8. Pleuridium Rabenhorst, Deutschl. Krypt.-Fl. 2(3): 79. 1848.
[Greek, pleura, side or rib, and -idium, diminutive, alluding to sporophytes often emerging laterally from perichaetium]
Kwok Leung Yip
Plants gregarious to loosely tufted, yellow-green. Stems 0.3-0.7 cm, simple or branched; rhizoids at base, smooth. Leaves erect-spreading or appressed when dry, proximal leaves linear to deltoid, distal leaves oblong to lanceolate with subulate to acuminate tips, margins plane and undifferentiated, entire to serrate or abruptly toothed; costa subpercurrent to excurrent, sometimes roughened on the abaxial surface, stereid bands one or two, guide cells centrally located, confluent with the cells of the leaf lamina; basal laminal cells quadrate-rectangular, median cells irregular, rhomboidal to trapezoidal, marginal cells slightly longer than adjacent cells; distal cells elongated. Specialized asexual reproduction by innovations and rhizoidal tubers. Sexual condition monoicous; perigonia occur proximal to perichaetium with antheridia generally naked in leaf axils (paroicous), or in small buds in axils of stem leaves (autoicous); perichaetia single, terminal on mature stem, with 3-8 perichaetial leaves similar to the distal cauline leaves. Seta short, erect or curved. Capsule cleistocarpous, erect, immersed, orange to brownish, ovoid to elliptical; stomata 4-10, superficial, restricted to base of capsule. Calyptra mostly persistent, cucullate. Spores spherical, papillose to short-spinose.
Species 21 (4 in the flora): worldwide except Antarctica.
Pleuridium is related to Ditrichum, as demonstrated by hybridization (L. E. Anderson 1980) and by similarities in gametophyte morphology and molecular characters (C. La Farge et al. 2000). Pleuridium is also closely related to Cleistocarpidium by the immersed cleistocarpous capsules but C. palustre has mitrate calyptras and pale whitish capsules, which are asymmetrical and stomatose to the equator. In Pleuridium, sporophytes may emerge laterally from the perichaetium, not because of an actual lateral origin or position, but owing to the curvature of a short seta (H. A. Crum and L. E. Anderson 1981), or because an innovation originated proximally and developed beyond the perichaetium (Chen P. C. et al. 1963), in some species of Archidium originally included in the genus (J. A. Snider and W. D. Margadant 1973). Rhizoidal tubers were reported for the genus by T. Arts and S. Risse (1988). The “central accessory cells” as used here are equivalent to Begleiter or hydroid cells of other authors.
Bryan, V. S. 1956. Chromosomes and systematic position of the inoperculate mosses, Pleuridium and Bruchia. Amer. J. Bot. 43: 460-468. Snider, J. A. 1994b. Pleuridium. In: A. J. Sharp et al., eds. The moss flora of Mexico. Mem. New York Bot. Gard. 69: 92-95.