Flower:

Flowers are 1 to 2 inches across, 4 white notched petals with a spot of pale yellow at the base of each petal. There are 8 long yellow stamens surrounding a style with a cross-shaped stigma in the center. Behind the flower are light green to reddish sepals and a long light green to reddish calyx tube that resembles a flower stalk. One plant can have several flowers, with 1 to a few at the end of branching stems. The flowers open in the evening and close up in the morning.

Leaves:

Leaves are very narrow with a pointed tip and no leaf stalk, alternately attached. The leaves along the main stem are up to 3 inches long and ¼ inch wide with tiny widely-spaced teeth; they are rather shorter on the branches and teeth aren't visible. The stem is smooth and nearly white.

Notes:

The flower photos posted here were taken early in the morning. They were starting to close before 9am and were completely closed well before noon. The stems of Nuttall's Evening Primrose are said to be covered in a peeling white bark, though I did not observe this peeling myself. This species is easy enough to ID even when the flowers are closed, as the white stems and long, narrow leaves are quite distinct. The location where these images were taken was a tiny postage stamp of remnant prairie along the railroad right-of-way at Long Lake Regional Park in Ramsey County. As part of a city redevelopment project, the railroad destroyed the spot in early 2009 after dismantling  a section of the tracks. This species has not been seen there since. The last (and only) herbarium record for Ramsey County was collected in 1895, so these plants you see here may have been the last of their kind in Ramsey County. Life is fleeting, and all such native remnants are at risk.