Monographs Details: Taxithelium
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 medium-sized, in somewhat lustrous, ± soft, yellow-green, thin, often extensive mats. Stems creeping, subpinnately branched, the branches usually complanate-foliate; in cross-section with small thick-walled green cells surrounding larger thinner-walled cells, central strand absent; pseudoparaphyllia foliose; axillary hairs with 1-2 short brown basal cells and 1 to several elongate hyaline distal cells. Stem and branch leaves ± similar, the stem leaves more acuminate, somewhat larger, and less papillose than branch leaves, all leaves erect- to wide-spreading, not falcate-secund, the lateral and ventral leaves often differentiated from one another, lanceolate to ovate, obtuse to acuminate, concave; margins serrulate, usually plane; costa short and double or absent; cells linear, sometimes flexuose, mostly seriately pluripapillose, rarely (extralimitally) prorulose or smooth, thin- to firm-walled, not or scarcely porose; alar cells few and quadrate in basal angles, or inflated and colored. Asexual propagula ? none. Autoicous or dioicous. Perichaetial leaves erect, mostly lanceolate, acuminate; margins serrulate to serrate, plane; costa mostly none; cells linear, seriately pluripapillose or smooth; alar cells not differentiated. Setae elongate, slender, smooth, usually reddish; capsules inclined, asymmetric, ovoid to cylindric, constricted below the mouth when dry and empty; exothecial cells subquadrate to short-rectangular, indistinctly collenchymatous; annulus not differentiated; operculum conic to obliquely conic-rostrate; peristome double, exostome teeth narrowly triangular, shouldered, bordered, on the front surface with a zig-zag median line, cross-striolate below, coarsely papillose above, trabeculate at back; endostome with a high basal membrane, segments papillose, broad, keeled, perforate, almost as long as the teeth, cilia usually single. Spores spherical, finely papillose. Calyptrae cucullate, naked, smooth.
Discussion:Taxithelium Spruce ex Mitt., J. Linn. Soc., Bot. 12: 496. 1869; Taxithelium Spruce, Cat. Muse. 14. 1867, nom. nud.
Hypnum sect. Omalia subsect. Sigmatella Müll. Hal., Syn. Muse. Frond. 2: 263. 1851; Hypnum sect. Sigmatella (Müll. Hal.) Müll. Hal., J. Mus. Godeffroy 3(6): 86. 1874; Trichosteleum sect. Sigmatella (Müll. Hal.) A. Jaeger, Ber. Thätigk. St. Gallischen Naturwiss. Ges. 1876-77: 411. 1878; Sigmatella (Müll. Hal.) Müll. Hal., Bot. Jahrb. Syst. 23: 328. 1896, hom, illeg., non Kütz., Alg. Aq. Dulc. Germ. 2. 1833 [n.v.], nom. rej. [= Nitzschia Hassall, nom. cons., Bacillariophyceae].
Discussion. Taxithelium is very easy to recognize, and fortunately so, because T. planum is one of the most common and weediest mosses in lowland tropical America. The pluripapillose laminal cells with the papillae seriately arranged immediately separate the genus from all others among our Sematophyllaceae. The Brazilian species were treated by Buck (1985).
Renauld and Cardot (1901) divided Taxithelium into three subgenera on the basis of leaf papillosity: Polystigma (= Taxithelium), Monostigma, and Oligostigma. All the West Indian species belong to subgen. Taxithelium, T planum being the type species of the genus. Our three species, though, are placed in two different sections, sect. Taxithelium for T planum and T. portoricense, and sect. Aptera for T. pluripunctatum. Microscopically these three species seem to be easily accommodated in a single genus, but macroscopically the two sections are very different. The largest concentration of species of Taxithelium is found in Malesia (Damanhuri & Longton, 1996).