An order of aquatic flowering plants, division Magnoliophyta Angiospermae), in the subclass Alismatidae of the class Liliopsida (monocotyledons). The order consists of the single family Hydrocharitaceae, with about 100 species. Within the subclass the order is marked by its inferior, compound ovary with basically laminar placentation. The ovules are scattered over the walls of the individual carpels, which are weakly connate to form partial partitions (intruded placentas) in the ovary. The aquarium plant Vallisneria spiralis, called tape grass or eelgrass, is not a grass but belongs to the Hydrocharitaceae.