Illinois Pondweed

Potamogeton illinoensis and P. gramineus are often difficult to separate. Certainly, in the extreme of each they are easily separated, but they continually grade into each other. Features to look for are the acute-mucronate apex of the submersed leaves of P. illinoensis and the acuminate apex for P. gramineus. Also, the number of veins seems to work as well.

Three hybrids, Potamogeton illinoensis ´ P. nodosus (= P. ´ faxonii Morong), P. amplifolius ´ P. illinoensis (= P. ´ scoliophyllus Hagström), and P. gramineus ´ P. illinoensis [= P. ´ spathuliformis (J. W. Robbins) Morong], have been described.

Rhizomes present. Cauline stems terete, without spots, 28--120 cm; nodal glands absent. Turions absent. Leaves both submersed and floating or the floating absent, ± spirally arranged. Submersed leaves sessile or petiolate, lax; stipules persistent, conspicuous, convolute, free from blade, light brown to red-brown, not ligulate, 1--8 cm, not fibrous, not shredding at tip, apex acuminate; petioles if present 0.5--4 cm; blade red-brown to light green, elliptic to lanceolate or rarely linear, often arcuate, 5--20 cm ´ 2--45 mm, base acute, margins entire, often crispate, apex not hoodlike, acute-mucronate, lacunae in 2--5 rows each side midrib; veins 7--19. Floating leaves: petioles continuous in color to apex, 2--9 cm; blade adaxially light green, elliptic to oblong-elliptic, 4--19 cm ´ 20--65 mm, base cuneate, apex round-mucronate; veins 13--29. Inflorescences emersed, unbranched; peduncles not dimorphic, terminal or axillary, erect to ascending, cylindric, 4--30 cm; spikes not dimorphic, cylindric, 25--70 mm. Fruits sessile, grayish green to olive green, obovoid to ovoid, laterally compressed, abaxially keeled, laterally ridged, 2.5--3.6 ´ 2.1--3 mm, abaxial keel well developed, lateral ridges without points; beak erect to slightly recurved, 0.5--0.8 mm; sides without basal tubercles; embryo with 1 full spiral. 2n = 104.

B.C., Man., N.B., N.W.T., Ont., Que.; Ala., Ark., Calif., Colo., Conn., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., Md., Mass., Mich., Minn., Mo., Mont., Nebr., N.H., N.J., N.Y., N.C., Okla., Ohio, Oreg., Pa., S.C., S.Dak., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Mexico; West Indies; Central America; South America.

Flowering and fruiting summer--fall.

Alkaline waters of streams, rivers, lakes, ponds, and sloughs; 0--2700m.

Potamogeton angustifolius Berch. & Presl, Rostlin
2 : Alismac. 19. 1821.
Potamogeton heierophyllus elongatus Mert. & Koch, in Rohling, Deuts. Fl. ed. 3. 1 : 845. 1823.
Potamogeton Zizii Roth, I$num. PI. Germ. 1 : 531. 1827.
Potamogeton lucens connecticutensis Robb. in A. Gray, Man. ed. 5. 488. 1867.
Potamogeton angustifolius methyensis Morong, Mem. Torrey Club 3 2 : 30. 1893.
Potamogeton angustifolius connecticutensis A. Benn. Jour. Bot. 39 : 199. 1901.
Potamogeton Zizii porrectifolius A. Benn.; Asch. & Graebn. in Bugler, Pflanzenreich 4 11 : 83.
1907. Potamogeton Zizii gracilis A. Benn.; Asch. & Graebn. in Engler, Pflanzenreich 4 1 * : 83. 1907.
Stems branching; floating leaves petioled, sometimes wanting; blades coriaceous, rarely somewhat translucent, elliptic-ovate or elliptic, acute or acuminate or sometimes mucronate at the apex, narrowed at the acute base, 3.5-9 cm. long, 1.5-3.8 cm. wide; petioles shorter than the blades or longer ; submerged leaves sessile or the upper p etiolate ; blades pellucid, lanceolate or somewhat spatulate, and minutely serrulate towards the acute, sometimes cuspidate apex, 5-15 cm. long, 0.5-2.8 cm. wide, or rarely wider; primary nerves 7, sometimes with a few intermediate secondary nerves; stipules 2-keeled, obtuse, 1.3-3 cm. long, axillary and free from the petioles; spikes cylindric, 2.3-4.5 cm. long, densely flowered, rarely interrupted ; peduncles as thick as the stem or thicker, 6-12 cm. long ; nutlets smooth, 3-keeled when mature; embryo mostly an incomplete spiral, the apex pointing directly towards the base or sometimes inside it.
Type locality : Bohemia.
Distribution : Maine to California, south to Florida, Mexico, and Guatemala ; also in

Potamogeton illinoensis Morong, Bot. Gaz. 5 : 50. 1880
+
Stem branched ; floating leaves petiolate ; blades coriaceous, thick, ovate or elliptic-
t
ovate, mucronate at the apex, rounded or subcordate at the base and sometimes decurrent, 8-14 cm. long, 4-8 cm. wide ; petioles 6-10 cm. long, sometimes winged ; submerged leaves petioled ; blades mostly lanceolate-oblong, acute or acuminate at the apex and narrowed or sometimes decurrent at the base, 8-18 cm. long, as wide as or a little wider than the floating leaf -blades ; petioles winged, 1-2.8 cm. long, rarely longer ; stipules 2.5-4.5 cm. long, obtuse, 2-keeled; spikes cylindric, many-flowered, 2.5^.8 cm. long; peduncles as thick as or thicker than the stem, 5-9 cm, long ; nutlets smooth, 3-keeled, the middle keel sharper and more prominent than the smaller lateral ones ; embryo a complete spiral,, the curved apex pointing inside the base.
Type locality : Oquawka, Illinois.
Distribution : Western Illinois and eastern Iowa.

Potamogeton illinoensis, commonly known as Illinois pondweed or shining pondweed, is an aquatic plant. It provides food and cover for aquatic animals.

It is generally not weedy in its native range, but it is a troublesome noxious weed in Viedma, Río Negro, Argentina, where it is an introduced species.