The Subclass Alismatidae, includes 4 orders, 16 families and about 500 species.  In terms of species diversity it is the smallest subclass of the Magnoliophyta.  The 'basal' monocots are interpreted and classified in different ways (see the Flowering Plant Gateway) and the approach used by Cronquist centers on the gynoecium and apocarpy as a central, unifying feature for this group.  Aquatic adaptations of all but the Triuridales also provides a tentative link to the archaic dicot element (Magnoliidae - Nymphaeales) placed by Cronquist as a possible extant link to the monocots.

Alismatales
     Alismataceae
Najadales
     Potamogetonaceae
     Zosteraceae