The acorn is the nut of the oak tree and its close relatives, in the family Fagaceae. Acorns usually contain a seedling surrounded by two cotyledons (seedling leaves), enclosed in a tough shell known as the pericarp, and borne in a cup-shaped cupule. This acorn, with a length of 25 millimetres (1 in), was photographed in Keila, Estonia.Photograph credit: Ivar Leidus