General:
Perennial aquatic from a rhizome; stems 20-100 cm long, simple or branched.

Leaves:
In whorls of 4 or 5, 1-2.5 cm long, comb-like and dissected into 13-21 threadlike segments, very gradually transitional to the bracts; bracts conspicuous, oblong triangular, 7-10 mm long and from comb-like to lightly sharp-toothed; winter buds absent.

Flowers:
Inflorescence simple to conspicuously forked, submersed to emergent; flowers subtended by small, whitish, erose to comb-like bracteoles 1-2 mm long; sepals whitish, more or less sharp-toothed to comb-like; petals soon deciduous, about 2.5 mm long; stamens 8, the anthers about 2 mm long; stigmas feather-like.

Fruits:
Mericarps 4, about 2 mm long, glabrous.

If more than one illustration is available for a species (e.g., separate illustrations were provided for two subspecies) then links to the separate images will be provided below. Note that individual subspecies or varietal illustrations are not always available.