Acorn

Acorns from small to large of the Willow Oak, Quercus phellos (very small, at center); the Southern Red Oak, Quercus falcata; the White Oak, Quercus alba; the Scarlet Oak, Quercus coccinea; from southern Greenville County, SC, USA. Scale bar at upper right is 1 cm.

Diagram of the anatomy of an acorn: A.) Cupule B.) Pericarp (fruit wall) C.) Seed coat (testa) D.) Cotyledons(2) E.) Plumule F.) Radicle G.) Remains of style. Together D., E., and F. make up the embryo.

The acorn, or oaknut, is the nut of the oaks and their close relatives (genera Quercus and Lithocarpus, in the family Fagaceae). It usually contains a single seed(occasionally two seeds), enclosed in a tough, leathery shell, and borne in a cup-shaped cupule. Acorns are 1–6 cm (0.39–2.36 in) long and 0.8–4 cm (0.31–1.57 in) broad. Acorns take between 6 and 24 months (depending on the species) to mature; see  the list of Quercus species for details of oak classification, in which acorn morphology and phenology are important factors.

https://en.m.wikipedia.org/wiki/Acorn