Platyhypnidium

• Authority

  Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.

• Family

  Brachytheciaceae

• Scientific Name

  Platyhypnidium

• Description

  Genus Description - Plants medium-sized to rather robust, in ± lustrous, ± rigid, green to golden or brownish or dark-green to blackish, often extensive mats. Stems creeping to floating, irregularly and usually scarcely branched, the branches elongate, sometimes ± ascending; in crosssection with small thick-walled cells surrounding larger thinner-walled cells, central strand present; paraphyllia none; pseudoparaphyllia broadly foliose; axillary hairs with a single short brown basal cell and 2-5 elongate hyaline or brown distal cells. Stem and branch leaves not differentiated except sometimes in size, erect- to wide-spreading, sometimes homomallous, often ± contorted when dry, broadly ovate to oblong-ovate, acute to broadly obtuse, sometimes constricted at apex, shallowly concave, sometimes ± striate; margins serrulate throughout, more distantly so below, plane; costa single, relatively slender, tapering distally, ending above midleaf, sometimes projecting as a prickle; cells long-hexagonal to linear, smooth, thin- to firm-walled, not porose, becoming shorter in the upper margins and in a small area at extreme apex, there becoming rhombic; alar cells gradually but usually ± distinctly differentiated, sometimes extending to the costa, enlarged, short-rectangular, firm-walled, sometimes ± porose. Asexual propagula none. Autoicous. Perichaetia inconspicuous, sometimes very numerous along stems; leaves spreading, oblong-lanceolate to oblong-ovate, gradually to ± abruptly short-acuminate, often rounded, concave; margins serrulate above, entire below, plane; costa none or single and ending ca. midleaf; cells linear, often ± flexuose, smooth, firm-walled, not or somewhat porose, becoming shorter, broader and thinner-walled toward the insertion; alar cells not differentiated. Setae elongate, sometimes stout, smooth, orange to reddish, often flexuose, ± twisted; capsules inclined to horizontal, ± arcuate, asymmetric, cylindric with a short neck; exothecial cells subquadrate to short-rectangular, thin- to firm-walled, sometimes somewhat collenchymatous, stomata round-pored; annulus well developed, broad; operculum obliquely long-rostrate; exostome teeth reddish brown, triangular, shouldered, ± strongly bordered, on the front surface cross-striolate below, coarsely papillose above, trabeculate at back; endostome usually densely and finely papillose, with a high basal membrane, segments keeled, perforate, ca. as long as the teeth, cilia in groups of 1-3, nodose, somewhat shorter than the segments. Spores spherical, smooth to finely papillose, sometimes rather large. Calyptrae cucullate, naked, smooth.

• Discussion

  Platyhypnidium M. Fleisch., Musci Buitenzorg 4: 1536. 1923. Platyhypnum sect. Rhynchohypnum Loeske, Hedwigia 50: 243. 1911, nom. nud. Discussion. Platyhypnidium is characterized by aquatic plants that are only little-branched. The leaves are very broad with acute to obtuse apices. Unlike those broad-leaved species of Eurhynchium (the genus to which species of Platyhypnidium are often referred by North American authors), the area of short apical leaf cells is restricted to a small marginal band rather than to a large area beyond the apex of the costa. Additionally, although the costa may often project apically, it gradually tapers above. An additional differentia from Eurhynchium is that the branch and stem leaves, although somewhat different in size, are not differentiated in shape or areolation. Platyhypnidium is probably most closely related to Rhynchostegium as evidenced by its autoicous sexuality and gradually narrowed costa. It differs, however, in the broad, nonacuminate leaves, noncomplanate habit, and aquatic habitat. Platyhypnidium was described to include 17 species, from all over the globe. Fleischer (1923) did not designate a type species but only discussed a single one, P muelleri (Sande Lac.) M. Fleisch., in detail. Therefore, I here designate it as the lectotype as it seems the only reasonable choice.