Homalothecium
in P. Bruch and W. P. Schimper, Bryol. Europ. 5: 91, plates 456, 457. 1851.
Plants small to large, in loose to dense tufts or mats, green to rich golden or more rarely pale yellow and whitish. Stems creeping, ± densely terete-foliate, regularly or more rarely irregularly pinnate, branches densely terete-foliate; central strand present; pseudoparaphyllia with apex acute to acuminate; axillary hairs of 3–6 cells. Stem leaves erect-appressed when dry, erect, erect-spreading, or rarely erect-appressed when moist, imbricate, lanceolate or narrowly triangular, weakly to strongly concave, strongly longitudinally plicate; base not to broadly decurrent; margins serrulate or entire, teeth sometimes sharp, recurved, especially in alar region; apex gradually tapered or acuminate; costa ending in distal lamina, occasionally percurrent, moderate to stout, terminal spine present or absent; alar cells quadrate to short-rectangular, rarely elongate; laminal cells elongate to linear, walls moderately thick; basal cells small, as wide or slightly wider than more distal cells, walls strongly incrassate, region opaque across base. Branch leaves not differentiated, or somewhat narrower. Sexual condition dioicous or phyllodioicous; perichaetial leaf acumen straight or more rarely reflexed. Seta red-brown, rough or rarely smooth distally. Capsule inclined, horizontal, or erect, red-brown, cylindric or rarely ovoid-oblong, straight or ± curved; annulus separating by fragments or scarcely separating; operculum long-conic and gradually rostrate, sometimes conic; peristome xerocastique, then perfect or slightly modified, or hygrocastique. Calyptra naked. Spores 10–21 µm.
Distribution
North America, nw Mexico, Europe, w, c Asia, n Africa, Atlantic Islands, Mediterranean climate areas.
Discussion
Species ca. 10 (9 in the flora).
A. J. Grout (1928–1940, vol. 3), as well as many early botanists, accepted a narrow circumscription of Homalothecium, placing some species in the genus Camptothecium. These taxa differ by peristome reduction in Homalothecium; the peristome is perfect or only slightly reduced in Camptothecium. However, H. Robinson (1962) combined the two genera in Homalothecium, and this decision was followed by E. Lawton (1965, 1971) and subsequent American authors. Recent molecular phylogenetic studies (S. Huttunen and M. S. Ignatov 2004; Huttunen et al. 2008) demonstrated that peristomial reduction took place independently in more than one lineage of the genus. The genus was monographed by H. Hofmann (1998), and its phylogeny was reconstructed by Huttunen et al. The latter analysis demonstrated that Homalothecium megaptilum, treated by W. B. Schofield (1968) in the separate genus Trachybryum, is nested in the Homalothecium clade, supporting the opinions of D. H. Norris and J. R. Shevock (2004) and Ignatov and Huttunen (2002), who included it in Homalothecium. The recently described H. californicum has a number of common features with H. megaptilum, but as an unrelated species, is an example of convergent evolution, and thus provides additional evidence for merging Homalothecium with Trachybryum.
Selected References
Lower Taxa
Key
| 1 | Plants very large; stems regularly pinnate, branches to 15-20(-30) mm; stem leaves 3.5-4.5 mm; alar cells elongate. | Homalothecium megaptilum |
| 1 | Plants small to large; stems irregularly branched, or if regular then branches 5-15 mm; stem leaves to 2.5(-3.3) mm; alar cells subquadrate or ovate, occasionally with few elongate cells | > 2 |
| 2 | Leaf margins with several recurved teeth proximally; Atlantic coast (Newfoundland) or Pacific coast | > 3 |
| 2 | Leaf margins without or occasionally with recurved teeth proximally; Pacific coast and Rocky Mountains | > 4 |
| 3 | Plants small to medium-sized; branches 3-5 mm; leaf apices long-acuminate, acumina filiform, providing hyaline-villous aspect; Pacific coast. | Homalothecium nuttallii |
| 3 | Plants medium-sized to large; branches 5-7 mm; leaf apices acuminate or gradually tapered, acumina occasionally short-filiform, not providing hyaline-villous aspect; Atlantic coast (Newfoundland). | Homalothecium sericeum |
| 4 | Basal laminal cells in 5-8 rows; branch leaf distal laminal cells prorate on abaxial surface. | Homalothecium arenarium |
| 4 | Basal laminal cells in 1-3(-5) rows; branch leaf distal laminal cells smooth | > 5 |
| 5 | Stems ± remotely pinnate, very unequal so branching appears not pinnate; branch leaves erect and somewhat spreading when dry. | Homalothecium fulgescens |
| 5 | Stems regularly pinnate; branch leaves usually appressed when dry | > 6 |
| 6 | Stem leaf alar cells subquadrate, walls relatively thin, regions translucent, of 15-20 × 10-15 cells, arranged immediately along margins. | Homalothecium aureum |
| 6 | Stem leaf alar cells ovate, walls thick, regions opaque to moderately translucent, of few cells, or if more, arranged mostly along submarginal folds, separated from margin | > 7 |
| 7 | Plants large, usually green; stem leaf alar regions 15-25 cells wide; capsules straight. | Homalothecium californicum |
| 7 | Plants medium-sized, usually golden yellow or light green; stem leaf alar regions 6-10 cells wide; capsules straight or curved | > 8 |
| 8 | Capsules curved; operculum conic; endostome cilia long; branch leaf alar cells subquadrate to irregularly ovate, walls moderately incrassate. | Homalothecium aeneum |
| 8 | Capsules straight or slightly curved; operculum long-conic to rostrate; endostome cilia short or absent; branch leaf alar cell shape irregular, walls strongly incrassate. | Homalothecium nevadense |