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Oenothera (evening primrose; family Onagraceae) A genus of flowering plants that have spirally arranged leaves and 4-petalled, large, usually yellow to orange flowers in a leafy spike. The seeds, unlike those of some other genera in the family, are not plumed. The unusual chromosome morphology and genetic mechanism of Oenothera have been the subject of much study by cytogeneticists. There are about 80 species, native to N. America, but some are much cultivated in gardens for the flowers, both as species and as hybrids, and are now well naturalized in Europe and elsewhere.