Plants with a prostrate, leafless rhizome with descending, hyaline, leafless stolons and aerial leafy shoots; growth via tetrahedral apical cell; central strands of hydroids present. Leafy shoots 3-ranked, isophyllous or anisophyllous with the reduced row of leaves dorsal; oil bodies present; rhizoids absent. Sexual condition dioicous or monoicous; antheridia and archegonia with a common pattern of development at least in early ontogenetic stages; acrogynous or anacrogynous. Sporophyte protective structure a shoot-calyptra, a true calyptra, or a perigynium; perianth absent. Sporophytes with capsule wall 1-stratose with each cell traversed by a single, annular, longitudinal thickening band.
Genus 1, species 7 (1 genus, 1 species in the flora): temperate, subtropical and tropical regions, nearly worldwide, but disjunct and local.
Haplomitriaceae constitutes the sole family of the Haplomitriales (= Calobryales), with Haplomitrium Nees representing the sole extant genus of the family. The Paleozoic fossil genus, Gessella C. Poulsen from Denmark is also considered a member of the Haplomitriaceae.
SELECTED REFERENCE Bartholomew-Began, S. E. 1991. A morphogenetic re-evaluation of Haplomitrium Nees (Hepatophyta). Bryophytorum Bibliotheca. 41. Berlin.
1. HAPLOMITRIUM Nees, Naturg. Eur. Leberm., 1: 109. 1833, name conserved • Greek aploos, simple and mitrion, little cap, in reference to the shoot calyptra serving as the sole protective structure for the sporophyte
Calobryum Nees; 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.
SELECTED REFERENCES Bartholomew-Began, S. E. 1991. A morphogenetic re-evaluation of Haplomitrium Nees (Hepatophyta). Bryophytorum Bibliotheca. 41. 297 pp., 508 figs. Berlin. Engel, J. J. 1981. Haplomitrium monoicum, a remarkable new species of Calobryales (Hepaticae) from New Caledonia, together with a reclassification of subg. Haplomitrium. Ann. Missouri Bot. Gard. 68: 668--676. Schuster, R. M. 1967. Studies on Hepaticae XV. Calobryales. Nova Hedwigia 13: 1--63, 12 plates.
1. Haplomitrium hookeri (Smith) Nees, Naturg. Eur. Leberm., 1: 111. 1833
Jungermannia hookeri Smith, Sowerby’s Engl. Bot., 36, plate 2555. 1813; Scalia hookeri (Smith) Lindberg; Scalius hookeri (Smith) Gray
Varieties 2 (1 in the flora): North America, Eurasia, New Zealand.
1a. Haplomitrium hookeri var. hookeri
Plants 5--16(--27) ´ 1.4--2.6 mm. Stolons unbranched. Leafy shoots erect, terminal branching rare; isophyllous with all leaves transversely inserted or slightly anisophyllous with lateral leaves transverse, subtransverse, or succubous. Leaves ovate to orbicular or rhomboidal, entire or shallowly lobed, 1-stratose except for a restricted bistratose basal field. Oil bodies 12--26 per cell, 2--4 µm; homogeneous to finely granular. Specialized asexual reproduction by gemmae (rare). Sexual condition dioicous. Gametangia axillary or supra-axillary in loose terminal clusters, in scattered undifferentiated or slightly modified bracts or free-standing along the axis; antheridia 2--4 to 6(--15) per bract or single and naked, yellow-orange; archegonia 1--3(--10) per bract or single and naked. Sporophyte protective structure a true calyptra. Seta with central row of enlarged cells present. Capsule 1.2--3.0 ´ 0.6--0.8 mm, dehiscence along (1--)2--4(--5) complete or incomplete sutures; elaters free, attenuate, 2-spiralled becoming 1-spiralled at the tips or 1-spiralled throughout. Spores 26--36 µm, exine verruculate with limited fusion of the verruculae.
Capsules mature late summer--early fall. Soil in hygric areas such as sandy depressions, moist heath, or margins of ditches, often growing at bases of boulders in alpine meadows in the southern part of its range, among heathers at cliff bases or on rocky slopes in mesic to somewhat dry, exposed sites on the crests of barren, alpine ridges in the northern part of its range; 900--1900 m; B.C.; Greenland (T. Böcher 1963); Alaska, Colo., Maine (R. M. Schuster 1966, 1967), N.H. (A. W. Evans 1917), Oreg.; Wash.; Europe; Asia (including Indonesia).
The virtually cosmopolitan range of H. hookeri, its wide variety of ancestral traits, and the great plasticity of these traits suggest that H. hookeri is most likely the ancestral progenitor of the genus. Being the sole Haplomitrium species known from North America, H. hookeri is easily distinguished from other hepatics within the flora by its branched rhizomatous habit with erect leafy shoots, descending, leafless stolons, and the absence of rhizoids. It is further distinguished by the production of a true calyptra in lieu of a perianth, and massive sporophyte in which the capsule wall is 1-stratose with each cell traversed by a single annular longitudinal band. Worldwide, H. hookeri is distinguished from all other Haplomitrium species by its unique spore morphology, i.e., it is the only Haplomitrium species with verruculate spore exines with limited fusion of the verruculae. Distinguishing vegetative gametophytic markers include erect leafy shoots with entire to lobed leaves on the same shoot, and lateral leaves with small, basal 2-stratose zones. Fertile shoots are dioicous with clustered or scattered, axillary, supra-axillary, or free-standing gametangia and true calyptrae. Taxonomic difficulties arise from the plasticity of gametophytic characters and are enhanced by artificial environmental conditions which induce morphological variability (S. E. Bartholomew-Began 1991; T. Furuki 1986).
SELECTED REFERENCES Gottsche, C. M. 1843. Anatomische-physiologische Untersuchungen über Haplomitrium Hookeri N. v. E. mit Vergleichung anderer Lebermoose. Nova Actorum Acad. Caes. Leop.-Carol. Nat. Cur. 20: 267--289, pl. XIII--XX. Worley, I. A. 1969. Haplomitrium hookeri from Western North America. Bryologist 72: 225--232.
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.
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