River Weeds

Submerged, annual or perennial, herbs, growing firmly attached to rocks in swift-flowing water, often resembling mosses, liverworts or algae. Stems well-developed, floating, often woody. Leaves with floating, often linear to filiform, dissected laminae and/or with small moss-like or reduced and bract-like ones. Flowers bisexual, solitary or cymose, actinomorphic or zygomorphic. Spathella present and persistent (Sphaerothylax) or 0 (Tristicha). Perianth segments 2-3 or 0. Stems 1-many. Ovary 1-3-locular. Fruit a septicidal capsule. Seeds minute, numerous.