1. Mitreola petiolata (J.F. Gmel.) Torr. & A. Gray (lax hornpod)

Cynoctonum mitreola (L.) Britton

Pl. 447 i, j Map 2029

Plants annual. Stems 8–40 cm long, erect or ascending, usually branched, glabrous. Lowermost leaves short-petiolate, the median and upper leaves sessile. Leaf blades 6–50 mm long, elliptic to very narrowly elliptic, broadest at about the midpoint, mostly narrowed or tapered to a sharply pointed tip (rarely blunt), narrowed or tapered at the base, glabrous. Inflorescences usually branched. Calyces 0.8–1.2 mm long, the lobes elliptic to ovate. Corollas 1.5–3.0 mm long, tubular to somewhat cup-shaped, white or slightly pinkish-tinged, the lobes erect or somewhat curved inward, the throat with a dense ring of short hairs. Stamens not exserted. Pistils initially united to the summit but soon becoming separated from the tip to below the midpoint and thus appearing 2-horned, the style initially 1, but often becoming split longitudinally and appearing as 2 units. Fruits 3–4 mm long, deeply 2-lobed, dehiscing longitudinally between the lobes. Seeds 0.2–0.4 mm in diameter, elliptic to nearly circular in outline, flattened, the attachment point in the middle of a concave face, the other face convex and rounded, brown, somewhat shiny. 2n=20. June–November.

Uncommon, known from historical collections in Butler and Dunklin counties and a single more recent station in Ripley county (southeastern U.S. west to Missouri and Texas; Mexico, Central America, South America, Caribbean Islands, Asia, Africa, Australia). Margins of ponds, swamps, and fens; also ditches.

Nigh and Ladd (1987) rediscovered this species in the open marly portion of a Ripley County fen more than 80 years after it was last seen in Missouri. J. B. Nelson (1980) summarized the nomenclatural history of the genus.