Monographs Details: Thuidium
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants large and coarse, in typically stiff, dark-green to golden, loose mats. Stems arched to creeping, mostly regularly 2-3-pinnate, mostly frondose; in cross-section with small thick-walled cells surrounding larger thinner-walled cells, central strand small; paraphyllia abundant or rarely sparse, filamentous to foliose, branched, the cells papillose; pseudoparaphyllia foliose; axillary hairs usually with a single short brown basal cell and mostly 2 elongate hyaline distal cells. Stem and branch leaves strongly differentiated, stem leaves mostly spreading when moist, usually broadly ovate, acuminate, concave, sometimes plicate, often short-decurrent or subauriculate; margins serrulate, often recurved at least below; costa single, usually strong, sometimes filling the acumen; cells quadrate to short-rectangular, mostly unipapillose, sometimes pluripapillose, always just at back, thin- to thick-walled, sometimes becoming more elongate in the acumen; alar cells not differentiated. Branch leaves smaller than stem leaves, usually spreading, lanceolate to oblong to ovate, obtuse to short-acuminate, usually concave, not plicate, not or slightly decurrent; margins serrate to serrulate, usually plane; costa single, weaker than in stem leaves, sometimes projecting as a spine at apex; cells mostly isodiametric, mostly unipapillose, sometimes pluripapillose, always just at back, the papillae sometimes curved, thin- to thick-walled, the apical cell sometimes differentiated; alar cells not differentiated. Asexual propagula none. Dioicous. Perichaetia conspicuous; leaves greatly enlarged, mostly erect, lanceolate, long-acuminate, sometimes plicate; margins serrulate, typically ciliate, plane; costa single, usually strong, sometimes ± excurrent; cells rectangular, more elongate than in vegetative leaves, often smooth. Setae elongate, stout, smooth, reddish; capsules horizontal to pendent, asymmetric and arcuate, cylindric; exothecial cells quadrate to short-rectangular, thin- to thick-walled, not collenchymatous; annulus differentiated; operculum conic, often obliquely rostrate; exostome teeth yellow to yellow-brown, shouldered, bordered, on the front surface cross-striolate below, sometimes with overlying papillae, papillose above, trabeculate at back; endostome mostly with a high basal membrane, segments keeled, not to narrowly perforate, cilia in groups of 2-3, nodulose. Spores spherical, usually finely papillose. Calyptrae cucullate, naked or sparsely hairy, smooth or roughened.
Discussion:Thuidium Bruch & Schimp. in Bruch, Schimp. & W. Gümbel, Bryol. Eur. 5(fasc. 49/51, Monogr. 1); 157. 1852; Hypnum sect. Thuidium (Bruch & Schimp.) Sull. in A. Gray, Manual, ed. 2, 667. 1864; Hypnum sect. Cyrtohypnum subsect. Thuidium (Bruch & Schimp.) Hampe, Ann. Sci. Nat. Bot. V, 5: 310. 1866; Hypnum subgen. Thuidium (Bruch & Schimp.) Hobk., Syn. Brit. Mosses 145. 1873.
Hypnum sect. Tamariscina Brid., Muscol. Recent. Suppl. 2: 135. 1812; Thuidium sect. Tamariscina (Brid.) Bruch & Schimp. in Bruch, Schimp. & W. Gtimbel, Bryol. Eur. 5(fasc. 49/51, Monogr. 1): 163. 1852.
Hypnum sect. Theliphyllum subsect. Tamariscella Müll. Hal., Syn. Musc. Frond. 2: 482. 1851; Thuidium sect. Tamariscella (Müll. Hal.) Mitt., Trans. Linn. Soc. London, Bot. II, 3: 190. 1891, nom. illeg.; Tamariscella (Müll. Hal.) Müll. Hal., Nuovo Giom. Bot. Ital. 23: 601. 1891.
Discussion. Thuidium is characterized by pinnately branched stems, often frond-forming and usually with abundant paraphyllia. The papillose, ± isodiametric cells are also helpful markers. The genus is easily recognized, but the species present difficulties. Many of them are recognized on only slight morphological differences. One might justifiably recognize many of the species at a varietal level, but they seem consistently different and I have chosen the specific rank here.
In the Americas we think of Thuidium as well differentiated from Cyrto-hypnum by the larger plant size and unipapillose leaf cells. However, in Asia, and to a lesser extent in the American tropics, pluripapillose large plants and unipapillose small plants are not unusual (Watanabe, 1972). Although this certainly seems to confuse the distinctions between the two genera, two groups are still apparent. Large, dioicous plants, with leaf cells papillose only at back, with 3-celled axillary hairs, and usually with well-branched paraphyllia characterize Thuidium. Not only within this genus but throughout the family paraphyllium morphology seems to be among the most stable characters, both in terms of size and branching and also cell shape, size, and papillosity. Although not as easy to observe as branch leaf characters, I have found paraphyllium characters, together with stem leaf features, more reliable.
Gier’s (1980) revision of Thuidium in Latin America is only of marginal use—the lack of original synonymy provides an indication of this.