Flower:

Dense cylindrical spike held above the surface of the water, ½ to 2 inches (to 5 cm) long at the tip of the stem and arising from the axils of floating leaves. Spikes have 5 to 12 whorls of flowers, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a green to orange-brown, ladle-shaped, sepal-like appendage.

Leaves and stems:

Both submersed and floating leaves are produced, though floating leaves are sometimes absent. Floating leaves are firm, green, elliptic, 1 to 3+ inches long, up to ¾ inch (2cm) wide, toothless, rounded to bluntly pointed at the tip, tapering at the base to a stalk up to 1.5 times as long as the blade but is often shorter than the blade. Blades have 11 to 41 veins.

Submersed leaves are alternate on opposite sides of the stem (2-ranked), bright green to reddish, linear and ribbon-like, 2 to 8 inches long, 2 to 10 mm (to 3/8 inch) wide, bluntly to sharply pointed at the tip, and stalkless. The midvein is flanked by several narrow rows of large, empty cells (known as the lacunar band), giving leaves a 3-striped appearance, the central stripe more translucent than the edges and about 1/3 the leaf width.

At the base of the leaf is a membranous appendage (stipule), not connected to the leaf blade, pale brown to reddish, up to 1 inch long, the tip somewhat rounded and not shredding. Stems are somewhat flattened, unbranched or few-branched. Colonies are often formed from long rhizomes. Vegetative buds (turions) are not produced. Glands at the leaf nodes are mostly absent.

Fruit:

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

Achenes are irregularly oval to nearly round, 2.5 to 4.5 mm long, with a slightly bumpy keel along the back edge usually flanked by a pair of more obscure lateral keels. The beak is minute and erect.

Notes:

Ribbon-leaf Pondweed is common in northern Minnesota, usually in the shallow, quiet waters of soft water lake and pond margins, most often in mucky substrates, less often in deeper or flowing water. It is distinguished by elliptic floating leaves up to 3 inches long, tapering at base (not rounded), rounded to bluntly pointed at the tip, on stalks not much longer than the blades (often shorter); submersed leaves strongly 2-ranked, stalkless, flexible and ribbon-like, 2 to 8 inches long, up to 10 mm (3/8 inch) wide, appearing 3-striped with a broad lacunar band along each side of the midrib, the central stripe totaling about 1/3 the leaf width. The floating leaves are sometimes absent, especially in deeper water.

Floating leaves resemble those of some other Pondweeds, but their blades may be heart-shaped or more consistently rounded at the base, more sharply pointed at the tip, leaf stalks may be consistently much longer than the blade; submersed leaves may be stalked, more elliptic, have wavy edges, have narrower lacunar bands (the central band much less than 1/3 the leaf width), and/or are smaller, more consistently less than 3 mm wide.