Onagraceae is quite a large family, with 17 US genera and 281 species, well represented in all parts of the country. Most plants are herbs, producing relatively large and colorful flowers, singly or in clusters. Leaves are usually undivided but often toothed, with an opposite or alternate arrangement along the stems. Flowers typically have four sepals and four petals, all separate, while at the center are four or eight stamens and a style topped by a four-lobed stigma. The receptacle beneath the flower is enlarged to form a narrow tube, the hypanthium, that can be several inches long (for example in the oenothera genus), while beneath this is the ovary, also often extended, such as in the epilobium genus. Some species have flower parts numbering in the range two to seven rather than four. The common name evening primrose family refers to the habit of flowers opening in the evening, closing during the following morning.