An achene (Greek ἀ, a, privative + χαίνειν, chainein, to gape;[1] 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 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.[1]
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Typical achenes are the fruits of buttercup, buckwheat, and cannabis.
The achenes of the strawberry are familiar, where the "seeds" are achenes. Technically, the strawberry is an aggregate fruit with an aggregate of achenes, and what is eaten is accessory tissue, so this is an aggregate accessory fruit.
A rose also produces achenes. Each fruit, called a rose hip, holds a few achenes.
A winged achene, such as in maple, is called a samara.
Some achenes have accessory hair-like structures that cause them to tumble in the wind, similar to a tumbleweed; this type sometimes is called a "tumble fruit" or diaspore. An example is Anemone virginiana.
A caryopsis or grain is a type of fruit that closely resembles an achene, but differs in that the pericarp is fused to the thin seed coat in the grain.
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.
Fruits of sedges are sometimes considered achenes although their one-locule ovary is actually a compound ovary.
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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