Botany: 1 genus, Hydrostachys; 22 species; Madagascar, southern and central Africa; aquatic, growing on rocks in fast-moving water.
Phylogenetic placement of Hydrostachys has been problematic. Hydrostachys was formerly grouped with other aquatic plants, such as the Podostemaceae. Later it was placed in Hydrangeaceae but that makes Hydrangeaceae paraphyletic.
In the Apg3 classification Hydrostachys is placed in its own family Hydrostachyaceae in the Order Cornales. Hydrostachys though shares few morphological similarities with other Cornales.
In the Plant theory Hydrostachyaceae is placed in Cornales. As an aquatic it is placed in Subphase 1.