Hydrostachyaceae, plant family in the order Cornales, composed of a single genus (Hydrostachys) of some 20 species. Most members of the family are aquatic herbs native to central and southern Africa and Madagascar. The leaves form rosettes that can be highly divided and usually have small scaly or fringed appendages. The unisexual flowers are very small and are tightly arranged along a spike inflorescence. The male flowers have a single stamen, and the female flowers have two elongate styles. The fruit is a capsule with many tiny seeds.