Podostemaceae (riverweed family) is a family in the order Malpighiales. It comprises about 46 genera and ca 300 species of more or less thalloid aquatic herbs. Riverweeds adhere to hard surfaces (generally rock) in rapids and waterfalls of rivers. They are found mostly in tropical and subtropical areas worldwide. Many species are found in a very small geographic area (perhaps even just a single river or waterfall). Riverweeds are submerged when water levels are high, but during the dry season they live a terrestrial existence, flowering at this time. Their root anatomy is specialized for the purpose of clinging to rocks, and in fact details of the root structure are one of the ways of classifying riverweeds.