§ 2. Leaves all submersed and similar, mostly sessile, membranaceous and dilated, lanceolate, oblong, or oval; stipules obtuse, becoming loose.
14. P. lùcens, L. Stem thick, branching, sometimes very large; leaves more or less petioled, oval or lanceolate, mucronate, often rough serrulate, frequently shining; peduncles often elongated; fruit roundish and compressed, with obtuse margins, slightly keeled; embryo circularly incurved above.—Ponds, N. Eng. to Fla., west to the Pacific. Aug., Sept. (Eu.)
Var. (?) Connecticuténsis, Robbins. Stem flexuous; leaves all submersed, nearly sessile, lanceolate, acuminate, crisped, not shining nor serrulate; fruit larger, distinctly keeled; nutlet thick and hard.—Lake Saltonstall, East Haven, Conn.
15. P. prælóngus, Wulf. Stem very long, branching, flexuous; leaves lance-oblong or lanceolate (sometimes 7´ long), half-clasping, obtuse with a boat-shaped cavity at the extremity, thence splitting on pressure; stipules scarious, very obtuse; spikes rather loose-flowered; peduncles very long (sometimes reaching 20´); fruit obliquely obovate, compressed, sharply keeled when dry; style terminating the nearly straight face; curve of the embryo oval and longitudinal.—Ponds and large rivers, N. Scotia to Mass., west to Minn. and Iowa. Sept., Oct.—Stem white; foliage bright green. (Eu.)
16. P. perfoliàtus, L. Stem branching; leaves orbicular, ovate or lanceolate from a cordate-clasping base, usually obtuse and often minutely serrulate; peduncles short, cylindrical; fruit irregularly obovate, obtusely margined; embryo incurved in an oval.—Ponds and slow streams, common. N. Scotia to Fla., west to Minn. and Iowa. Sept., Oct. (Eu.)
Var. lanceolàtus, Robbins. Larger; leaves long-lanceolate from a cordate clasping base and acuminate, wavy, 3–4½´ long; peduncles thickened upward.—Same range as the species, and extending west to the Pacific.
17. P. críspus, L. Stem compressed; leaves linear-oblong, half-clasping, obtuse, serrulate, crisped-wavy, 3-nerved; fruit long-beaked; upper portion of the embryo incurved in a large circle.—Flowing and stagnant waters, Mass. to N. J. and Va., west to western N. Y. June, July. (Eu.)
18. P. Mýsticus, Morong. Stem very slender and irregularly branching, nearly filiform (1–3° high); leaves oblong-linear (½–1½´ long by 2–3´´ wide), 5–7-nerved, finely undulate and entire, obtuse or bluntly pointed, abruptly narrowing at base, sessile or partly clasping; spikes few, capitate (4–6-flowered). on erect peduncles (1–2´ long); fruit (immature) obovate, small (hardly ¾´´ long), obscurely 3-keeled on the back, a little beaked by the slender recurved style.—Mystic Pond, Medford, Mass.
§ 3. Leaves all submersed and similar, mostly membranaceous and sessile, linear or setaceous.
[*] Stipules free from the sheathing base of the leaf.