1. Ammi majus L. (bishop’s weed)

Pl. 202 a, b; Map 838

Plants annual. Stems 20–80 cm long, erect or ascending, glabrous. Leaves alternate and also basal (basal rosette usually present at flowering), glabrous, short- to long-petiolate, the sheathing bases not inflated. Leaf blades 2–20 cm long, oblong to broadly triangular-ovate in outline, those of the basal and lowermost stem leaves ternately or pinnately 1 time compound, the leaflets 10–20 mm long, elliptic-lanceolate, narrowed at the base, rounded or narrowed to a sharp point at the tip, finely toothed along the margins; the blades of the median and upper stem leaves 2 times pinnately dissected, the ultimate segments 2–20 mm long, narrowly linear, narrowed to sharply pointed tips. Inflorescences terminal and sometimes also axillary, compound umbels, mostly long-stalked, the stalks roughened. Involucre of numerous 1 or 2 times pinnately dissected bracts, these mostly longer than the rays, spreading to reflexed at flowering, with thin, papery margins and sharply pointed tips. Rays numerous, 2–7 cm long, roughened. Involucel of numerous entire bractlets, these mostly slightly shorter than the flower stalks, with thin, papery margins and sharply pointed tips. Flowers mostly numerous in each umbellet, the stalks 3–12 mm long. Sepals minute triangular teeth. Petals ovate to obovate, broadly rounded to more commonly notched or 2-lobed at the tip, white. Ovaries glabrous. Fruits 1.5–2.5 mm long, oblong-elliptic in outline, flattened laterally, glabrous, dark brown, each mericarp with 5 shallow, angled ribs lacking wings. 2n=22. May–July.

Introduced, known thus far only from the city of St. Louis (native of Europe, Asia; widely cultivated as an ornamental, escaped sporadically in the U.S.). Railroads and open, disturbed areas.

Plants of A. majus, particularly the immature fruits, reportedly contain furanocoumarins and related compounds similar to those found in Heracleum and may cause phototoxic dermatitis in some individuals. The plants are sometimes grown for the cut-flower trade or for use in dried flower arrangements.