaphyllia. However, he then postulated that paraphyllia arose from pseudoparaphyllia, a conclusion that seems incongruous. Pseudoparaphyllia are very difficult to observe when a stem is densely matted with paraphyllia, however, with patience they can be found. Rohrer (1985b) described the pseudoparaphyllia of Hylocomium as undifferentiated from the paraphyllia, but this does not conform to my observations.