4. Thuidium Schimper in P. Bruch and W. P. Schimper, Bryol. Europ. 5: 157, plates 481-486. 1852. • [Genus Thuja and Latin -idium, diminutive, alluding to branched habit].
Howard A. Crum
Plants large. Stems creeping to arched-ascending, 2- or 3-pinnate; paraphyllia mostly foliose, lanceolate or filiform-branched; axillary hairs 2-6(-7)-celled. Stem leaves ovate; margins plane, reflexed, or revolute, papillose-serrulate proximally, often serrulate distally by projecting cell ends; costa ending before apex; laminal cells rounded- to oblong-hexagonal, rhombic, or elliptic, coarsely 1-papillose abaxially, papillae sometimes 2-fid, appearing multipapillose. Branch leaves ovate; margins incurved, erect, or recurved, entire to denticulate; apex usually acute; costa ending before apex, not covered with cells apically; laminal cells rounded, hexagonal, elliptic, or oblong-rhombic, 1-papillose. Sexual condition dioicous; perichaetial leaves lanceolate or ovate-lanceolate, margins sometimes ciliate, costa ending near apex, laminal cells smooth or somewhat papillose. Seta smooth or rarely papillose. Capsule inclined to horizontal, curved-cylindric, asymmetric; annulus 2-4-seriate; operculum conic or rostrate from convex-conic base; endostome segments not perforate, cilia in groups of 2-4.
Species ca. 230 (4 in the flora): North America, Mexico, West Indies, Central America, South America, Europe, Asia.
Thuidium is characterized by abundant, 2- or 3-pinnate paraphyllia; dioicous sexuality; dimorphic stem and branch leaves with short, thick-walled cells that are papillose abaxially and nearly always 1-papillose; stem leaves with base more or less plicate and laminal cells more or less uniform; apical cells of branch leaves usually truncate and multipapillose; and setae nearly always smooth. The capsule is smooth, with a short neck. The genus is treated here in a sense exclusive of Abietinella, Cyrto-hypnum, and Rauiella (see W. R. Buck and H. A. Crum 1990). Abietinella is relatively robust and has 1-pinnate branching, abundant paraphyllia, dioicous inflorescences, laminal cells 1-papillose on both surfaces, and 3- or 4-celled, brown axillary hairs. Cyrto-hypnum and Rauiella are small with autoicous inflorescences, paraphyllia short and not much branched, and laminal cells papillose on both surfaces (most commonly multipapillose). Cyrto-hypnum has mostly 2-pinnate branching, 2-celled axillary hairs, leaves incurved when dry, laminal cells plane, and setae often roughened. Rauiella has 1-pinnate branching, 3-celled axillary hairs, leaves not incurved when dry, strongly bulging laminal cells, and smooth setae.