7. P. hýbridus, Michx. Floating leaves oval to lance-oblong (the largest 10´´ long, 6´´ wide), often acute, longer than the filiform petioles, with about 5–7 nerves beneath deeply impressed; submersed leaves very numerous, almost setaceous (1–3´ long, very rarely ½´´ wide); stipules obtuse; emersed spikes 4–7´´ long; submersed spikes 1–4-flowered, their peduncles (of their own length) frequently recurved; fruit minute, about 8-toothed on the margin; embryo coiled 1½ turns.—Shallow stagnant waters, N. Brunswick to Fla., west to Mich., Mo., and N. Mex. June–Aug.

[*][*] Submersed leaves lanceolate, rarely oval or linear, membranaceous; spikes dense, many-flowered, on stout peduncles.

8. P. ruféscens, Schrad. Stem simple; floating leaves (often wanting) 2–5´ long, rather thin, wedge-oblanceolate, narrowed into a short petiole, 11–17-nerved; submersed leaves almost sessile, lanceolate and lance-oblong, smooth on the margin, fewer-nerved; stipules broad, hyaline, obtuse, upper ones acuminate; spike 1–2´ long, often somewhat compound; fruit obovate, lenticular, pitted when immature, with an acute margin and pointed with the rather long style; embryo incompletely annular.—In streams or ponds, N. Brunswick to N. J., west to Minn. and Tex. Aug., Sept. (Eu.)

9. P. flùitans, Roth. Stem often branching below; floating leaves thinnish, lance-oblong or long-elliptical, often acute, long-petioled, 17–23-nerved; submersed leaves very long (3–12´, by 2–12´´ wide), lanceolate and lance-linear, 7–15-nerved, coarsely reticulated; peduncles somewhat thickened upward; fruit obliquely obovate, obscurely 3-keeled when fresh, and distinctly so when dry, the middle one winged above and sometimes with 3–5 shallow indentations; the rounded slightly curved face surmounted by the short style; nutlet with the sides scarcely impressed; upper part of the embryo circularly in-curved. (P. lonchites, Tuckerm.)—In streams or rarely in ponds, N. Brunswick to N. J., west to Minn. and Iowa. Aug., Sept. (Eu.)

10. P. púlcher, Tuckerm. Stem simple, black-spotted; leaves of three kinds; floating ones becoming very large (4½ by 3½´), roundish-ovate and cordate or ovate-oblong, 25–37-nerved, all alternate; upper submersed ones (3–5) usually lanceolate, acute at base and very long-acuminate, 10–15-nerved, very thin, cellular each side of the midrib, undulate, short-petioled; lowest (2–4 near the base of the stem) thicker, plane, oval or oblong with a rounded base, or spatulate-oblong, on longer petioles; stipules rather short and obtuse; peduncles thicker than the stem; fruit with a rounded back and angular face, pointed, distinctly 3-keeled when fresh, sharply so when dry; nutlet with two deep dorsal furrows, and a sinus below the angle in front; sides flat; embryo circularly much incurved above.—Ponds, Vt. to Ga. and Mo. July, Aug.

11. P. amplifòlius, Tuckerm. Stems simple, of very variable length; floating leaves (sometimes wanting) large, oblong or lance-ovate, sometimes slightly cordate at base, abruptly acutish, 30–50-nerved, on rather long petioles; submersed leaves often very large (reaching 7´ by 2´), lanceolate or oval, acute at each end, usually much recurved, undulate, mostly on short petioles; stipules very long and tapering to a point, soon becoming loose; peduncles thickened upward, in deep water much elongated; fruit very large (over 2´´ long), rather obliquely obovate, 3-keeled, with a broad stout beak; nutlet slightly impressed on the sides; upper part of the embryo curved into a ring.—Ponds and rivers, N. Eng. to N. J., west to Minn. and Kan. Aug., Sept.

12. P. Illinoénsis, Morong. Stem stout, branching towards the summit; floating leaves opposite, oval or ovate (2–5´ long by 1½–2´ broad), 19–25-nerved, rounded or subcordate at base, with a short blunt point at apex, on short petioles, submersed leaves rather few, oblong-elliptical, acute at each end, usually ample (largest 8´ by 1½´); stipules coarse, obtuse, strongly bicarinate (2´ long); peduncles often clustered at the summit (2–4´ long), thickening upward; fruit roundish-obovate (1¾–2´´ long), 3-keeled on the back, middle keel prominent; nutlet flattened and slightly impressed on the sides, obtuse or pointed at base; apex of embryo directed transversely inward.—Streams and ditches, western N. Y. to Ill., Iowa, and Minn. Very near the last.

13. P. heterophýllus, Schreb. Stem slender, very branching below; floating leaves mostly thin, variable, but with a short blunt point, 9–15-nerved, usually 1–2´ long and 6–9´´ wide; submersed ones usually lanceolate or linear-lanceolate, acuminate or cuspidate, narrowed toward the base, about 7-nerved on the stem and 3-nerved on the branches; upper ones petioled, lower sessile; stipules obtuse, loose; peduncles somewhat thickened upward; fruit small, roundish, compressed, scarcely keeled; embryo annular above. (P. gramineus, Fries.)—Still or flowing water, common. Varies exceedingly in its submersed leaves, peduncles, etc.; the var. graminifòlius (Fries), growing in rapid streams, with stems much elongated and less branched, and the flaccid submersed leaves 2–7´ long by 2–10´´ wide.

Var. (?) myriophýllus, Robbins. Sending up from running rootstocks many short repeatedly dichotomous and densely leafy stems; fertile stems very slender; floating leaves small, delicate, lance-oblong, on long filiform petioles; submersed stem-leaves larger, early perishing; those of the branches (deep-green) linear-oblanceolate, very small (¾–1´ long), acute, sometimes minutely serrulate; spike slender, loosely-flowered, much shorter than the thickened peduncle.—Apponaug Pond, R. I., without fruit.

13a. P. Zízii, Mert. & Koch. Resembling P. lucens, but smaller, much branched at base; upper leaves coriaceous or subcoriaceous, long-petioled and sometimes emersed, the others subsessile, all usually numerous, undulate and shining; peduncle elongated. (P. lucens, var. minor, Nolte. Also P. gramineus, var. (?) spathulæformis, Robbins; P. spathæformis, Tuckerm.; "P. varians, Morong.")—N. Eng to Fla., and westward. Connecting with the next section. (Eu.)