Plants: to 40 cm, yellow-red, yellowish green, green, dark green, or brown. Stems: slender to occasionally robust, rigid; stem and branch apices swollen, flaccid, short-attenuate, sharply angled; axillary hairs to 820 µm, 6–10 cells, basal cell quadrate, red, distal cells long-cylindric, hyaline. Leaves: monomorphic, imbricate to erect-appressed when dry, erect to erect-spreading in 3 ranks when moist, ovate, oblong-ovate, ovate-lanceolate, or suborbiculate, keeled and conduplicate, concave, variously curved along keel from insertion to apex, 2–8 mm; margins occasionally broadly reflexed on one side proximally; apex acute, broadly acute, or rounded obtuse; medial laminal cells 100–150 × 12–18 µm. Perigonia: with leaves 1.2–1.4 mm. Perichaetia: with leaves oval to suborbiculate, 2–3 mm, apex obtuse. Seta: 0.2–0.3 mm. Capsule: immersed to slightly emergent, ovoid to subcylindric, 2–2.7 mm; operculum obtuse-conic, 0.7–1.5 mm; endostome trellis perfect. Calyptra: 1.3–1.6 mm. Spores: 14–20 µm.
Rock, sticks, logs, roots in streams, ponds, pools, ditches, swamps, floodplains, seasonally dry. low to high elevations (0-3300 m). Greenland, St. Pierre and Miquelon, Alta., B.C., N.B., Nfld. and Labr., N.S., Ont., P.E.I., Que., Yukon, Alaska, Ariz., Calif., Colo., Conn., Del., Idaho, Ill., Maine, Mass., Mich., Minn., Mont., Nev., N.H., N.J., N.Mex., N.Y., Ohio, Oreg., Pa., R.I., S.Dak., Utah, Vt., Wash., Wis., Wyo., Europe, Asia, n, s Africa, Atlantic Islands (Iceland).
Fontinalis antipyretica is stenotypic in eastern North America, where most collections conform to the gigantea expression that differs in its robust size and broad, overlapping leaves strongly curved along the keel. Most atypical western plants are variations on the oreganensis expression: medium-sized plants, weakly keeled stem leaves straight to slightly bent along the keel, apices frequently concave or plane and subobtuse or obtuse, and branch leaves occasionally concave or plane. The oreganensis expression is common along the west coast, but occurs sporadically throughout the range of the species. Fontinalis howellii and the oreganensis expression of F. antipyretica sometimes intergrade; both have stem leaves with keels typically straight beyond the basal curve and concave branch leaves. Fontinalis howellii differs in having larger plants; an abrupt transition from stem to branch leaf form; long, slender branch leaves; and loosely terete-foliate stem or branch apices that when dry lack the swollen, triangular appearance of stem or branch apices in F. antipyretica. The gracilis expression is slender (leaves 2–4 mm), loosely foliate, with lax to flaccid stems and weakly keeled (often concave) leaves. Plants of F. antipyretica found in swiftly moving streams often have leaves completely split along the keel and can be mistaken for F. hypnoides.
Plants: to 35 cm, reddish, yellow-red, golden brown, or pale green. Stems: medium to robust, rarely slender, rigid; stem and branch apices firm, loosely foliate, not angled; axillary hairs 600–750 µm, 6–10 cells, basal cell quadrate to short-rectangular, red, distal cells long-cylindric, hyaline. Leaves: strongly dimorphic. Stem: leaves erect to erect-spreading when dry or moist, ovate-lanceolate to lanceolate, sharply or weakly keeled and conduplicate, keels straight beyond basal curve, 4–7 mm; margins plane, sometimes erect to somewhat involute at apex; apex acute to narrowly obtuse; medial laminal cells 110–180 × 12–16 µm. Branch: leaves erect-spreading when dry, spreading and sharply 3-ranked when moist, lanceolate to linear-lanceolate, not keeled, concave to tubular-concave, 3–6 mm; margins involute; apex long-acuminate, acute, or somewhat rounded. Perigonia: with leaves 0.7 mm. Perichaetia: with leaves oval to suborbiculate, 2–3 mm, apex obtuse. Seta: 0.2–0.3 mm. Capsule: immersed to slightly emergent, oblong to subcylindric, 2–2.5 mm; operculum conic, 1 mm; endostome trellis perfect. Calyptra: 2 mm. Spores: 12–16 µm.
Logs, sticks, shrubs, swamps, pools, ponds, streams, streamlets, springs, wet rock in seeps, often seasonally dry. low to high elevations (0-1600 m). B.C., Calif., Idaho, Mont., Oreg., Wash.
Wet plants of Fontinalis howellii have spreading, sharply three-ranked branch leaves; when dry the stem and branch apices are firm and loosely terete-foliate. Typical F. howellii differs from other keeled species of Fontinalis in having strongly and abruptly dimorphic leaves: stem leaves keeled-conduplicate with straight keels after the basal curve; branch leaves linear-lanceolate to lanceolate and concave to tubular-concave. Permanently submerged plants (chrysophylla expression) differ from the typical form in their smaller size and mostly concave stem leaves. The oreganensis expression of Fontinalis antipyretica is similar but differs from F. howellii in having often smaller plants; some transitional leaves between stem and branch leaf forms; shorter, broader branch leaves; and triangular stem and branch apices that when dry have a swollen aspect very different from the firm, loosely foliate stem and branch apices in F. howellii. All collections referred to F. howellii from eastern North America and recent reports from Germany represent the oreganensis expression of F. antipyretica. The only other Fontinalis taxon with tubular-concave leaves is the cymbifolium expression of F. novae-angliae. It differs from F. howellii in having consistently concave stem leaves.