1. Ceratophyllum (hornwort, coontail)

Plants perennial herbs, monoecious, lacking roots. Stems to 1 m or more long, freely branched. Leaves whorled, sessile or nearly so. Leaf blades divided 1 or more times dichotomously into 3–10 narrowly linear segments, rarely undivided, the margins with small, broadly triangular teeth, especially toward the segment tip, each with a minute, spinelike or bristlelike tip. Stipules absent. Inflorescences of solitary or less commonly 2 or 3 flowers in the leaf axils (staminate and pistillate flowers usually at different nodes), these sessile or very short-stalked, each subtended by a small involucre (interpreted as a perianth by some authors). Involucres of 8–12 bracts, these 0.5–1.5 mm long, linear to narrowly obovate, similar in appearance to the tips of the leaf divisions. Flowers hypogynous, the sepals and petals absent (but see above). Staminate flowers with 3 to numerous stamens, the filaments short, the anthers with 2 or 3 short teeth at the tip. Pistillate flowers with 1 pistil composed, the ovary superior, with 1 ovule. Style 1, short, tapered, grooved longitudinally along one side, the stigmatic region a small pouch at the base of the groove. Fruits achenes, the body 3.5–5.5 mm long, 2.5–3.5 mm wide, somewhat flattened, oblong-ovate in outline, green to olive-green or dark green, with a slender terminal spine (the elongated stigma) and 2 or more additional spreading spines along the rim, these shorter than to longer than the body. Six species, worldwide.

The taxonomy of Ceratophyllum has received three modern studies (Lowden, 1978; Wilmot-Dear, 1984; Les, 1986), which have recognized three, two, and six species respectively. The present treatment follows that of Les (summarized in Les, 1993), whose analyses include the largest and most diverse data set, including morphometric, developmental, chemosystematic, and molecular studies.

Species of Ceratophyllum are unrooted, submerged aquatics that are suspended below the water surface. In Missouri, reproduction is mostly by fragmentation of the branching stems, and fertile specimens are encountered rarely. The genus is unusual in that staminate flowers shed anthers, which trap small exuded air bubbles in their apical teeth and have about the same buoyant density as the surrounding water. These reach the pistillate flowers for pollination seemingly by accident as they are carried in and around the plants by small currents. Coontails have been used in folk medicine as purgatives, diuretics, and as treatments for jaundice, rheumatism, and skin disorders. The fruits and foliage are consumed by waterfowl, which help to disperse the plants, and the plants also provide valuable cover for small fish and aquatic invertebrates. Some species are sold for horticultural use in pools and aquaria, but the plants can become pests, growing so quickly and densely that they crowd out other plants and interfere with fishing and boating.