Bryum viridissimum Dickson
Plants to 2.3 cm. Stem leaves loosely-erect and ± spirally twisted to somewhat contorted, sometimes falcate-secund in distal portions of stems when dry, erect-spreading to spreading when moist, oblong-lanceolate, lanceolate, or linear-lanceolate, 0.5--2 mm, acute, with gradual, 1--few celled apiculus, ± keeled below, decurrent; margins entire or with sharp, irregular teeth near apex, somewhat wavy; costa ending below apex with abaxial side covered with quadrate papillose cells only at apex; distal laminal cells 7--11 µm wide, with 2--4 clavate papillae, somewhat thick-walled; basal laminal cells pale-yellow, subquadrate to rectangular; gemmae clavate to elliptic. Sexual condition dioicous. Spores 11--15 µm.
Varieties at least 5 (4 in the flora); high elevations; widely scattered in the flora area, from Alberta. and Quebec south to Massachusetts and California; North and Central America; Europe; w Asia; Africa; Atlantic Islands (the Canaries).
Zygodon viridissimus in the flora includes Northern Hemisphere Zygodon phenotypes with (1) ovate to elliptic gemmae, (2) narrowly lanceolate, loosely erect leaves, (3) the abaxial surface of the costae in the distal 1/3--1/2 of the leaf covered with papillose, quadrate cells, (4) dioicous sexual conditions, and (5) rudimentary (or no) peristome. The species is generally found on tree trunks and in crevices on vertical rock faces, usually at high elevations (up to 4000 m in western N. America).