4. Ditrichum heteromallum (Hedwig) E. Britton in N. L. Britton et al., N. Amer. Fl. 15: 64. 1913.
Weissia heteromalla Hedwig, Sp. Musc. Frond., 71. 1801; Didymodon homomallus Hedwig; Ditrichum homomallum (Hedwig) Hampe; D. zonatum (Bridel) Kindberg; D. zonatum var. scabrifolium Dixon
Plants in loose to dense tufts, yellowish green. Stems to 1 cm, simple, seldom branched. Leaves 1.5-3 mm, erect-spreading, sometimes slightly secund, from an ovate to oblong base tapering gradually to a long channelled subula, lamina 2-stratose distally; margins plane, 1-stratose proximally, 2-stratose in the middle to distal parts; costa broad, occupying most of the subula, in section with a distinct abaxial and poorly developed adaxial stereid band; cells of subula and distal lamina elongate-rectangular, longer in the leaf base, smooth or sometimes papillose at both ends, especially near leaf apices. Specialized asexual reproduction by rarely produced rhizoidal tubers. Sexual condition dioicous. Seta reddish brown, 1-2.5 mm. Capsule erect, reddish brown, oblong to cylindric, symmetric, 0.5-1.5 mm; peristome teeth pale orange, about 300 µm, lightly papillose; operculum conic-rostrate, blunt, 0.4-0.5 mm. Spores 10-15 µm, finely papillose.
Capsules mature summer (Jun-Aug). Soil; low to high elevations (50-1500 m or higher); B.C.; Alaska, Oreg., Wash.; South America (Colombia); Europe; Asia (China, Japan, Taiwan); Atlantic Islands (Iceland).
Rhizoidal tubers were first reported for Ditrichum heteromallum by S. Risse (1985) from European material and have also been noted in Japanese material by H. Deguchi and T. Matsui (1986). They have the appearance of short filaments with swollen, contorted rhizoid cells. Ditrichum zonatum (Bridel) Kindberg, including the var. scabrifolium Dixon, appears to be only a small form of D. heteromallum with shorter, appressed leaves that are often more 2-stratose, especially near the base, and leaf cells that are sometimes papillose at the ends. We have not seen specimens that support the Wisconsin report by F. Bowers and S. Freckman (1979) and believe the record to be dubious.