Vegetative key to the families

Submerged plants rooted in mud. Leaves submerged or floating on the surface or sometimes the leaf blades held erect above the water surface. Freshwater or marine (Amphibious plants are not included in this group)
Leaves opposite or whorled on an elongated stem
Either; leaves alternate on an elongated stem (sometimes opposite when subtending flowers); or, leaves tufted on a short stem or arising from a rhizome