ACENA, in botany, a genus of the monogynia order belonging to the tetrandria class of plants; the characters of which are these: The calyx is a perianthium consisting of four leaves, which are ovate, concave, equal, and persistent; there is no corolla: The stamina consists of four equal middle-sized filaments opposite to the calyx; the anthers are quadrangular, twin, erect: The pistillum has an inversely-ovate hispid germ; the stylus is small, and inflected on one side; and the stigma is a small thickish coloured membrane, divided into many segments: The pericarpium is an inversely-ovated dry one-celled berry covered with prickles bent backwards: The seed is single. There is only one species, a native of Mexico.