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The Quercus genus is quite old, being one of the early angiosperms of the Miocene epoch (26-12 million years ago). Over time, oaks have divided into two main lineages, with an intermediate subgenus for less genetically distinct species. The red oaks (Erythrobalanus) are characterized by pointed leaves with bristles or spines that can be either lobed or unlobed. The acorns have a hairy inner shell and mature in two years (except for California live oaks, Q. agrifolia which mature in one year) on the twigs of the first year's growth. The smooth bark is dark gray, black or brown, with reddish brown wood. The white oak (Leucobalanus) leaves are rounded and smooth but can also be lobed or unlobed. Acorns mature in one year and have a smooth inner shell. The wood is light brown or yellow and the bark is scaly or rough brown to light gray. The leaves of the intermediate oaks (Protobalanus) are unlobed, although some may have green spines or teeth. The inner shell of the acorns can be either smooth or hairy, but does not mature until the second year. The bark can be either scaly or rough, with a wide color range. The wood is generally light brown and not as commercially valuable as that in the other oak families.

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Science EncyclopediaScience & Philosophy: Nicotinamide adenine dinucleotide phosphate (NADP) to Ockham's razorOaks - Evolution, Biology And Ecology, Diseases, Distribution, Historic Importance, Acorns, Wood, Ecological Significance - Economic importance