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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 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 referred to 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.
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.
An utricle is like an achene, but it has a compound ovary, sometimes with several seeds. In addition, the ovary of the fruit becomes bladder-like or corky.
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. Many cypselas (e.g. dandelion) have calyx tissue attached that functions in biological dispersal of the seed.
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