1. Justicia americana (L.) Vahl (water willow)

J. americana var. subcoriacea Fernald

Pl. 195 a–c; Map 799

Aerial stems 30–100 cm long. Leaf blades 1–16 cm long, the lowermost leaves often reduced, mostly narrowly elliptic or lanceolate, rarely broader, tapered at the tip and base. Inflorescences axillary headlike clusters, the inflorescence stalk slightly shorter than to longer than the subtending leaf. Calyces 4–8 mm long, the lobes narrowly lanceolate. Corollas 8–15 mm long, the lower lip white or pale purple with purple markings, the upper lip usually light purple, rarely white. Fruits 8–13 mm long. Seeds 2.0–3.5 mm long, reddish brown to brown, lacking a differentiated margin, the surfaces papillose. 2n=26. May–October.

Common nearly throughout the state, but uncommon or absent from portions of the Glaciated Plains and Mississippi Lowlands Divisions (eastern U.S. and adjacent Canada west to Wisconsin and Texas). Usually emergent aquatics along banks of spring branches, streams, and rivers, less commonly margins of ponds, lakes, and sloughs; also ditches.

Water willow is a characteristic species of gravel bars and other stream banks throughout much of Missouri. The dense colonies of aerial stems frequently are unbranched and often do not flower every year.