Racomitrium lawtoniae Ireland, Bryologist 73: 707, figs. 1-12. 1971 (as lawtonae)

Plants rather robust or less often moderately sized, loosely tufted, coarse and rigid, slightly glist-ening, yellow, yellow-green, olivaceous, sometimes grayish, in the upper part, brown proximally. Stems 5-10(-15) cm, curved-ascending to erect, simple or sparingly, irregularly branched. Leaves erect-appressed to falcate-secund when dry, erect-spreading when wet, straight or curved, sometimes almost circinate, (3.5-)4-5.5(-6) × (0.5-)0.6-0.8(-0.9) mm; margins broadly recurved on one side and more narrowly on the other side to ¹/3-¹/2(-³/4) of the leaf length, 1-stratose throughout or seldom with some bistratose patches distally; pilose, awns hyaline, not flexuose, erect-recurved to squarrose, 0.4-1.5(-2.6) mm, rarely almost absent, flattened, with a distinct cell areolation, remotely sharply denticulate at the margins, distinctly decurrent; costa percurrent, strongly convex abaxially, reniform in the median and proximal portions, 75-100(-115) µm wide basally, 40-50 µm wide distally, 3(-5)-stratose basally with (3-)4-5 enlarged adaxial cells, 2(-3)-stratose distally, with 2-3(-4) enlarged adaxial cells; laminal cells 1-stratose throughout, smooth; basal laminal cells elongate, 25-55 × 9-11 µm, with incrassate, sinuose-nodulose lateral walls; alar cells not or slightly differentiated, sometimes slightly yellowish; supra-alar cells hardly differentiated, not forming a distinct basal marginal border; medial and distal cells rectangular, 12-35 × 5-7 µm. Inner perichaetial leaves (2-3) broadly ovate, convolute, yellowish haline, shortly obtusely pointed or pilose, with thick-walled, chlorophyllose distal cells. Seta 4.5-7.5 mm, straight, brown, twisted clockwise. Capsule obloid to short-cylindric, (1.2-)1.5-2.5 × 0.6-0.8 mm; operculum rostrate, straight, 1-1.5 mm; peristome teeth 250-480 µm, brown, deeply cleft into to branches, strongly papillose, erect. Spores 12-17 µm, granulose.

Humid or wet, mostly shaded acidic rocks, boulders, cliffs and rock outcrops near streams and waterfalls; low to moderate elevations (0-800 m); B.C.; Alaska, Wash.

Bucklandiella lawtoniae occurs along the coastal region from Kodiak Island and southern Alaska southwards to the Cascade Range in central Washington, with a highly isolated site on Attu Island in the Aleutians. The largest species of the genus, it can be recognized by its characteristic, long and not flexuose, erect-recurved to squarrose, hyaline awns that are distinctly decurrent. Some elongate and pilose plants of B. affinis may imitate B. lawtoniae but that species, apart from a number of differences in structural characters including the hyaline innermost perichaetial leaves, has erect-flexuose awns and leaf margins recurved on both sides towards the apex. The same differences exist between B. lawtoniae and B. heterosticha, in which, additionally, the costa is usually broad, 2-stratose, and canaliculate in the middle and distal parts of the leaf. Additional diagnostic characters of B. lawtoniae are undifferentiated alar cells and the absence of both a basal marginal border and a basal membrane in the peristome. The species appears to be strongly isolated in the genus and on account of its peculiar innermost perichaetial leaves it is positioned in a separate section.

SELECTED REFERENCE

Ireland, R. R. 1976. Alar cells of Rhacomitrium lawtonae. Bryologist 79: 238-241.