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An achene (Greek ἀ, a, privative + χαίνειν, chainein, to gape; also sometimes called 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 indehiscent (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 is called the "seed" is an achene, a fruit containing the seed. The seed-like appearance is owed to the hardening of the wall of the seed-vessel, which encloses the solitary seed so closely as to seem like an outer coat.
Some achenes have accessory hair-like structures that cause them to tumble in the wind in a manner similar to a tumbleweed. This type sometimes is called a tumble fruit or diaspore. An example is Anemone virginiana.
The fruit of the family Asteraceae is also so similar to an achene that it is often considered to be one, although it derives from a compound inferior ovary (with one locule). A special term for the Asteraceae fruit is cypsela (plural cypselae or cypselas). For example, the white-gray husks of a sunflower "seed" are the walls of the cypsela fruit.[clarification needed] Many cypselas (e.g. dandelion) have calyx tissue attached that functions in biological dispersal of the seed.