Serpoleskea (Hampe) Loeske
Plants very small and slender, in light- to dark-green or brownish mats. Stems creeping, freely and irregularly branched; branches erect-ascending; paraphyllia none; pseudoparaphyllia present or lacking. Leaves very small, 0.15--0.3(--0.5) mm, slightly concave, erect-appressed to loosely spreading, sometimes subsecund, lanceolate, acuminate; margins plane, entire to serrulate; costa none or very short, double, and indistinct; medial cells subquadrate to oblong-rhombic or short-rhomboidal, rather thick-walled; alar cells scarcely differentiated to subquadrate in small, inconspicuous groups. Perichaetial leaves pale, erect, sheathing at base, acuminate. Seta elongate, reddish to brownish. Capsule erect and symmetric or variously inclined and asymmetric, oblong-cylindric to oblong-obovoid, not or variously contracted below the mouth when dry, smooth; annulus of 1--2 layers of well-differentiated cells; operculum convex-conic to conic, bluntly apiculate to rostellate; stomata generally present in the short neck; peristome teeth lanceolate, yellow to yellow-brown, bordered, cross-striolate proximally, pale and papillose distally, trabeculate at back; endostome pale, finely papillose, with a moderately high basal membrane, keeled, not or narrowly perforate segments, cilia none or 1--3 and rudimentary to well developed. Calyptra naked. Spores small, 8--13 µm, finely papillose.
Species 10 (4 in the flora): Europe, Asia, North America, South America.
Plants of this genus resemble Amblystegium but are even smaller and have leaves virtually ecostate and the capsule is variously inclined and asymmetric but not slenderly cylindric, curved-asymmetric, or greatly contracted below the mouth and at the neck when dry. The genus seems best allied to the Hypnaceae because of its ecostate, often secund leaves and capsule shape. L. S. Cheney's (1897) treatment of the genus Amblystegium, in a broad sense, is a useful reference for the species of Platydictya. Illustrations are presented in that work, as well as by H. A. Crum and L. E. Anderson (1981).
According to L. Hedenäs (1987), Platydictya confervoides and P. subtilis should be treated as a separate genus, Serpoleskea, owing to brown, smooth rhizoids located below the leaves in contrast to the purple, granular-papillose, axillary ones of P. jungermannioides. However, I find the species too interrelated and too difficult to distinguish from one another to allow generic segregation.
Crum, H. A. and L. E. Anderson. 1981. Mosses of Eastern North America. Vol. 2, pp. 142--147. New York. Brotherus, V. F., Musci. In: A. Engler, Die natürlichen Pflanzenfamilien. Ed. 2. Vol. 11, pp. 341--342. Leipzig. Cheney, L. S. 1897. North American species of Amblystegium. Bot. Gaz. 24: 236--291. Hedenäs, L. 1987. North European mosses with axillary rhizoids, a taxonomic study. J. Bryol. 14: 429--439. Söderstrom, L., K. Karttunen & L. Hedenäs. 1992. Nomenclatural notes on Fennoscandian bryophytes. Ann. Bot. Fenn. 29: 119--122.