1	"Male flowers with a single stamen and perianth segment; leaves with finely toothed margins, in subopposite pairs"	Najas
1	"Male flowers with 3–9 stamens and perianth segments in two whorls of 3; leaves with entire margins, or if finely toothed then in whorls"	2
2	"Marine plants; leaves opposite, arranged along a thin, creeping stem"	Halophila
2	"Freshwater plants; leaves all basal or whorled, never in pairs"	3
3	"Stem reduced or stoloniferous such that all leaves appearing basal"	4
3	"Stem and branches distinct with internodes developed; leaves whorled"	6
4	"Stems floating on or suspended in water, rooted in substrate or not; some leaves with aerenchyma on abaxial surface; peduncles mostly short (less than 5 cm); plants monoecious"	"Hydrocharis {Hydrocharis laevigata}"
4	"Stems rooted in substrate; leaves without aerenchyma on abaxial surface; peduncles mostly elongate (more than 5 cm); plants dioecious or flowers bisexual"	5
5	"Leaves elliptic or narrowly ovate to suborbicular (juvenile leaves linear); flowers bisexual; stem reduced, not stoloniferous"	"Ottelia {Ottelia ovalifolia}"
5	"Leaves linear; plants dioecious; stem stoloniferous"	"Vallisneria {Vallisneria australis}"
6	"Leaf margin coarsely toothed with fine teeth; leaf whorls often well-spaced along branches; stamens 3; stigmas 3, threadlike and undivided"	"Hydrilla {Hydrilla verticillata}"
6	"Leaf margin minutely toothed; leaf whorls moderately to densely crowded on branches; stamens 9 or 10; stigmas 3, broad distally and deeply 2–4-lobed at apex"	7
7	"Leaves usually 4 or 5 per whorl, 15–30 mm long; staminal filaments at least 3 times as long as anthers; inner perianth 6–12 mm long, 2–3 times as long as outer whorl"	"Egeria {Egeria densa}"
7	"Leaves usually 3 per whorl, 6–12 mm long; staminal filaments much shorter than anthers; perianth 2.6–5 mm long, inner and outer whorls ± equal"	"Elodea {Elodea canadensis}"