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The meaning of «oak»

An oak is a tree or shrub in the genus Quercus (/ˈkwɜːrkəs/;[1] Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks.[2] The common name "oak" also appears in the names of species in related genera, notably Lithocarpus (stone oaks), as well as in those of unrelated species such as Grevillea robusta (silky oaks) and the Casuarinaceae (she-oaks). The genus Quercus is native to the Northern Hemisphere, and includes deciduous and evergreen species extending from cool temperate to tropical latitudes in the Americas, Asia, Europe, and North Africa. North America contains the largest number of oak species, with approximately 90 occurring in the United States, while Mexico has 160 species of which 109 are endemic. The second greatest center of oak diversity is China, which contains approximately 100 species.[3]

Oaks have spirally arranged leaves, with lobate margins in many species; some have serrated leaves or entire leaves with smooth margins. Many deciduous species are marcescent, not dropping dead leaves until spring. In spring, a single oak tree produces both male flowers (in the form of catkins) and small female flowers,[4] meaning that the trees are monoecious. The fruit is a nut called an acorn or oak nut borne in a cup-like structure known as a cupule; each acorn contains one seed (rarely two or three) and takes 6–18 months to mature, depending on their species. The acorns and leaves contain tannic acid,[5] which helps to guard from fungi and insects.[6] The live oaks are distinguished for being evergreen, but are not actually a distinct group and instead are dispersed across the genus.

The most recent classification of Quercus divides the genus into two subgenera and eight sections.[7] These divisions support the evolutionary diversification of oaks among two distinct clades: the "Old World" clade, including oaks that diversified mainly in Eurasia; and the "New World" clade, for oaks that diversified mainly in the Americas.[8]

The advent of molecular techniques for phylogenetic analysis transformed understanding of oak relationships, initially by uncovering molecular support for the diphyletic division of Quercus into Old World and New World clades.[8] These techniques have proved highly useful in resolving fine-scale relationships among 2–5 oak species, particularly groups known to hybridize, but until recently the larger emphasis on this narrow approach prevented systematists from making large-scale determinations about oak history.[9] As the capacity for sampling across wider swaths of oak species rose, so has resolution at the section and species level across the oak tree.[10]

Further advances in oak systematics are expected to arise from next-generation sequencing techniques, including a recent project to sequence the entire genome of Quercus robur (the pedunculate oak).[11] The recent completion of that genome has uncovered an array of mutations that may underlie the evolution of longevity and disease resistance in oaks.[12] In addition, the generation of RAD-seq loci for hundreds of oak species has allowed for the construction of the most highly detailed oak phylogeny to date, although the high signal of introgression across the tree poses difficulties for deriving an unambiguous, unitary history of oaks.[13]

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