Plant: perennial herb; stems to 150 cm tall, producing branches mostly along their upper half Leaves: to 15 cm long, to 2 cm wide, sessile, narrowly elliptic to lanceolate; margins shallowly toothed; upper leaves with prominently clasping bases INFLORESCENCE: cymose Flowers: pedicellate, with (0-)2-6 lanceolate, several-toothed to -lobed bracts, these mostly arising from the ovary; petals white, but drying pale yellow, 25-50 mm long, 5-12 mm wide, with trichomes at apex only; staminodia 5, equal to the petals; stamens with broad filaments common in several outer whorls; style 15-30 mm long; stigmatic papillae inconspicuous Fruit: capsules cylindric, (15-)20-30 mm long. SEEDS horizontal, winged, oval to obovate, flattened; testa cells with straight to curved adjoining walls, the surface walls with 3-5 bumps Misc: On various substrates, especially roadsides; 1850 m (6000 ft); Aug-Sep REFERENCES: Christy, Charlotte M. 1998. Loasaceae. J. Ariz. - Nev. Acad. Sci. 30(2): 96.
Biennial or perennial herb to 1 m tall Leaves: alternate, stalkless to short-stalked, 4 - 10 cm long, 1.5 - 2 cm wide, usually smaller near top of plant, narrow and inversely egg-shaped with a pointed to rounded tip, coarsely toothed, covered with stinging barbed hairs. Flowers: borne near branch tips, white to cream, with 100 to 200 stamens, opening in afternoon, closing near sunset. The ten petals are 2 - 5 cm long, narrow and inversely egg-shaped, and usually do not overlap when open. Fruit: a cylindric capsule, 2 - 3 cm long, 8 - 10 mm wide, opening at the tip. The seeds are flat with well-developed wings. Stems: single to few, erect, often branching on upper half, yellowish to whitish.
Similar species: Mentzelia decapetala is a coarser plant with longer petals (4 - 7 cm) that tend to overlap and flowers that remain open until around midnight.
Flowering: July to August
Habitat and ecology: Introduced from farther west, this species grows near railroads.
Occurence in the Chicago region: non-native
Etymology: Mentzelia is named after German botanist Christian Mentzel (1622 - 1701). Nuda means naked.