Duckweeds are a group of small aquatic herbs known as Lemnaceae that grow in fresh water. They are the smallest flowering plants on earth. They are just an oval or flattened disk with no stem or leaves and may or may not have simple roots. Duckweed grows in most temperate and tropical regions of the world, and can be found floating in thick mats in quiet streams or ponds containing high levels of organic matter. Given ideal conditions they can double their mass in less than two days.