Achene

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.

The fruits of buttercup, buckwheat, and cannabis are typical achenes.

The achenes of the strawberry are sometimes mistaken for seeds. The strawberry is an aggregate fruit with an aggregate of achenes, and what is eaten is accessory tissue.

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 in a manner 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.

...
Wikipedia