Potamogeton nodosus Poiret, longleaf pondweed. Aquatic perennial herb, clonal, rhizomatous, fibrous–rooted, not rosetted, mostly submersed with floating leaves; shoots with only cauline leaves with long petioles and stems and rhizomes with long internodes, glabrous; rhizomes cylindric, internodes to 190 × 3 mm, white; adventitious roots nodal, slender.
Stems ± cylindric, to 210 × 3 mm, white to green lacking purple spots; with elongate air canals in “cortex” (aerenchyma).
Leaves alternate distichous, simple, petiolate, with stipules; stipule 1, attached across node, V–folded along midvein, 35—65 mm long, translucent with many parallel veins; of submersed leaf petiole ± hemicylindric with rounded edges, 40—105 mm long, with aerenchyma, blade linear–oblanceolate to narrowly elliptic to narrowly oblanceolate, 18—53 × 6—9 mm, long–tapered at base, entire, blunt acute at tip lacking fine point at tip, midrib giving rise to 8 ascending, parallel lateral veins, midrib slightly raised on both surfaces; of floating leaf petiole ± hemicylindric with rounded edges, 20—210 mm long, with aerenchyma, blade elliptic, 28—95 × 20—37 mm, tapered at base, entire, broadly acute to obtuse or rounded at tip, midrib giving rise to 20 ascending, parallel lateral veins with veins slightly raised on lower surface, upper surface flat, water repellent, and many stomates, lower surface wettable and lacking stomates.
Inflorescence spike, axillary, emergent at anthesis, many–flowered, ca. 30—40 × 5 mm in flower, without bracts, glabrous; peduncle stemlike, cylindric, 40—60 mm long; rachis ridged, 26—35 mm long.
Flower bisexual, radial, 3—3.5 mm across; sepals 4, clawed and sharply bent inward 90° at top of claw, green, claw cylindric, 0.6—0.7 mm long, limb fan–shaped to broadly triangular, 2—2.5 mm, horizontal, with raised midvein; petals absent; stamens 4, opposite tepals, sessile; anthers basifixed, dithecal, 1—1.4 mm long, pale yellow, longitudinally and outward dehiscent; pollen pale yellow; pistils 4 in whorl; ovaries subsessile, superior, kidney–shaped, ca. 1.1 × 0.6 mm, green, glabrous, each 1–chambered with 1 ovule; styles absent; stigma ridgelike from midpoint upward across top, purplish, papillate.
Fruits drupes, 1—4 per flower, plumb D–shaped to ovoid, 3.6—4.2 × 3 × 2.5 mm, reddish to reddish brown.
A. C. GIbson