An aquatic flowering plant (family Araceae) that floats on or just below the surface of still or slow-moving water. There is no obvious stem or leaf; there are one or more simple roots or none, and flowers comprise one or two stamens and one pistil; Wolffia duckweeds are the smallest-known flowering plants, 1 mm across, with the smallest-known flowers, 0.3 mm across. There are five genera and several species.