An achene (Greek ἀ, a, privative + χαίνειν, chainein, to gape; also sometimes referred to as akene and occasionally achenium or achenocarp) is a type of simple dry fruit produced by many species of flowering plants. Achenes are monocarpellate (formed from one carpel) and (they do not open at maturity). Achenes contain a single seed that nearly fills the pericarp, but does not adhere to it. In many species, what we think of as the "seed" is actually an achene, a fruit containing the seed. The seed-like appearance arises from the fact that the wall of the seed-vessel hardens and encloses the solitary seed so closely as to seem like an outer coat.