An order of flowering plants, division Magnoliophyta (Angiospermae), in the subclass Asteridae of the class Magnoliopsida (dicotyledons). The order consists of three small families with about 50 species, most of which are aquatics or small herbs of wet places, and have much reduced vascular systems. The flowers are small and solitary in the axils of leaves or bracts. The perianth is nearly or completely absent. The pistil consists of two carpels united to form a compound, unilocular or four-chambered ovary, or sometimes the pistil appears to be of a single carpel. The Callitrichales are placed in the Asteridae largely on the basis of their embryology and phytochemistry. The ovules are anatropous and tenuinucellular and have a single integument, and the plants generally produce iridoid substances.