Monographs Details: Hypnum
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants small to robust, in often lustrous, soft or stiff, green to golden, yellowish or brownish, sometimes reddish, lax or dense mats. Stems creeping or sometimes ascending, irregularly branched to regularly pinnate, sometimes frondose, sometimes complanate-foliate, rarely julaceous; in cross-section sometimes with a unistratose hyalodermis of enlarged thin-walled cells, subtended by small firm- or thick-walled cells surrounding larger thinner-walled cells, central strand present or absent; pseudoparaphyllia foliose; axillary hairs with 1-2 short brown basal cells and 2-4 elongate hyaline distal cells. Stem and branch leaves similar, crowded, typically falcate-secund, sometimes erect-spreading, oblong-lanceolate to lanceolate-ovate, symmetric, gradually short-acuminate to subulate, concave, rarely plicate, sometimes decurrent; margins entire to serrulate above, erect or reflexed, especially at base, rarely revolute; costa short and double or absent; cells mostly linear to linear-flexuose, smooth, thin- to firm-walled, becoming thicker-walled and porose toward the insertion; alar cells differentiated in basal angles, small and quadrate in ± triangular areas or enlarged and ± inflated in well-defined areas with a small group of small subquadrate cells above them, hyaline or colored. Asexual propagula none. Autoicous or dioicous. Perichaetial leaves differentiated, mostly lanceolate, long-acuminate, often plicate; margins mostly entire, sometimes toothed above, plane; costa usually none; cells linear, smooth, often firm- to thick-walled and porose; alar cells not or scarcely differentiated. Setae elongate, smooth, usually reddish, mostly twisted; capsules inclined to horizontal, rarely suberect, usually asymmetric, usually arcuate, long-cylindric; exothecial cells quadrate to rectangular, firm-walled, not or rarely weakly collenchymatous; annulus usually differentiated; operculum conic to short-rostrate; peristome double, exostome teeth shouldered, bordered, on the front surface cross-striolate below, papillose above, trabeculate at back; endostome with a high basal membrane, segments broad or seldom narrow, keeled, not or narrowly perforate, ca. as long as the teeth, cilia usually in groups of 1-3, rarely rudimentary, sometimes nodulose, not appendiculate. Spores spherical to oval, mostly finely papillose. Calyptrae cucullate, naked, smooth.
Discussion:Hypnum Hedw., Sp. Musc. Frond. 236. 1801.
Hypnum subgen. Stereodon Brid., Bryol. Univ. 2: 550. 1827; Stereodon (Brid.) Mitt., J. Linn. Soc., Bot. 4: 87. 1859.
Hypnum sect. Omalia subsect. Cupressina Müll. Hal., Syn. Musc. Frond. 2: 289. 1851; Cupressina (Müll. Hal.) Müll. Hal., Nuovo Giorn. Bot. Ital. II, 3: 119. 1896.
Hypnum sect. Drepanium Schimp., Syn. Musc. Eur. 618. 1860; Drepanium (Schimp.) C. E. O. Jensen in Lange, Meddel. Grønland 3: 326. 1887.
Caribaeohypnum Ando & Higuchi, Cryptog. Bryol. Lichénol. 5: 9. 1984.
Discussion. Hypnum is characterized by usually lustrous plants with falcate-secund leaves. The leaves have a short, double costa and differentiated alar cells. Hedwig (1801) used Hypnum in a very broad sense, including species that are now housed in many genera and families. The modern usage of the genus is assured by the conservation of H. cupressiforme as the lectotype.
Caribaeohypnum was described (Ando & Higuchi, 1984) for a single species, C. polypterum. It was primarily characterized by plicate leaves, a strong double costa, and a reduced peristome. The reduced sporophytic features were considered paramount in the erection of the new genus, even though Hypnum was admitted as the closest relative. In a genus the size of Hypnum it is reasonable to expect various reduction series and to chop them off as generic segregates is deceptive when trying to understand the group’s phylogeny. Hypnum s.l. will probably one day be split apart and H. polypterum will not go with the generitype, H. cupressiforme. However, to recognize Caribaeohypnum at this premature time only belies H. polypterum’s relationships.