APG 3 places Potamogetonaceae in Alismatales:
Potamogetonaceae contains pondweeds.
Potamogetonaceae are distinguished from other pondweeds by their four tepals and four stamens.
In Potamogeton, leaf arrangement tends to be alternate.
In Groenlandia, leaf arrangement tends to be opposite
The inflorescence is usually a stalked spike or an axillary cluster of flowers.
The ovary is superior. The fruit is a drupe or a berry.
Potamogetonaceae are aquatic plants most of which have more or less petiolate leaves with midrib and cross veins; there is usually a conspicuous basal ligule. The inflorescence is densely spicate and the flowers have small tepals appearing to be borne on the backs of the stamens.
plantfamilies 