Any of about 850 species of small herbaceous plants that make up the genus Oxalis, native mostly to southern Africa and tropical and South America. Most members are familiar garden ornamentals. The name (Greek for acid) reflects the plant's sharp acidic taste. The common wood sorrel (O. acetosella) of eastern North America and Britain is a small, stemless plant with cloverlike three-part leaves, whose leaflets fold back and droop at night. The flowers have five white, purple-veined petals. After the fruit splits open, the fleshy coat of the seed curls back elastically, ejecting the true seed.