Zannichellia palustris L.: Horned pondweed. Zannichelliaceae (horned-pondweed family). Native perennial. Submersed plant; branches freely from slender rhizomes in the substrate and grows to 3.3 ft (1 m). It has very slender, flattened, grass-like stems and leaves. Leaves are opposite (unlike Potamogeton species), sometimes whorled, with 2 to several emerging per node. The stalkless blade is flat, strap-like, 1 - 4 in (2.5 - 10 cm) long, with smooth edges. A thin sheathing stipule at each node surrounds the stem and leaf bases. Flowers and later fruits are borne underwater in short clusters at the nodes. The inconspicuous flowers are inside small bracts, 0.1 in (2.5 mm) long. The curved fruits are hard, rough-surfaced and ridged, 0.125 in (3 mm) long, and topped by a prong or horn about half as long as the fruit itself. The plant can form mats at the surface.