Pontederiaceae is a family of flowering plants. The APG II system, of 2003 (unchanged from the APG system, of 1998) places the family in the order Commelinales, in the commelinid clade, in the monocots. It is a small family of heterostylous aquatic plants, occurring in tropical and subtropical waters. Charles Darwin was interested in the specialized form of heterostyly found in the family, known as tristyly. Not all of the species are heterostylous. The family probably contains fewer than three dozen species.