Bot. Zeitung (Berlin) 5: 99. 1847 ,
Nearly worldwide, mainly in temperate regions.
Species 15 (3 in the flora).
Acaulon is similar to species of Microbryum in the bulbiform habit, leaves red in KOH, but its species are even smaller in size, and the capsules are spheric, lacking an apiculus. Papillae are absent in two of the three species. Although spore size and ornamentation have figured in taxonomy in the past, according to H. A. Crum and L. E. Anderson (1981) there is much overlap; but, similarly, their use of differences in margin recurvature and seta length do not hold. Taxa belonging to the old, superficially similar Phascum have been recently placed largely with Tortula and Microbryum (R. H. Zander 1993). I. G. Stone (e.g., 1989) has studied the genus extensively in Australia, and Casas de Puig (e.g., C. Casas de Puig et al. 1990) and her students have done the same for the Iberian Peninsula.
|1||Leaves awned, laminal cells papillose abaxially.||Acaulon schimperianum|
|1||Leaves cuspidate or muticus, laminal cells smooth||> 2|
|2||Plants often three-angled, about 1 mm, leaves keeled, seta about as long as the diameter of the capsule, spores about 30 µm, finely papillose.||Acaulon triquetrum|
|2||Plants flattened-globose or three-angled, about 2 mm, leaves broadly channeled, seta short, about 0.3 the diameter of the capsule, spores 30-50 µm, smooth or papillose.||Acaulon muticum|