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
Submerged leaves finely dissected into filiform segments
Leaves divided pinnately (Myriophyllum) Haloragaceae
Aquatic plants or growing on mud. Petals absent in female flowers Myriophyllum
All leaves alternate. Emergent leaves linear to lanceolate, sometimes pinnatisect, usually less than 10 mm long. Flowers solitary, ± sessile. Male petals cream, c. 2 mm long
At least submerged leaves whorled or opposite