acorn nut

The acorn, or oak nut, is the nut of the oak tree (genera Quercus, Lithocarpus and Cyclobalanopsis, in the family Fagaceae). It usually contains a single seed (rarely two seeds), 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 or 24 months (depending on the species) to mature; see List of Quercus species for details of oak classification, in which acorn morphology and phenology are important factors.