Monographs Details: Leptodictyum
Authority: Buck, William R. 1998. Pleurocarpous mosses of the West Indies. Mem. New York Bot. Gard. 82: 1-400.
Description:Genus Description - Plants slender to rather robust, in usually dull, rarely lustrous, soft to flaccid, mostly yellow-green, thin mats. Stems creeping, irregularly and freely branched; in cross-section without a hyalodermis, with a poorly differentiated outer layer of small thick-walled cells surrounding somewhat larger thinner-walled cells, central strand present, small; paraphyllia absent; pseudoparaphyllia foliose; axillary hairs with 1(-2) short brown basal cells and several elongate hyaline distal cells. Stem and branch leaves similar, usually wide-spreading, rarely erect, not falcate-secund, distantly spaced, ovate-lanceolate to oblong-lanceolate, acute to acuminate, not channeled above, sometimes ± contorted when dry; margins mostly entire or serrulate at apex, plane; costa single, slender, ending 1/2-2/3 the leaf length, rarely longer; cells long-hexagonal to linear, smooth, thin-walled, becoming broader toward the insertion; alar cells not or poorly differentiated. Asexual propagula none. Autoicous. Perichaetia enlarged, conspicuous; leaves erect, lanceolate to oblong-ovate, acuminate, sometimes plicate; margins mostly entire, plane; costa single, longer than in vegetative leaves, ending near the acumen, cells linear, smooth, becoming shorter and broader toward the insertion. Setae elongate, smooth, red-brown to yellow-brown, slightly twisted when dry; capsules suberect to strongly inclined, sometimes arcuate, asymmetric, cylindric, constricted below the mouth when dry, exothecial cells ± rectangular, firm-walled, not collenchymatous, stomata long-pored; annulus differentiated in several rows, operculum conic to conic-short-rostrate; exostome teeth yellow-brown, shouldered, bordered, on the front surface cross-striolate below, coarsely papillose above, trabeculate at back; endostome with a high basal membrane, segments keeled, not or narrowly perforate, cilia in groups of 1-4, nodulose to appendiculate. Spores spherical, finely papillose. Calyptrae cucullate, naked, smooth.
Discussion:Leptodictyum (Schimp.) Warnst., Krypt.-Fl. Brandenburg 2: 867. 1906; Amblystegium subgen. Leptodictyum Schimp., Syn. 595. 1860; Amblystegium sect. Leptodictyum (Schimp.) Monk, in Rabenh., Rabenh. Krypt.-Fl., ed. 2, 4(Suppl. [Laubm. Eur.]): 730. 1927.
Discussion. Leptodictyum is characterized by soft or flaccid plants typically with distantly spaced, wide-spreading leaves. The single costa mostly extends about 2/3 the leaf length, and the leaf cells are relatively long. The alar cells are only poorly, if at all, differentiated.
Leptodictyum may be distinguished from Hygroamblystegium by the shorter, more slender costa and thinner-walled, longer leaf cells. The more distantly spaced leaves also aid in separation. From Amblystegium, Leptodictyum is distinguished by the much longer leaf cells and more poorly differentiated alar cells.
The plants seem to be adapted to low-light, high-humidity (or aquatic) habitats. Although plants are sometimes found in seemingly xeric conditions, their occurrence usually indicates a habitat at least seasonally submerged (perhaps by flash floods), much like the habitat favored by Philonotis.
I cannot agree with Kanda’s (1975) broad concept of Leptodictyum. His inclusion of dioicous species and species with filamentous pseudoparaphyllia and channeled leaf apices in the genus seems to argue for heterogeneity.