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J. f. Bot. 1800. 1(2): 294. 1801.
    Dicranum bergeri Bland. in Sturm, Deutschl. Fl. 2(10): 7. 1809.
    Dicranum stenodictyon Kindberg in Macoun, Bull. Torr. Bat. Club 16: 92. 1889.
    Dicranum rugosum Kindberg, Ottawa Natural. 4: 61. 1890.

Plants in yellow green to green cushions or tufts, to 12 cm high, glossy to almost lustreless. Leaves symmetrical to slightly falcate, undulate at least above the middle, loosely erect when moist, keeled distally, to 6 mm long, 5–7: 1, neither plicate nor fragile, lanceolate to elliptic lanceolate. Median leaf cells smooth, to 10 µm wide, 2–6: 1, in regular but non-linear rows, rectangular, strongly pitted. Marginal cells more narrow but not otherwise differentiated. Cells near the leaf apex isodiametric to elliptic, irregular in shape with the long axes radiating at broad angles from the costal region, often distally prorate on the abaxial surface. Basal juxtacostal cells linear to narrowly rectangular, to 10 µm wide, more strongly pitted than median cells. Alar cells to 20 µm wide; well demarcated, often extending to costa, inflated, thin- to rather thick- walled. Costa filling 1/3–1/5 of leaf base, ending a few cells below the bluntly acute apex, sometimes almost percurrent, smooth proximally but often somewhat crispate near its apex. Costa cross-section at mid-leaf with a single layer of guide cells, with obvious dorsal and ventral stereid bands but with no epidermis. Margin not decurrent, plane to erect, serrulate to serrate in distal 1/2, unistratose throughout. Rhizoids brown to red brown, to 18 µm wide at base, smooth, mostly dichotomously branched with the ultimate branches strongly curled, arising on stem from immediately below leaf insertion. Axillary hairs to 500 µm long, composed of up to twelve similar long and cylindric cells but with a single short basal brown cell. Stem cross-section homogeneous with no central strand and with even the outer cortical cells rather large, but somewhat pigmented.
    Pseudautoicous with male plants bud-form, in the axils of leaves close to the perichaetia. Perichaetial leaves partially sheathing base of seta, abruptly short acuminate. Monosetous with the seta to 30 mm, yellow to yellow-brown, straight and smooth. Urn inclined to horizontal, arcuate, smooth to very slightly sulcate, to 2.5 mm long, 4–5: 1, not strumose. Operculum rostrate, about as long as urn. Annulus not differentiated. Peristome red-brown, inserted significantly below capsule mouth, to 1500 µm long, hi- to trifid to near base, vertically striate. Suboral exothecial cells quadrate to elliptic in up to 5 rows, to 15 pm wide, thick-walled with lumen: wall ratio about 2–3: 1. Median exothecial cells to 30 µm wide, not in regular vertical rows, quite irregular in size and shape, rectangular 1–2: 1, thick-walled with lumen: wall ratio about 3: 1. Spores smooth to minutely papillose, to 20 µm.
   
    Dicranum undulatum is listed in this flora on the basis of a single seemingly persistent clone growing without sporophytes in low shrubbery in front of the life sciences building at the University of California, Berkeley. This plant is ordinarily a plant of boggy sites north of the state of California, and its presence on the Berkeley campus is obviously due to accidental introduction.

The nearly erect, hardly falcate leaves of D. undulatum will distinguish it from nearly all expressions of our other three California species of Dicranum. The undulate leaves of D. undulatum are unique when compared to those other three species although D. howellii may sometimes show slight distal undulations. With a handlens, the rather blunt apices of the leaves of D. undulatum can be seen as very different from the subulate apices of our other Dicranum.

Elevation by latitude plot for Dicranum undulatum
   in California

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