Oak
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An oak is a hardwood tree or shrub in the genus Quercus of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisphere; it includes some 500 species, both deciduous and evergreen. Fossil oaks date back to the Middle Eocene. Molecular phylogeny shows that the genus is divided into Old World and New World clades, but many oak species hybridise freely, making the genus's history difficult to resolve.
Quercus robur habit
Illustration of Quercus lusitanica showing staminate (left) and pistillate flowers, which mature into acorns (right)
Catkins of Quercus alba containing the staminate or 'male' flowers
Acorn
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The acorn, or oaknut, is the nut of the oaks and their close relatives. It usually contains a seedling surrounded by two cotyledons, enclosed in a tough shell known as the pericarp, and borne in a cup-shaped cupule. Acorns are 1–6 cm long and 0.8–4 cm on the fat side. Acorns take between 5 and 24 months to mature; see the list of Quercus species for details of oak classification, in which acorn morphology and phenology are important factors.
Acorns from small to large of the Willow Oak, Q. phellos (very small, at center); the Southern Red Oak, Q. falcata; the White Oak, Q. alba; the Scarlet Oak, Q. coccinea; from southern Greenville County, SC, USA. Scale bar at upper right is 1 centimetre (0.39 in).
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.
Ponies eating acorns. Acorns can cause painful death in equines, especially if eaten to excess amounts.