Potamogeton L.

Inflorescence usually more than 2-flowered. Carpels 4. Mature fruit ± sessile Potamogeton
Leaves sessile or nearly so, all alike, submerged, membranous
Leaves lanceolate or oblong to linear. Spikes ± interrupted. Flowers few. Fruiting carpels distinctly beaked
Leaves lanceolate to linear-lanceolate, usually 3–6 cm long, with serrate and usually undulate or crisped margins, 3–5-veined, without faint intermediate veins; ligule obtuse. Fruit with a beak c. as long as the fruit. Spikes c. 1 cm long, few-flowered. Widespread. Fl. spring–summer Potamogeton crispus
Leaves almost or quite flat, with entire margins, linear to narrow-oblong. Fruit with a beak shorter than the fruit.