Pterogonium gracile (Hedw.) Sm.
Plants in rather loose mats, whitish-green to green, pinnately branched with the branches inserted at an acute angle and parallelling the stem. Primary stems short and prostrate with secondary stems julaceous, to 8 cm long, decumbent to somewhat ascending, often strongly flagelliform. Leaves to 1.6 mm, 3–5:1, symmetric and straight, not plicate, ovate to ovate-lanceolate, broadly acute to short acuminate but the stolon leaves very long acuminate, cordate and somewhat clasping at base, imbricate when dry but spreading at a 45–90*o. angle when moist. Margins plane or somewhat incurved near base, decurrent at base, serrulate throughout except at the apex which is sharply dentate, often with multicellular teeth. Median cells prorate dorsally especially in distal 1/2 of leaf, 5–6 µm wide, 6–10:1, thick-walled (lumen/wall ratio 0.5–4:1) and pitted throughout. Alar cells quadrate and, at least in part, transversely elongate, thick-walled, extending nearly to costa and an equal distance up the margin, grading into laminal cells. Apical cells short-rhomboidal, sometimes isodiametric. Costa extending about 1/4 to 1/2, forked distally or divided to the base. Rhizoids dark brown, to 15 µm, smooth, clustered on stem at abaxial costal insertion.
Dioicous with similar male and female plants. Perigonia nearly spherical, bud-like, densely cloaking the large branches. Perichaetia on larger branches, the bracts 2–3 mm long, ecostate or with a short costa, strongly convolute around the seta, abruptly acuminate from an ovate-lanceolate base. Seta 8–16 mm long, erect, exserted. Capsule brown, nearly symmetric, erect to inclined, not distorted or sulcate when dry. Exothecial cells irregularly quadrate to short rectangular, 1–2:1, to 25 µm wide, with a lumen/wall ratio about 5:1. Urn to 3.0 mm, 3–4:1. Operculum conic to apiculate, about 1/4 as long as urn. Exostome horizontally to obliquely striate below becoming smooth to lightly papillose above, white, erect when dry. Endostome segments narrow, about half as long as exostome and with low basal membrane and no cilia. Spores smooth, 12–20 µm.
This plant is usually recognizable by the branching pattern of its julaceous axes. The branches diverge from the stem at an acute angle, parallel the stem and terminate together so that it resembles the foot of a bird. Microscopically, the prorate cells and the weak costa allow identification. Many plants, however, have only a few prorations on distal portions of smaller leaves, and the costa sometimes exceeds mid-leaf. Care should therefore be taken to examine several leaves from different portions of the plant.
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