In Greek mythology, Demeter is the goddess of grain and agriculture, the pure nourisher of the youth and the green earth, the health-giving cycle of life and death, and preserver of marriage and the sacred law. Her daughters are Persephone, by Zeus and a goddess known only as Despione (the mistress), who was important in the Arcadian Mysteries, and a horse, Arion, by Poseidon. The Roman equivalent is Ceres, from whom the word "cereal" is derived.