Mueller's Isopterygiopsis Moss

Isopterygiopsis muelleriana

Isopterygiopsis muelleriana (Schimp.) Z. Iwats., J. Hattori Bot. Lab. 33: 379. 1970. Plagiothecium muellerianum Schimp., Syn. Musc. Eur. 584. 1860; Isopterygium muellerianum (Schimp.) A. Jaeger, Ber. Thätigk. St. Gallischen Naturwiss. Ges. 1876–77: 441. 1878. Type: Austria?, 30 Sep 1851, J. Müller (holotype: BM!).

Plants in thin mats, glossy, yellowish green. Stems to 2 cm long, 0.8–2.0 mm wide, complanate-foliate, appearing ventrally concave because of curvature of upturned leaves, simple or irregularly branched; hyalodermis of large epidermal cells, enclosing 1 to several layers of small, thick-walled cortical cells and several large, thin-walled cells in the center. Asexual reproductive bodies not seen on Latin American plants, known on plants from elsewhere as axillary clusters of 2–6 celled, smooth, cylindrical or fusiform, green to yellowish green brood bodies in leaf axils on stems and branches. Leaves 0.5–1.0 mm long, 0.2–0.3 mm wide, somewhat rigid, crowded and overlapping, erect-spreading and rigidly complanate, smooth, ovate- to oblong-lanceolate, symmetric, acuminate, often abruptly so; margins plane, entire or minutely serrulate; costa none or short and double; median cells 50–80 × 2–5 μm, smooth or prorulose at upper ends on dorsal leaf surface; alar cells not differentiated or sometimes with 1–3 short-rectangular cells on margins. Sex organs and sporophytes not seen on Latin American plants, known to be dioicous from regions where it is fertile and possesses sporophytes. Setae 0.6–1.2 cm long, brown to reddish brown; capsules 0.5–1.5 mm long, erect or rarely ± cernuous, straight, ellipsoid to ovoid, contracted below the mouth when dry; operculum conic to obliquely rostrate; annulus present, deciduous, of 2–3 rows of cells. Spores 8–12 μm in diameter.

Distribution and ecology: Rare; known only from a single Mexican collection (Figure 8); occurring on a bluff at 3139 m.

Discussion: Isopterygiopsis muelleriana is a species that is rare or rarely collected in Latin America. It is best recognized by its plants that are complanate-foliate, appearing concave on the upper surface because of the upturned leaves, and by the stems that have a hyalodermis with large, thin-walled cells (best seen in cross section) with papillose rhizoids clustered in the leaf axils. Other helpful features distinguishing the species, are the ovate- to oblong-lanceolate, nondecurrent leaves, often abruptly acuminate with margins that are entire or minutely serrulate, the median leaf cells that are 50–80 × 2–5 μm, often prorulose at the distal ends on the dorsal leaf surface, the few differentiated alar

cells, with 1–3 short-rectangular ones on the margins. In addition, the plants are dioicous, and they sometimes possess smooth, cylindrical or fusiform brood bodies clustered in leaf axils on the stems and branches.

Specimen examined: MEXICO. Distrito Federal: Desierto de los Leones near Mexico City, 3139 m, Sharp 64 (CANM, MEXU, MICH, TENN).