Gymnostomum Nees & Hornsch.

The following generic description is modified from Zander (1993).

Plants yellow-green to olive-green above, light to dark brown below, forming turves or cushions, usually on calcareous soil or rock. Stems simple or branched, sparsely to densely tomentose below, in cross-section rounded-triangular to rounded-pentagonal, with central strand usually present, sclerodermis present (in N.Z. species) or absent, and hyalodermis present or absent (in N.Z. species). Leaves erect-spreading to spreading-recurved when moist, appressed to appressed-incurved when dry, usually ligulate, rounded-obtuse to broadly acute; margins plane or occasionally recurved below, entire or papillose; upper laminal cells clear in outline to obscure (in N.Z. species), subquadrate, thin- to firm-walled, pluripapillose with papillae simple or complex; upper marginal cells not differentiated; lower laminal cells weakly differentiated, rectangular, 2–4:1, smooth. Costa usually ending 2–5 cells below apex or occasionally percurrent; adaxial superficial cells quadrate to short-rectangular and papillose; abaxial superficial cells elongate and papillose; in cross-section with 1 or 2 stereid bands. Axillary hairs 3–10 cells in length, the basal 1–2 usually brown. Laminal KOH colour reaction usually yellow to yellow-orange, occasionally red in patches.

Dioicous. Perichaetia terminal, with inner leaves ovate-lanceolate from a broadened sheathing base. Perigonia terminal, bulbiform. Setae elongate. Capsules ovoid to ellipsoid. Operculum rostrate or rostrate from a conical base. Peristome absent. Spores 9–15 µm, smooth (in N.Z. species) or papillose.

A widely distributed genus of some 24 species (Zander 1993​​​​​​​). The single N.Z. species is widespread in temperate regions of the world.

Four species described by C. Knight in Gymnostomum are not retained in the genus.

Gymnostomum areolatum C.Knight, Trans. & Proc. New Zealand Inst. 7: 355, 1875. (WELT-Knight M044539!) is synonymous with Tortula areolata (C.Knight) Fife.

Gymnostomum knightii Schimp. ex C.Knight, Trans. & Proc. New Zealand Inst. 7: 354, 1875. (WELT-Knight M044541!) is synonymous with Geheebia ceratodontea (Müll.Hal.) J.E.Beever.

Gymnostomum patulum C.Knight, Trans. & Proc. New Zealand Inst. 7: 354, 1875. (WELT M044538!) is synonymous with Weissia patula (C.Knight) Fife.

Gymnostomum sulcatum C.Knight, Trans. & Proc. New Zealand Inst. 7: 355, 1875. (WELT-Knight M044540!) is synonymous with Zygodon intermedius Bruch & Schimp.

Weissiaacutifolia R.Br.bis, was placed in the synonymy of Gymnostomum calcareum (as G. calcareum var. longifolium) by Dixon (1923, p. 116), but the holotype (CHR 335700!) and an isotype in Dixon’s herbarium (BM 000845878!) are both referable to Trichostomopsis australasiae​​​​​​​ (Hook. & Grev.) H.Rob.

Trichostomum linearifolium R.Br.bis was placed in synonymy of Gymnostomum calcareum by Dixon (1923, p.116), but the holotype (CHR 335723!) and an isotype in Dixon’s herbarium (BM 000876953!) are both referable to Tortella knightii (Mitt.) Broth.

The Australian Gymnostomum calcareum var. australe was mistakenly recorded for N.Z. in Index Muscorum (Wijk et al. 1962), and subsequently by Zander (1993).  These reports are perhaps due to misinterpretations of the entry “Pac.: Austral. or. (Victoria)" cited by Paris (1900). Comparison of type material (MEL 1002903) with representative European and N.Z. specimens confirms that the variety is not worthy of recognition.