12. Isopterygium Mitten, J. Linn. Soc., Bot. 12: 21, 497. 1869. • [Greek isos, equal, and pteron, wing, alluding to complanate leaves].
Robertr. Ireland Jr.
Plants small to medium-sized, in thin to dense mats, whitish, yellowish, or green, glossy. Stems creeping, simple or sparingly and irregularly branched; hyalodermis absent, central strand usually absent; pseudoparaphyllia filamentous, <1 (or 2)-seriate at base>. Stem and branch leaves similar, erect-spreading to squarrose, ovate or lanceolate, not plicate; base not decurrent or rarely 1 or 2 cells decurrent; margins plane to erect, serrulate to entire proximally, serrate to serrulate distally, or rarely entire throughout; apex acuminate; costa double and short or sometimes ecostate; alar cells usually clearly differentiated, quadrate to rectangular, rarely transversely elongate; laminal cells smooth. Specialized asexual reproduction sometimes present as filamentous, multicellular, branched brood bodies on stems and branches, cells papillose. Sexual condition autoicous [rarely dioicous]; perichaetia at base of stems, leaves oblong-lanceolate, apex gradually acuminate. Seta yellow, brown, or reddish brown. Capsule inclined to cernuous, sometimes erect, ovoid to ellipsoid, straight or arcuate when mature, usually contracted below mouth and sometimes wrinkled at neck when dry; annulus absent; operculum conic to short-rostrate; peristome double; exostome teeth with external surface cross striolate proximally, papillose distally; endostome basal membrane high to low, segments keeled, cilia shorter than segments, in groups of 1-3, sometimes absent. Calyptra naked. Spores spheric to ovoid, smooth or minutely papillose.
Species 120-140 (2 in the flora): North America, Mexico, West Indies, Central America, South America, s Europe (Italy), Asia, Africa, Pacific Islands, Australia.
Isopterygium occurs predominately in terrestrial habitats at low elevations in the subtropics and tropics. Many of the names in this genus will undoubtedly prove synonymous with those of other taxa after revision. Plants of this genus have complanate-foliate stems with small, thick-walled cortical cells and larger, thinner walled inner cells; the smooth rhizoids arise on the ventral surfaces of stems and branches just below the junctures of leaves; and the axillary hairs have one short-rectangular basal cell and one elongate apical cell. The leaves are crowded, imbricate, often asymmetric, and flat or somewhat concave, sometimes with pitted basal laminal cells. The perigonia are scattered along the stems; the setae are usually twisted; the capsules are smooth; and the exostome teeth are bordered and internally trabeculate.
SELECTED REFERENCE Iwatsuki, Z. 1987. Notes on Isopterygium Mitt. (Plagiotheciaceae). J. Hattori Bot. Lab. 63: 445-451.