Preston, C.D. 1995: Pondweeds of Great Britain and Ireland
but modified
1Leaves orbicular to elliptical, more than 7 mm wide
1'Leaves filiform to linear, up to 4 mm wide
2All leaves submerged, amplexicaul
2'With many floating petiolate leaves
3Floating leaves translucent; fruits 1.5-1.75 mm long
3'Floating leaves opaque; fruits 3-4 mm long
4Leaf lamina arising directly from the node
4'Leaf lamina arising from the top of a sheath (which surrounds the stem above the node)