* Upper leaves floating on the surface of the water.

P. natans: aquatic; leaves opaque, ovate or oblong, usually rounded, sometimes cordate or tapering at the base, marked by several longitudinal nerves, the submerged ones thinner and narrower, all stalked or reduced to a mere stalk; spike dense and cylindrical. - Stagnant or running waters. Fl. June, July.

P. heterophyllus resembles this, but is smaller, and has the lower submerged leaves sessile or nearly so, linear, and 1-3 nerved; while P. lucens, which is larger, has the lower leaves lance-shaped, with 5-7 longitudinal nerves.

** Leaves all submerged. † Leaves alternate.

P. perfoliatus: aquatic; leaves broadly ovate, thin, obtuse, completely clasping the stern, the auricles united on the opposite side, spike shortish. - Rivers and ponds. Fl. July.

P. crispus: aquatic; leaves thin, narrow-oblong or broadly linear, obtuse, waved and sinuated on their edges; spikes small. - Ponds, streams and ditches. Fl. June.

P. pusillus: aquatic; stems thread-like; leaves narrow-linear, 1-3 inches long, not dilated at the base into a scarious sheath; spike short, close. - Pools and still waters. Fl. June, July.

P. pectinatus: aquatic; stems thread-like; leaves narrow linear, 2-3 inches long, dilated at the base into a rather long sheath, scarious at the edge; spike slender, interrupted. - Pools and still waters. Fl. June, July.

†† Leaves opposite.

P. densus: leaves short, ranged in two rows on opposite sides of the stem, submerged, thin, broadly lanceolate, folded and clasping the stem at their base; spikes 2-4-flowered, becoming reflexed. - Shallow pools and ditches. Fl. June, July.