The Podostemaceae ("river-weeds", including Tristichaceae of some authors) are the largest family of strictly aquatic angiosperms. Recent results based on molecular analysis indicate that Podostemaceae belong to the Malpighiales clade in the eurosids I group, most closely related to Clusiaceae (Hypericaceae). Tristichoideae and Weddellinoideae are acceptable as podostemaceous subfamilies which are clearly distinguishable from the large subfamily Podostemoideae (Cusset & Cusset 1988, 1989; Kita and Kato 2001). Hydrostachyaceae are not a sister group to Podostemaceae. New molecular evidence is given in favor of the view that Hydrostachyaceae belong to the Cornales clade within asterids.
Podostemaceae range worldwide on all continents with tropical climate. Most of the members occur in tropical and subtropical rivers and waterfalls. Many of the c. 48 genera and c. 280 species are endemic to small geographical areas or even a single river or waterfall. Only one species, Tristicha trifaria, is widespread and occurs in the Old and New World. 41% of all Podostemaceae genera worldwide are monotypic. In tropical America, there are 19 genera and c. 165 species. The largest Neotropical genera are Apinagia (c. 50 spp.), Marathrum (25), and Rhyncholacis (25). The very high degree of endemism may well be a taxonomic artifact because few species have been sufficiently studied; what has been accepted as a set of species by one taxonomist may be viewed as 'environmental forms' by others!
Podostemaceae are haptophytes normally fixed to a hard substrate (usually rock) in rapids and waterfalls, usually in sunny places. In most Podostemaceae flowering takes place when the water level drops. The substrate does not seem to be important as long as it is hard. Most reports state that they grow in oligotrophic and oxygen-rich water.