Achene

An achene is a kind of simple dry fruit produced by many species of flowering plants. Achenes are "monocarpellate" (shaped from one carpel) and indehiscent (they do not open at maturity). Achenes have a single seed that almost fills the pericarp, however does not adhere to it. In many species, what we think of as the "seed" is really an achene, a fruit containing the seed.