1. Egeria densa Planch. (giant elodea, Brazilian waterweed)

Pl. 89 b, c; Map 336

Anacharis densa (Planch.) Victorin

Elodea densa (Planch.) Caspary

Plants dioecious. Stems sparsely branched, 30–200 cm long. Leaves mostly in whorls of 4–6 (sometimes 3 near base of stems), 12–40 mm long, 1.5–5.0 mm wide, linear to narrowly oblong (sometimes narrowly ovate near base of stems), the tips pointed, the margins minutely toothed, without a spongelike mass of enlarged cells on the undersurface. Spathes from the upper leaf axils, 7–12 mm long, the bracts fused along 1 side. Staminate flowers 2–4 per spathe, the stalks 3–8 cm long, the nectary green, 3-lobed, central. Sepals 2–4 mm long, boat-shaped, oblong-elliptic. Petals 7–11 mm long, obovate, white. Stamens 9, the filaments not united. Pistillate flowers not produced in North American plants, similar in morphology to the staminate flowers, but with petals 4–8 mm long, the styles deeply 3-lobed, and with a small nectary at the base of each lobe. 2n=24, 48. April–June.

Introduced, escaped from cultivation at a single station in Oregon County (native of South Americ a, widely escaped from cultivation). Submerged aquatic in still and moving water.

At its only Missouri station, this species was introduced into the Eleven Point River when floods scoured nearby ponds of a commercial grower of aquarium plants (Whitley et al., 1990). The species is perhaps the most popular of aquarium plants. Throughout its North American range, it occurs as plants with only staminate flowers, which spread by fragmentation of the brittle stems, and can become a nuisance weed. It should be expected sporadically in ponds and rivers elsewhere in the state.