Lemna gibba, the gibbous duckweed, swollen duckweed, or fat duckweed, is a species of Lemna (duckweed). It has a simple plant body, known as a thallus, which floats on the surface of the water and measures 3 - 5 mm in diameter. A single root hangs down into the water. Found in a wide range of still or slow-flowing water bodies, this common duckweed can also grow on mud or damp rocks.