19. P. zosteræfòlius, Schum. Stem branching, wing-fattened; leaves linear and grass-like (commonly 4´ by 1½´´), abruptly pointed, with many fine and 3 larger nerves; stipules (seen young) oblong, very obtuse; spikes cylindrical, 12–15-flowered, not half as long as the peduncle; fruit obliquely obovate, somewhat keeled and with slight teeth on the back, the sides not impressed, the face arching and terminated by the short style; summit of the large embryo lying transverse to the fruit. (P. compressus, Fries; not L.?)—Still and slow-flowing waters, N. Eng. to N. J., Iowa, and Minn. Aug., Sept. (Eu.)
20. P. Híllii, Morong. Stem slender, widely branching, flattish; leaves linear, acute (1–2½´ long by ½–1¼´´ wide), 3-nerved, the lateral nerves delicate and near the margin; stipules whitish, striate, obtuse (3–5´´ long); spikes capitate (3–6-fruited), on short spreading or recurved peduncles; fruit as in the last.—Mich. and western N. Y.
21. P. obtusifòlius, Mertens & Koch. Stem flattened, very branching; leaves linear, tapering toward the base, obtuse and mucronate or very acute, 3- (rarely 5-) nerved; stipules elongated, very obtuse; spike ovate, continuous, 5–8-flowered, about the length of the peduncle; fruit oval, apiculate with the style, not keeled when fresh, upper portion of embryo coiled inward and lying transverse to the fruit.—Slow streams and ponds, Canada and N. Eng. to western N. Y. and Mich. Sept., Oct. (Eu.)
22. P. pauciflòrus, Pursh. Stem filiform, flattish and very branching; leaves narrowly linear (1–2´ long and seldom ½´´ wide), acute, obscurely 3-nerved; stipules obtuse; spikes capitate, 1–4- (usually 2-) flowered, on short club-shaped peduncles; fruit roundish-lenticular; the back more or less crested; upper portion of the embryo incurved in a circle.—Still or stagnant waters, N. Brunswick to Ga., Iowa, Minn., and westward.
Var. Niagarénsis, Gray. Stem often longer (1–3°); leaves larger (1½–3½´ long by 1´´ wide or less), 3–5-nerved at base, very acute and mucronate, narrowed to the subpetiolate base. (P. Niagarensis, Tuckerm.)—Running water, Great Lakes to S. C.; also in Cal.
23. P. pusíllus, L. Stem slender, flattish or nearly cylindrical, often very branching; leaves narrow- or setaceous-linear, acuminate, acute or subacute, 1–3-nerved, furnished with translucent glands on each side at the base; stipules at first obtuse; spikes interrupted or capitate, 2–8-flowered, on rather long peduncles; fruit obliquely elliptical, scarcely keeled; apex of embryo incurved and directed obliquely downward.—Pools and ditches, N. Scotia to N. J., west to Minn. and Mo., and westward.—Leaves sometimes almost setaceous (var. tenuissimus, Koch).
Var. polyphýllus, Morong. Dwarf form (3–5´ high), divaricately branching from the base, very leafy throughout; leaves very obtuse, not cuspidate, 3-nerved; non-flowering but abundantly provided with propagating buds which are formed on the thickened and hardened ends of the branches and closely invested by imbricated leaves.—In a shallow pool, S. Natick, Mass.
24. P. mucronàtus, Schrad. Resembling P. pusillus, but stem less branching; leaves broader (almost 1´´ wide), often 5-nerved; spikes interrupted. (P. pusillus, var. major, Fries.)—N. Brunswick to western N. Y., Mich., and Minn. July. (Eu.)
25. P. gemmíparus, Robbins. Stem filiform, branching, terete, varying greatly in height; leaves hair-like, sometimes not as broad as the stem, often with no apparent midrib, tapering to the finest point (1–3´ long), bi-glandular at base; stipules ½–1´ long; spikes few (3–6-flowered), interrupted, on long filiform peduncles; propagating buds very numerous; fruit like that of P. pusillus, very rare. (P. pusillus, var.? gemmiparus, Robbins.)—Slow-moving streams and still water, Mass. Aug., Sept.
26. P. Tuckermàni, Robbins. Very slender and delicate from a creeping rootstock, of a fine light green; stem filiform with several short and repeatedly dichotomous leaf-bearing branches; leaves thin and flat, but setaceous and tapering to near the fineness of a hair (1–4´ long and {1/3}´´ extreme width), obscurely 1–3-nerved, with a few coarse reticulations; stipules rather persistent below, {1/3}´ long, obtuse; peduncle solitary, very long, rather thickened upward; spike 4–8-flowered, in fruit continuous, oblong; fruit thick-lenticular, obscurely 3-keeled; nutlet slightly impressed on the sides; shell thick and hard; embryo nearly annular.—Cold ponds, White Mountains of N. H., N. Y., and N. J.