Meesia triquetra, the three-ranked hump-moss, is a moss that occurs all around the northern hemisphere in higher latitudes.
Description
Meesia triqueta grows in small tufts or cushions. The plants, often large, are acrocarpous and dioicous. In color they are dark-green to grass-green above, occasionally red-brown below due to dense rhizoids. The plant's stems are not branched or little branched, pale-brown to yellow-brown, closely foliate, and 2–14 cm high. Its leaves are decurrent, squarrose when moist, triangular to ovate to lanceolate, somewhat crispate, 2–3½ mm long, and tristichous. The leaf margins are sharply serrate or denticulate to the base or nearly so, plane or sometimes recurved in the middle of the leaf. The costa is relatively narrow, subpercurrent to percurrent, gradually tapering at distal end. The leaf's apex is acute to acuminate. The lamina is unistratose throughout. The perigonia of Meesia triqueta' are terminal in discoid heads. The seta is straight and smooth, brown to yellow-brown, and very long. The capsule is asymmetrical, somewhat pyriform, 2¾–5½ mm long including the neck; the hypophysis is long, moderately well-defined, not much wrinkled when dry; the urn is oblong to short-cylindric, arcuate and asymmetric, brown to yellow-brown, up to 4 mm long, and when dry, wrinkled but not regularly sulcate. The operculum is short-conic; the endostome cilia are often rather well-developed. The plant's spores are finely papillose. Meersia triquetra is easily recognized by its distinct three-ranked leaf arrangement and for being dioicous. It may be distinguished from M. uliginosa by its squarrose leaves; narrow costa relative to the size of the leaf base; serrate, planar leaf margins; and acute leaf tips. In contrast, M. uliginosa a wide costa, entire, revolute leaf margins, and blunt apices. Other Meesia species also have blunt apices.
This species has a circumboreal distribution: it is found in Northern Europe, northern Asia, Greenland, Canada, and the northern United States, Some discoveries have been reported from Oceania. These mosses occur in wetland sites, specifically, within wet woods in the wettest portions of what are called "extreme rich fens". Montagnes describes this species as a "rich fen indicator of high fidelity." Associates include Scorpidium spp. and Drepanocladus revolvens. The fire ecology of this plant is not known; however, fens rarely burn. Excess soot from a nearby fire, however, might negatively affect habitat quality. Fire return intervals in conifer bogs, a somewhat similar mire-type habitat, are estimated to be about once every 150–200 years. Fire does significant damage to peat, but the bog must be dry in order to burn; typically, bogs are not dry enough.
