I have always thought of the evening primroses as particularly romantic plants. Many of the Oenothera have richly-scented blooms that open in twilight. Their light-coloured petals make them gleam in the dusk light like rising moons. John Rusk's (aka John Rusk@Flickr) photo of Oenothera elata subsp. hookeri captures the romance of Hooker's evening primrose perfectly. Thanks John!
Some sources list four subspecies of Oenothera elata. Oenothera elata subspecies hirsutissima occurs in the western United States into the Mexican states of Baja California and Durango, while subspecies elata is found in central Mexico south into Central America. Subspecies hookeri occurs in moist coastal sites in California. Subspecies texensis is known from a single specimen collected in Texas (as of 1987), though not all public databases incorporate this last subspecies.
Oenothera elata is tall for an evening primrose, reaching heights of well over one meter. It sends up near-erect flowering stems, which bear flowers that open from the bottom up. The bisexual flowers are about 8 cm across. They are yellow and darken to orange with age. Each flower has four heart-shaped petals and four smaller sepals that may be fused in pairs. One unusual feature of evening primrose flowers are their X-shaped stigmas that extend out beyond the petals and stamens. Today's photo shows this property--the stamens look particularly unusual when extending beyond the furled petals of the spent flowers.
The flowers open in the evenings, timed for Oenothera elata's main pollinators: hawk moths (often Hyles lineata). One of the most amazing aspects of evening primroses is that their flowers open very quickly, some in under a minute. The Yosemite National Park website has a great video of an Oenothera elata subsp. hirsutissima flower opening. In the paper, Flower opening and closure: a review, van Doorn and van Meeteren explain that opening Oenothera flowers release their petals suddenly. This occurs because the cells at the margins of the sepals are connected in a zipper-like structure. As the petals grow, they break through the zipper and quickly spill out. Depending on the species of Oenothera, flower opening might be triggered by light levels or relative humidity. Other species of evening primrose appear to have an endogenous (internal) rhythm that triggers opening.