Flower:

Dense cylindrical spike held above the surface of the water, 1¾ to 3¾ inches (to 9.5 cm) long at the tip of the stem and arising from the axils of floating leaves. Spikes have 6 to 10+ whorls of flowers, each flower with a 4-parted style surrounded by 4 stamens, each stamen with a green, ladle-shaped, sepal-like appendage.

Leaves and stems:

Both submersed and floating leaves are produced, more or less spirally arranged. Floating leaves are firm, green, oval-elliptic to egg-shaped, 1½ to 4+ inches long, 1 to 2 inches wide, toothless, mostly heart-shaped at the base or sometimes rounded, pointed to rounded at the tip, with 17 to 37 veins flanking the midrib. The leaf stalk is usually much longer than the blade, bent at a 90° angle where it meets the blade, and is conspicuously pale just below the blade.

Submersed leaves are ascending to spreading, sometimes partially floating on the water's surface, linear, 3 to 8 inches long, .7 to 2.5 mm (less than 1/8 inch) wide, stiff, pointed at the tip, somewhat narrowed at the base, stalkless, with 3 to 5 obscure veins. Submersed leaves may be reduced to bladeless stalks, round to elliptic in cross-section, which can disintegrate by fruiting time.

At the base of the leaf is a membranous appendage (stipule), not connected to the leaf blade, 1¾ to 4+ inches long, the tip pointed to rounded and not shredding, initially green turning translucent straw-colored to whitish. Stems are round, often rusty-spotted, mostly unbranched. Colonies may be formed from creeping 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, green to greenish-brown when mature.

Achenes are irregularly oval, 3.5 to 5 mm long, lack distinct keels, and are slightly concave on the sides. The beak is short, erect or curved at the tip.

Notes:

Floating Pondweed is common in lakes and ponds of Minnesota, the floating leaves often mixed in among other aquatic species or smaller groups in deeper water. Pulling up a stem with floating leaves you'll discover there are few submersed leaves persisting, though newer leaves may be found on new branches growing from the lower axils later in summer. Floating leaves are rarely not produced. Floating Pondweed is readily identified by floating leaves with mostly heart-shaped bases and long leaf stalks that are conspicuously pale just below the blade (see image below), and usually bent where it meets the blade, though this last characteristic may be less pronounced in flowing waters than calm waters. Submersed leaves, when persisting, are stiff and very narrow, and achenes lack any distinct keels along the outer edge.

Floating leaves resemble those of some other Pondweeds, but their leaf stalks may be shorter than the blade, are a consistent color for the entire length and not bent where it meets the blade, submersed leaves may be broader or have more flexible and ribbon-like blades, and achenes may have distinct ridges. A curious thing I noted in both our own images and others on the web: floating leaves are commonly indented in lines that do not correspond to the veins. The lines are straight and parallel to the midrib (like the blade was pleated) where the veins radiate from the base where it meets the stalk. While this oddity has been noted on some other species, it seems most pronounced on P. natans.