Immersed aquatic herbs, with jointed stems and sheathing stipules. Leaves flat, immersed or floating. Flowers perfect or imperfect, inconspicuous, naked, or with a free scalelike calyx. Ovaries 1-celled, 1-seeded, solitary or 2-4, and distinct.
* Flowers perfect, in spikes or clusters.
1. Potamoge'ton. Flowers with sepals, stamens, and sessile ovaries each 4.
* * Flowers monoecious or dioecious, axillary, naked, monandrous.
2. Nai'as. Flowers dioecious. Ovaries solitary and naked. Stigmas 2 or i, awl-shaped. Staminate flower (of 1 stamen) enclosed in a membranous spathe. Stems floating. Leaves linear, opposite, somewhat crowded into whorls, sharp-toothed, sessile and spreading at the base.
3. Zannichell'ia. Flowers moncecious, usually both kinds in the same axil. Ovaries 2-5, from a cup-shaped involucre. Leaves linear-thread-form, entire.
I. Zoste'ra. Flowers monoecious, the two kinds naked and sessile, arranged alternately in 2 vertical rows on the inner side of a leaflike enclosed spadix. Sterile flowers of single sessile 1-celled anthers; the fertile of single ovate-oblong ovaries. Stigmas 2, bristle-form, deciduous.
* Leaves of 2 sorts, the floating ones with a dilated petioled blade, different in form from the submerged ones.
1. P. natans, L. Submerged leaves grass-like or capillary; upper stipules very long, acute. Spikes cylindrical, all out of the water. Stem hardly branched. Floating leaves long-petioled, elliptical, with a somewhat heart-shaped base, with a blunt apex, 21-29-nerved.
2. P. Clayto'nii, Tuckerman. Stem compressed. Submerged leaves linear, 2-5 inches long, 2-ranked, 5-nerved; stipules obtuse. Floating leaves short-petioled, chiefly opposite, oblong, 11-17-nerved. Spikes all above water.
3. P. Spiril'lus, Tuckerman. Stems very slender. Floating leaves when present oval to lanceolate, about as long as the petiole; submerged leaves narrowly linear, or the upper ones broad-linear, or, lance-oblong. Emersed flowers in many-flowered spikes; submersed flowers usually solitary. Fruit either winged and 4-5-toothed, or wingless and entire.
4. P. rufes'cens, Schrad. Stem simple. Submersed leaves almost sessile, lanceolate and lance-oblong; floating leaves (often wanting) wedge-oblanceolate, narrowed into a short petiole, 11-17-nerved. Spike dense, many-flowered. Fruit obovate, lenticular, with acute margin, and pointed with the long style.
5. P. flu'itans, Roth. (P. lonchites, Tuckerm.) Stem often branching below. Submersed leaves very long, lanceolate and lance-linear, 7-15-nerved; floating leaves lance-oblong, or oblong-elliptical, long-petioled, 17-23-nerved. Spike dense. Fruit obliquely obovate, 3-keeled when dry, the middle one winged above. - In streams mostly.
6. P. amplifo'lius, Tuckerman. Submersed leaves large, lanceolate or oval, acute at each end, recurved, wavy; stipules long and tapering. Floating leaves large, oblong or lance-ovate, or slightly cordate, long-petioled, 30-50-nerved.
7. P. heterophyl'lus, Schreb. (P. gramineus, Fries.) Stem slender, very much branched below. Submersed leaves lanceolate or linear-lanceolate, acuminate or cuspidate, 3-7-nerved, upper ones petioled, lower sessile; floating leaves variable, with short blunt points, 9-15-nerved. Stipules obtuse.
8. P. Ziz'ii, Mert. and Koch. (P. lucens, var. minor, Nolte.) Much branched at the base. Upper leaves long-petioled and sometimes emersed, the others nearly sessile, all usually numerous, wavy and shining. Resembling the next species, but smaller.
* * Leaves all submersed and similar, mostly sessile, membranaceous and dilated. Stipules obtuse, becoming loose.
9. P. lucens, L. Stem thick, branching. Leaves petioled oval or lanceolate, mucronate, frequently shining. Fruit roundish, compressed, with blunt edges, slightly keeled.
10. P. praelon'gus, Wulfen. Stem very long and branching. Leaves all submersed and similar, lanceolate, half-clasping, with a boat-shaped cavity at the end. Spikes loose-flowered with very long peduncles. Fruit sharply keeled when dry. Stem white. - Ponds and large rivers.
11. P. perfolia'tus, L. Leaves all submersed, varying in width from orbicular to lanceolate, clasping by a heart-shaped base. Stem branching.
Var. lanceola'tus, Bobbins, has long-lanceolate acuminate leaves. Peduncles thickened upward.
* * * Leaves all submersed and similar, mostly sessile, linear or bristlelike.
12. P. compressus, Fries. (P. zosteroefolius, Schum., in Macoun's Catalogue.) Leaves linear, grass-like, sessile, abruptly pointed, with three large nerves and many fine ones. Stem branching, wing-flattened. Stipules free from the sheathing base of the leaf. Spikes cylindrical.
13. P. pauciflo'rus, Pursh. Stem filiform, flattish and much branched. Leaves narrowly-linear, acute, obscurely 3-nerved. Spikes capitate.
14. P. puisil'lus, L. Stem slender, somewhat flattish, often much branched. Leaves narrow- or setaceous-linear, 1-3-nerved, with translucent glands on each side at the base.
15. P. mueronat'us, Schrad. Like the last, but the stem less branching, and the leaves broader (nearly an inch wide), often 5-nerved.
16. P. pectina'tus, L. Leaves bristle-shaped. Stem repeatedly forking, filiform. Spikes interrupted, on long slender peduncles. Stipules united with the sheathing base of the leaf.
17. P. Robbin'sii, Oakes. Leaves narrowly lanceolate or linear, crowded in 2 ranks, recurved, serrulate, many-nerved. Stems rigid with numerous branches. Fruit keeled with a broadish wing. Stipules united with the sheathing base of the leaf.
L. Naiad. N. flex'ilis, Rostk. and Schmidt. Stems very slender. Leaves very narrowly linear, minutely serrulate.
Micheli. Horned Pondweed. Z. palus'tris, L. Fruit obliquely oblong, flattish and somewhat incurved. Style half as long as the nutlet.
L. Eel-grass. Grass-wrack. Z. mari'na, L. Leaves obscurely 3-5-nerved. - Atl sea-coast, in shoal water.
Flowers with a perianth coloured like a corolla.