[*] Submersed leaves reduced to narrowly grass-like or filiform sessile phyllodia.
[+] Stems rather stout; stipules free; spikes all emersed, cylindrical and densely fruited; fruits fleshy and turgid, obliquely obovate.
1. P. nàtans, L. Stem simple or sparingly branched; floating leaves all long-petioled, elliptical or ovate, somewhat cordate at base, obtuse but with a blunt point, 21–29-nerved; upper submersed leaves lanceolate, early perishing, the lower (later in the season) very slender (3–7´ long, barely 1´´ wide); upper stipules very long, acute; peduncle about the thickness of the stem; spikes 1–2´ long; sides of the turgid nutlet with a small deep impression in the middle; embryo coiled into an incomplete elliptical ring.—Ponds and ditches, N. Scotia to Va., westward across the continent. In deeper or flowing water the plant becomes more slender and often wholly submersed (var. prolíxus, Koch).—Aug., Sept. (Eu., Asia.)
2. P. Oakesiànus, Robbins. Stem more slender, much branched; floating leaves smaller (1–1½´ long), ovate- or oblong-elliptical, obtuse, fewer-(17–23-) nerved; lowest submersed ones almost capillary (only ¼–½´´ wide), continuing through the flowering season; spikes shorter (¾–1´ long), on peduncles much thicker than the stem; fruit smaller and more acute; sides of the turgid nutlet not at all impressed; curvature of the embryo nearly circular, its apex directed to a point above its base.—Ponds, and especially pools and stagnant ditches, Mass. to N. J.; also Anticosti. Aug.
3. P. Pennsylvánicus, Cham. Stems compressed, often simple from the creeping rootstocks; floating leaves chiefly opposite (1–3½´ long), 11–17-nerved, oblong, tapering into a short petiole, the lower gradually narrowing and passing into the submersed ones, which are very numerous and approximate, 2-ranked, linear (2–5´ long, and 1–2½´´ wide), 5–7-nerved, the lateral nerves slender and nearly marginal, the space within the inner nerves coarsely cellular-reticulated; stipules very obtuse; spikes numerous, about the length of the thickened peduncle; fruit round-obovate, flattish, 3-keeled when dry; nutlet distinctly impressed on the sides; curvature of the embryo transversely oval. (P. Claytonii, Tuckerm.)—Still or flowing water, N. Brunswick to S. C., west to N. Ind. and Minn. July, Aug.
[+][+] Like the preceding section, but all the parts small, slender and delicate, only the fertile plants producing floating leaves; spikes very small and few-flowered; propagated by autumn buds.
4. P. Vasèyi, Robbins. Very delicate; stem almost capillary; floating leaves obovate (3–5´´ long) and about the length of their filiform petioles, with 5 nerves deeply impressed beneath, cross-veins distinct; submersed leaves filiform-linear, very attenuate (1–2´ long, {1/8}–¼´´ wide) and acute; stipules not adnate, scarious, long, acute; spikes all emersed, few, interrupted-oblong, 3–5-flowered, on a thickish peduncle; fruit oblique, round-obovate ({2/3}´´ long), compressed, slightly sharp-margined, tipped with a distinct recurved style, the sides impressed and face acute; upper portion of the embryo circularly incurved, its apex transverse to the fruit.—Canada and N. Eng.; also Ill. The fruiting form, with floating leaves, rare; the submerged form apparently much more abundant.
5. P. lateràlis, Morong. Stem filiform, branching; floating leaves elliptical (4–6´´ long by 2´´ wide), with 5–7 nerves deeply impressed beneath, tapering at base into a somewhat dilated petiole shorter than the blade; submersed leaves linear, acute (1–3´ long by ¼–½´´ wide), 1–3-nerved, the mid-nerve with fine veins or cellular reticulations on each side, bi-glandular at base; stipules short; peduncles with a very peculiar lateral appearance, widely spreading at maturity, sometimes even recurved, often thicker than the stem; spikes often interrupted (2–4-flowered); fruit obliquely obovate (hardly 1´´ long), the back much curved, with two fine grooves upon it; embryo oval in its curve, the apex nearly touching the base.—Mass. and Mich.; rare. Undeveloped specimens resemble P. pusillus.
[+][+][+] Stems slender or filiform, much branched; floating leaves sometimes wanting; stipules adnate to the base of the leaf; spikes of two kinds, one emersed, cylindrical and many-flowered, on a club-shaped peduncle, the other submersed, globular and few-flowered; fruit flat, cochleate, with thin or scarcely any flesh and a thin nutlet; embryo spiral.
6. P. Spiríllus, Tuckerm. Floating leaves oval to lance-oblong and lanceolate (the largest 10´´ long, 4´´ wide), usually obtuse, about equalling the rather dilated petioles, with 5–many nerves beneath deeply impressed; upper submersed leaves either with or without a lance-oblong or broad-linear proper blade; the numerous lower ones narrow-linear, tapering toward the obtuse apex (¾–1½´ long, ¼–{2/3}´´ wide); stipules early lacerate; submersed flowers usually solitary on very short erect peduncles; fruit with the back either winged and with 4–5 distinct teeth or wingless and entire; embryo coiled 1¾ turns.—Rivers, and even far up small streams, N. Eng. to Va., west to Mich. and Mo. June–Aug.—Stem less slender than in the next.