Steereomitrium E. O. Campbell
Plants ca. 4.5--55 1--6.5 mm, pale to bright green. Rhizomes prostrate, branched, chlorophyllous. Stolons positively geotropic, unbranched (branched), hyaline. Leafy shoots erect, prostrate, or obliquely ascending, unbranched, or rarely with lateral basal or distal terminal branching, chlorophyllous. Lateral leaves transverse, subtransverse, or succubous depending on symmetry; ranging from subrotund to ovate or rhomboidal to lingulate-triangular, entire to shallowly lobed. Dorsal leaves transverse; similar to that of lateral leaves, or reduced in anisophyllous representatives. Oil bodies (3--)10--74 per cell. Gametangia axillary or supra-axillary, scattered along the stem in undifferentiated or slightly enlarged bracts, aggregated into loose terminal clusters, positioned at alternating swollen nodes, aggregated into well-defined rosettes on swollen discs, or free-standing on the stem. Sporophytes with foot obconiodal; capsule cylindrical, dehiscence variable; elaters attenuate, 350--700 7--11 µm at the broadest portion, with spiral thickenings; spores spherical, 15--36 µm. Germination exosporic, not precocious.
Species 7 (1 in the flora): temperate, subtropical, and tropical regions; disjunct and local throughout its range; North America, West Indies, South America, Europe, Asia (including Indonesia), Africa (Republic of the Congo, Rwanda), Australia (New South Wales, Tasmania), Pacific Islands (New Caledonia, New Zealand, Papua New Guinea).
The center of greatest species diversity is the Southern Hemisphere, specifically Australasia, with three species (H. ovalifolium, H. monoicum, and H. intermedium) being endemic to this region. However, the widest ranging species, H. hookeri, extends throughout the northern hemisphere. Worldwide, the genus displays a broad altitudinal range (0--3600 m). The taxon is restricted to soil habitats with the following exceptions: H. mnioides occurs on soil and rock, and H. blumii occurs exclusively on wood and rock.
Haplomitrium is a very ancient taxon that most likely shares a common ancestor with the Metzgeriales. R. M. Schuster (1967) recognized three subgenera (Haplomitrium, Calobryum, and Archibryum), while J. J. Engel (1981) and S. E. Bartholomew-Began (1991) recognized only two (Haplomitrium and Calobryum). The only North American species, H. hookeri, belongs to the subgenus Haplomitrium.
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Böcher, T. 1963. Phytogeography of Middle West Greenland. Meddel. Grønland 148: 1--289, 10 plates.
Evans, A. W. 1917. Notes on New England Hepaticae, XIV. Rhodora 19: 263--272.
Furuki, T. 1986. Gemmae of Haplomitrium hookeri (Smith) Nees. Hikobia 9: 495-496.
Frye, T. C. and L. Clark. 1943. Hepaticae of North America, Part II. Univ. Wash. Publ. Biol. 6: 166--336.
Poulsen, C. 1974. Further contributions to the knowledge of the Paleozoic of Slagelse no. 1, western Sealand. Dan. Geol. Unders. II. Raekke 101: 28--33.
Schuster, R. M. 1966. The Hepaticae and Anthocerotae of North America East of the Hundredth Meridian. Vol. 1. New York.