Flower:

Single yellow flowers in the upper leaf axils, ½ to 1 inch across, with 4 rounded petals, often quite 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 less than ¼ inch long, 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 narrowly oblong, linear, or widest above the middle, 1 to 3 inches long and not more than ¼ inch wide, stalkless, may be toothless but usually with small, sharp teeth around the edges and short hairy on the underside. Stems are erect, green to brown, covered in stiff, fine hairs, often clustered from a root crown but not much branched, and woody in the lower stem.

Fruit:

Fruit is a slender, cylindrical capsule in the leaf axils, slightly tapered at both ends, ½ to 1¼ inches long, covered with fine, short hairs.

Notes:

At first glance, Calylophus serrulatus looks like an Oenothera and not surprisingly, at one time it was known as Oenothera serrulata. Common in dry prairies, it is typically much shorter than the tall spiked stems of other common evening primroses and has much narrower leaves. A distinction with this species is that it blooms during the day where Oenothera flowers open in afternoon or evening, for pollination by night-flying insects.