2. Dichodontium pellucidum (Hedwig) Schimper, Coroll. Bryol. Eur. 12. 1856.
Dicranum pellucidum Hedwig, Sp. Musc. Frond., 142. 1801; Bryum flavescens Dickson ex Withering; Dichodontium flavescens (Dickson ex Withering) Lindberg; D. pellucidum var. americanum Lesquereux & James; D. pellucidum var. fagimontanum (Bridel) Schimper; D. subflavescens Kindberg
Stems 1.5-5.5(-8) cm. Leaves 1-4 mm, apex narrowly to broadly acute to rounded-obtuse; margins weakly to strongly recurved in proximal half or sometimes nearly to the apex, generally smooth near the insertion, margins of leaf apex strongly papillose but without teeth; marginal laminal cells weakly papillose by projecting distal cell ends or smooth in proximal half of the leaf; median leaf cells with horned papillae on both surfaces; costal cells adaxially elongate (3-6:1), often smooth, clearly different from adjacent laminal cells. Specialized asexual reproduction occasional, by mullticellular ovoid to ellipsoidal gemmae borne on branched, rhizoid-like structures in the leaf axils. Sexual condition dioicous. Seta 9-20 mm. Capsule 1.2-1.6 mm, not strumose, smooth when dry. Spores 13-20(-25) µm.
Capsules mature late fall-early spring. Moist soil on banks, wet cliffs, near streams, calcareous or acid rock; low to high elevations (to 2300 m); Greenland; Alta., B.C., N.B., Nfld. and Labr., N.S., Nunavut, Ont., P.E.I., Que., Yukon; Alaska, Ariz., Calif., Colo., Idaho, Mich., Minn., Mont., N.J., N.C., Ohio, Oreg., Pa., Tenn., Utah, Wash., Wis.; Europe; Atlantic Islands (Iceland).
The leaf margins of Dichodontium pellucidum are strongly serrate-denticulate in the distal 2/3-3/4 of the leaf, varying to minutely serrulate to nearly entire in the proximal part. In D. olympicum there are no distant marginal teeth in addition to the even denticulations from apex almost to the leaf insertion. In D. pellucidum, such teeth are always present even though the denticulations may be nearly absent. A distinct hydroid strand is visible in leaf sections in the proximal half of the leaf in the larger stems, disappearing in the distal half together with a general loss of anatomical differentiation.
The distinction of Dichodontium flavescens from D. pellucidum has been supported by J. Werner (2002), and accepted by D. H. Norris and J. R. Shevock (2004) for the California flora. Werner reviewed the literature regarding the separation of these two species and showed that there is no consensus of opinion, which ranges from no distinction (all D. pellucidum) to intergradation to separation. Analysis of the morphology of collections, including a few from the flora area, determined that the two taxa are inseparable morphologically. A similar, systematic review of the issue in the flora area, using additional characters, was made by B. C. Tan and W. B. Schofield (1980), who found no clear distinction between the two species, which is the view adopted here. Werner indicated that D. flavescens has a more elongate leaf (4:1) whereas that of D. pellucidum did not exceed 3:1, the capsules were longer in D. flavescens and erect (not inclined), the leaves were broadest at the base and less papillose with the costa wider proximally (to 100 µm in California specimens), whereas D. pelludicum was broadest above the base with much more coarsely papillose cells and with a much narrower costa (to 30 µm). The two variants are sympatric across the United States and they appear to have the same ecology and intergrade morphologically. Curiously, in the Japanese moss flora (A. Noguchi and Z. Iwatsuki 1987+), there is a similar distinction between an elongate variant (D. pellucidum var. pellucidum) and a shorter-leaved, “endemic” one (var. yezoense Noguchi), also apparently sympatric. Dichodontium nelsonii Kindberg described from Missouri is, according to B. H. Allen (2005), Bartramia pomiformis.
The rather smooth leaves of large-stemmed Dichodontium pellucidum without their typical robust papillae and generally from the northwestern United States may resemble Hyophila involuta. In H. involuta the width of the leaf at the insertion is nearly 1/2 the width of the leaf lamina at mid leaf. Its marginal teeth are located in the distal 1/4 of the leaf, not nearly to the insertion as in D. pellucidum. In D. pellucidum the leaf cells, in section, are convex and papillose, sometimes strongly so, on both sides of the leaf, whereas in H. involuta they only bulge on the adaxial side. Hyophila involuta has no papillae, and has clavate, stellate or dentate-elliptical “multi-horned” axillary gemmae, whereas the gemmae of D. pellucidum are simpler, cylindrical or globose with smooth walls and without projections.