in botany: A genus of the polygamia dioecia class; and in the natural method ranking under the 45th order, Umbellatae. The umbella of the male is compound; the involucrum consists of five leaves; the corolla has five petals; the stamens are five; and two pistils; the umbella of the hermaphrodite is simple; the involucrum is divided into four parts, is spinous, large, and contains many male flowers in the dish. There is but one species of arctopus, viz. the echinatus, a native of Ethiopia.