Stereodon revolutus Mitten
Plants rusty brownish green to yellowish green (occasionally dark green), medium-sized to slender, regularly to irregularly pinnate, or irregularly branched, erect, suberect, or creeping and firmly or loosely attached to substratum, 3--5 cm; branches 0.3--0.7 cm, the longest distant from apex, occasionally flagelliform. Stems yellow-green, lacking hyalodermis, but outermost cells often with thin walls collapsed inward, central strand weakly developed, pseudoparaphyllia foliose, lanceolate to ovate. Leaves of stem straight to falcate, secund, concave, weakly to strongly plicate, margin revolute from base to near apex, 1.1--1.8 × 0.4--0.6 mm, costa distinct to (rarely) absent, median cells short and wide 30--50 × (3--)4--5(--6) µm, thin or thick-walled, basal cells wider, thicker walled and porose sometimes yellowish, alar cells subquadrate, numerous, 8--15 in marginal row, forming clearly defined angles, 2--3 larger, hyaline, rectangular cells in decurrent portion; leaves of branches slightly smaller but otherwise similar. Sexual condition dioicous; antheridial and archegonial plants alike; inner perichaetial leaves erect, oblong-lanceolate, plicate, acumen short, almost entire, costa indistinct. Seta smooth, yellowish to reddish brown when mature, 1--2 cm. Capsule dull yellowish to warm brown, inclined to horizontal, oblong-cylindric, curved, 2--3 mm excluding conic operculum; annulus 2(--3)-seriate; cilia of endostome 2--3.
Varieties 2 (2 in flora): Greenland; North America, South America, Europe, Asia, Antarctica.
Hypnum revolutum is circumpolar in the Northern Hemisphere, but bipolar in distribution. It occurs at sea level in the northern portion of its range, but is most frequent at higher elevations. Strongly revolute margins are found in the genus only in H. revolutum, and the areolation in combination with the alar-cell differentiation usually make this species readily recognizable. Creeping plants tend to be pinnate, sometimes closely and regularly; suberect plants tend to be more loosely and irregularly branched.
