Achene
An achene (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.
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Examples
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.
Variations
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.
A 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. 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.
Gallery
 Cypselas on a dandelion "clock" (the matured capitulum) can disperse in the wind due to the hair-like calyx tissue above each ovary. |
 A microscopic view of a dandelion "clock" showing the receptacle and the cypselas. |
 Samaras of Acer buergerianum are achenes with large wing-like structures. |
 The diaspore of Clematis disperses in the wind, either as single achenes or as the entire aggregate of achenes. The achenes have long hairy appendages that developed from the style of the flower. |
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