An order of aquatic and semiaquatic flowering plants, division Magnoliophyta (Angiospermae), in the subclass Alismatidae of the class Liliopsida (monocotyledons). The order consists of 10 families and a little more than 200 species. The Potamogetonaceae, with about 100 species, are the largest family of the order, and the name Potamogetonales is sometimes used instead of Najadales for the group. The Najadales are Alismatidae in which the perianth, when present, is not differentiated into evident sepals and petals. Usually the flowers are not individually subtended by bracts. The evolutionary history of the order is in large part a story of floral reduction, associated with progressive adaptation to aquatic and eventually marine habitats. The Zosteraceae of this order are unique among flowering plants in that they grow submersed in the ocean, albeit in shallow water near the shore. Zostera marina, or eelgrass, is a common member of the family.