The acorn, or oak nut, is the nut of the oaks and their close relatives. It usually contains a single seed, enclosed in a tough, leathery shell, and borne in a cup-shaped cupule. Acorns vary from 1–6 cm long and 0.8–4 cm broad. Acorns take between about 6 and 24 months to mature; see List of Quercus species for details of oak classification, in which acorn morphology and phenology are important factors.
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