Haplomitrium hookeri var. hookeri
Authors: Sharon E. Bartholomew-Began
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.