duckweed

Any of a family of tiny plants found floating on the surface of still water throughout most of the world, except the polar regions and tropics. Each plant consists of a flat, circular, leaflike structure 0.4 cm/0.15 in or less across, with a single thin root up to 15 cm/6 in long below. (Genus chiefly Lemna, family Lemnaceae.)

The plants bud off new individuals and soon cover the surface of the water. Flowers rarely appear, but when they do, they are extremely small and are found in a pocket at the edge of the plant.