Lemnaceae was a family of flowering plants, also known as the duckweed family, as it contains the duckweeds or water lentils. The duckweeds are now known to be a branch of the arum or aroid family (Araceae), so the name 'Lemnaceae' is rapidly falling out of use among botanists. Now it is treated as a subfamily called Lemnoideae.
These plants are very simple, lacking an obvious stem or leaves, but consist of a small 'thalloid' or plate-like structure that floats on or just under the water surface, with or without simple rootlets. The plants have become highly reduced from their relatives in Araceae. Reproduction is mostly by budding, but occasionally a flower consisting of two stamens and a pistil is produced (some view the 'flower' as a pseudanthium, or reduced inflorescence, with three unisexual flowers, derived from the spadix in Araceae). The fruit is a utricle, a sac containing air and a seed designed to float. The flower of the Wolffia currently holds the record for being the smallest flower in world; measuring in at a size of 0.3 mm long.
Duckweed is an important food source for waterfowl and are eaten by humans in some parts of Southeast Asia (as khai-nam). Some duckweeds are used in freshwater aquariums and ponds where they may spread rapidly, although in a large pond they may be difficult to eradicate once established. The plants can provide nitrate removal (if cropped) and cover for fry. The plants are used as shelter by pond water species, such as bullfrogs and bluegills. The duckweeds are important in the process of bioremediation because they grow rapidly, absorbing excess mineral nutrients, particularly nitrogen and phosphates. A cover of duckweeds will reduce evaporation of water compared to a clear surface.