Flower:

Single yellow flowers in the upper leaf axils with 4 rounded petals ¼ to ½ inch (5 to 12+ mm) long, often wrinkled like crumpled tissue paper, with wavy edges that are sometimes a bit ragged, and a rounded notch at the tip. In the center are 8 yellow stamens surrounding a stigma with 4 disc-shaped lobes. The 4 sepals are shorter than the petals, triangular to egg-shaped with a prominent mid-rib, and reflexed down at bloom time. Flowers are stalkless but sit on top of a hairy, cylindrical ovary about 3/8 inch long that resembles a stalk.

Leaves and stems:

Leaves are alternate, narrowly oblong, linear, or widest above the middle, ½ to 3½ inches (to 9 cm) long, up to 3/8 inch (1 cm) wide, stalkless or nearly so, short-hairy on the underside, usually with small, sharp teeth around the edges, occasionally toothless. Stems are ascending to erect, green to brown, covered in stiff, fine hairs, single or multiple from the base, unbranched or few-branched, and woody in the lower stem.

Fruit:

Fruit is a slender, cylindrical capsule in the leaf axils, slightly tapered at both ends, up to about 1 inch (6 to 25 mm) long, covered with fine, short hairs. Inside are numerous seeds, 1 to 1.8 mm long, sharply angled.

Notes:

Yellow Sundrops, also known as Calylophus serrulatus, is common in dry prairies and rock outcrops. It is typically much shorter than the tall spiked stems of evening primroses and has much narrower leaves. Another distinction with this species is that it blooms during the day where most Oenothera flowers open in afternoon or evening, for pollination by night-flying insects. It does great in a garden and has a fairly long bloom period. Personally, I was disappointed when the musical name Calylophus was dropped in favor of boring Oenothera, but at least it's in Oenothera subsection Calylophus.