Hypnum undulatum Hedwig, Sp. Musc. Frond., 242. 1801; Neckeropsis undulata (Hedwig) Kindberg; Plagiothecium undulatum (Hedwig) Schimper; P. undulatum var. myurum Cardot & Thériot; P. undulatum subsp. subneckeroideum Kindberg; Stereodon undulatus (Hedwig) Mitten

Stems 15+ cm, 1-7 mm wide, complanate-foliate or sometimes julaceous, branches sometimes attenuate at apices; epidermal cells large, walls thin; rhizoids on leaves on underside of stems from base of stems and branches about 1/2 up and on base of costa and adjacent cells on abaxial surface, brown to reddish brown, smooth or minutely papillose. Leaves symmetric, concave, 2-5 × 1-2 mm; alar region triangular, of 1-3 longitudinal rows of decurrent cells, terminating at base in 1 cell; basal laminal cell walls pitted; medial cells 96-175 × 7-11 µm. Perigonia and perichaetia at base of stems, perigonial bracts broadly ovate-lanceolate, apex acute. Seta single, 2.5-4.5 cm, often curved, rarely circinate. Capsule exserted, light brown to orange-brown, 1.5-4 × 0.4-0.9 mm, smooth; stomata few, present in neck, superficial; operculum 0.8-1.2 mm. Spores 11-14 µm.

Capsules mature summer. Logs, stumps, base of trees, coniferous woods, boggy soil, soil and humus overlying rock; low to moderate elevations (0-1400 m); B.C.; Alaska, Calif., Idaho, Oreg., Wash.; n, w, c Europe; Asia (China); Pacific Islands (New Guinea).

Buckiella undulata is a large, distinctive species very common in the flora area from southern Alaska extending westward to Attu Island, south to northern California and inland to Idaho. Buckiella undulata is best recognized by its light green or whitish green color, large stems, often 15+ cm × 1-7 mm, strongly undulate, ovate to ovate-lanceolate large leaves, and large, inclined to pendulous, arcuate, cylindric, light to orange-brown capsule exserted on a long seta. Microscopically, the species is distinguished by laminal cells covered with granular, cuticular papillae, visible at 400± or greater magnification. Buckiella undulata was previously placed in the genus Plagiothecium in Plagiotheciaceae, but the cells of all species in that genus have completely smooth laminal cells.