Flower:

Separate male and female flowers on the same plant (monoecious), usually 1 of each in leaf axils all along the stem. All flowers are green, short-stalked, and lack petals or sepals. Male flowers consist of a single stamen. Female flowers consist of a cluster of 4 or 5 carpels (pistils), each with an elliptic to cylindric ovary, topped with a short style and a triangular or funnel-shaped stigma at the tip. A cup-shaped membrane surrounding the carpels quickly disintegrates.

Leaves and stems:

Leaves are all submersed, opposite or whorled, thread-like, ¾ to 4 inches (2 to 10 cm) long, .2 to 1 mm wide, stalkless, pointed at the tip, mostly 1-veined. At the base of the leaf is a pale, membranous appendage (stipule), not connected to the leaf blade, often rolled around the stem, and quickly disintegrates. Stems are thread-like, freely branching, round in cross-section. Colonies may from from slender rhizomes.

Fruit:

Fruit is a dry seed (achene), short-stalked, more or less banana-shaped with a knobby keel along the back and a straight beak at the tip. Commonly 2 to 4 are produced by each female flower, .

Achenes mature to brown, the body 1.7 to 3 mm long, the beak .7 to 2 mm long, the stalk .1 to 1.5 mm long.

Notes:

Horned Pondweed is a common aquatic with a world-wide distribution. In Minnesota, it is most often found in lakes with sandy or muddy bottoms, often in less than 1 foot of water. It's been noted as inhabiting cooler inlets or near cold, groundwater seepage areas, in brackish or fresh water. It is easily recognizable by the unique fruits, banana-shaped with a unicorn horn at the tip and wavy ridge along the back, which are produced all summer, as well as the opposite or whorled, thread-like leaves. Other Pondweeds with thread-like leaves are all alternate or sometimes basal.