Flower:

Cylindrical spike either submersed or held above the surface of the water, 1 to 2¾ inches (to 7 cm) long at the tip of the stem and arising from the axils of floating leaves. Spikes have 8 to 15 whorls of flowers that are tightly packed to slightly separated, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a greenish to yellowish to orange-brown, ladle-shaped, sepal-like appendage.

Leaves and stems:

Both submersed and floating leaves are produced, more or less spirally arranged, though floating leaves are often absent. Floating leaves are firm, green, elliptic, 1½ to 7+ inches long, ¾ to 2½ inches wide, toothless, abruptly tapered to an extended point at the tip, usually tapering at the base or slightly rounded, with 13 to 29 veins flanking the midrib. The leaf stalk is usually shorter than the blade.

Submersed leaves are thin, flat to somewhat arching, green to reddish, lance-elliptic, 1½ to 7½ inches long, to 1¾ inches (45 mm) wide, abruptly tapered to an extended point up to 4 mm (~1/6 inch) long at the tip, tapering at the base, often wavy along the edges. Leaves are stalkless or on a stalk up to about ¾ inch long, occasionally longer. The midvein is prominent with 2 to 5 narrow rows of large, empty cells (known as the lacunar band) along each side and flanked by 7 to 19 lateral veins.

At the base of the leaf stalk is a pale, membranous appendage (stipule), not connected to the leaf blade, 3/8 to 3¼ inches long, pointed to rounded and not shredding at the tip, and persists through summer. Stems are round, usually branched. Colonies may be formed from spreading rhizomes. Vegetative buds (turions) are not produced. Glands at the leaf nodes are absent.

Fruit:

Fruit is a dry seed (achene), the flowering spikes forming densely packed seed heads, gray-green to olive-brown when mature.

Achenes are irregularly oval to nearly round, slightly flattened, 3 to 4 mm long, with a narrow keel along the back edge flanked by a pair of less prominent lateral keels. The short, rounded, abrupt beak is erect.

Notes:

Illinois Pondweed is fairly common in central Minnesota, often forming large, loose colonies, found in the quiet waters of lakes and ponds or in slow to moderately fast moving rivers, in water up to 15 feet deep. Floating leaves are often not produced, especially in deeper water; fully submersed plants can still flower with the spikes all under water. Potamogeton illinoensis is distinguished by submersed leaves that are up to 8 inches long, flat to arching, generally elliptic, short-stalked (usually less than 1 inch) to nearly stalkless, abruptly tapered at the tip to a extended point up to 4 mm long, 7 to 19-veined, with stipules free from the blade and up to about 3 inches long; floating leaves, when present, have a similar tip on the leaf, stalks usually shorter than the blade, and are 13 to 29-veined; achenes are 3 to 4 mm long with a smooth keel along the outer edge and a pair of less prominent lateral keels.

Floating and submersed leaves resemble those of some other Pondweeds, notably Potamogeton nodosus, which has both floating and submersed leaves that are longer-stalked and pointed at the tip but not abruptly so, and P. gramineus, which has submersed leaves that have only 3 to 9 veins and are usually all stalkless, narrower and more ribbon-like but sometimes narrowly lance-elliptic, stipules are usually less than 1 inch long, and achenes are smaller, usually not more than 2.5 mm long. The broad, sometimes arching submersed leaves of P. illinoensis can resemble P. amplifolius, which has floating leaves rounded to heart-shaped at the base and stalks longer than the blades, and submersed leaves are typically broader, mostly arching, often folded, and have 19 to 49 veins.