Plants with shoots 5 25 × 0.75 1.2 mm, scattered among mosses or in crowded mats, green in shade or brownish black in exposed sites. Stems slender, 130 300 µm, branching irregular, terminal or ventral intercalary. Leaves distant to scarcely imbricate, spreading, flat or more often concave, as long as wide or slightly longer, 400 900 × 350 800 µm, 2-lobed 1/4 1/3, the sinus narrow, the lobes obtuse to rounded, entire; median leaf cells 22 27 × 25 30 µm, marginal cells 20 24 µm; cuticle smooth, walls evenly thickened; trigones absent; oil bodies 4 8 per cell, ovoid or spherical, 3 4 × 5 6 µm, finely granulate. Underleaves absent. Specialized asexual propagation by detached, unfertilized, inflated perianths that float on water; gemmae absent. Androecial bracts up to 8 pairs, contiguous, concave, 2-lobed. Gynoecia commonly with subfloral innovations; bracts similar to leaves in shape, slightly larger; perianths often present, exerted well above bracts, globose to oblong, inflated, large compared to the size of the shoot; mouth contracted, dentate lobulate; easily detached from stem.

Igneous rock outcrops subject to at least periodic seepage, often in direct sunlight; also around rock pools and in bogs; e, w, nw Greenland; Miquelon; Alta., Ark., B.C., Man., Nfld., N.S., Nun., Ont., Que., Yukon; Alaska, Calif., Colo., Conn., Maine, Mass., Mich., Minn., N. C., N.H., N.J., N.Y., Oreg., Pa., S.Dak., Tenn., Vt., Wash., Wyo.; Europe; Asia (Japan).

The variety Gymnocolea inflata var. heterostipa (Carrington & Spruce) K. Müller, a dubious taxon with frequent small intercalary branches and small underleaves, has been recognized from plants collected in East Greenland and Minnesota. This plant is doubtfully distinct and its gametophytic differences may be an environmentally induced variation. In Europe, where the variety was first recognized, no distinct limits have been found between this variety and the typical form of Gymnocolea inflata.