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''Ginkgo'' is a genus of highly unusual non-flowering plants. The scientific name is also used as the English name. The order to which it belongs, Ginkgoales, first appeared in the Permian,〔Royer et al (2003)〕 250 million years ago, possibly derived from "seed ferns" of the order Peltaspermales. The rate of evolution within the genus has been slow, and almost all its species had become extinct by the end of the Pliocene; the exception is the sole living species, ''Ginkgo biloba'', which is only found in the wild in China, but is cultivated across the world. The relationships between ginkgos and other groups of plants are not fully resolved. == Prehistory == The ginkgo is a living fossil, with fossils recognisably related to modern ginkgo from the Permian, dating back 270 million years. The most plausible ancestral group for the order Ginkgoales is the Pteridospermatophyta, also known as the "seed ferns", specifically the order Peltaspermales. The closest living relatives of the clade are the cycads,〔Royer ''et al''. (2003), p. 84.〕 which share with the extant ''G. biloba'' the characteristic of motile sperm. Fossils attributable to the genus ''Ginkgo'' first appeared in the Early Jurassic, and the genus diversified and spread throughout Laurasia during the middle Jurassic and Early Cretaceous. It declined in diversity as the Cretaceous progressed with the extinction of species such as ''Ginkgo huolinhensis'', and by the Palaeocene, only a few ''Ginkgo'' species, ''Ginkgo cranei'' and ''Ginkgo adiantoides'', remained in the Northern Hemisphere, while a markedly different (and poorly documented) form persisted in the Southern Hemisphere. At the end of the Pliocene, ''Ginkgo'' fossils disappeared from the fossil record everywhere except in a small area of central China, where the modern species survived. It is doubtful whether the Northern Hemisphere fossil species of ''Ginkgo'' can be reliably distinguished. Given the slow pace of evolution and morphological similarity between members of the genus, there may have been only one or two species existing in the Northern Hemisphere through the entirety of the Cenozoic: present-day ''G. biloba'' (including ''G. adiantoides'') and ''G. gardneri'' from the Palaeocene of Scotland.〔Royer ''et al''. (2003), p. 85.〕 At least morphologically, ''G. gardneri'' and the Southern Hemisphere species are the only known post-Jurassic taxa that can be unequivocally recognised. The remainder may have been ecotypes or subspecies. The implications would be that ''G. biloba'' had occurred over an extremely wide range, had remarkable genetic flexibility and, though evolving genetically, never showed much speciation. While it may seem improbable that a species may exist as a contiguous entity for many millions of years, many of the ginkgo's life-history parameters fit. These are: extreme longevity; slow reproduction rate; (in Cenozoic and later times) a wide, apparently contiguous, but steadily contracting distribution coupled with, as far as can be demonstrated from the fossil record, extreme ecological conservatism (restriction to disturbed streamside environments).〔Royer ''et al''. (2003), p. 91.〕 Modern-day ''G. biloba'' grows best in well-watered and drained environments,〔Royer ''et al''. (2003), p. 87.〕 and the extremely similar fossil ''Ginkgo'' favoured similar environments; the sediment records at the majority of fossil ''Ginkgo'' localities indicate it grew primarily in disturbed environments along streams and levees.〔 ''Ginkgo'' therefore presents an "ecological paradox" because, while it possesses some favourable traits for living in disturbed environments (clonal reproduction), many of its other life-history traits (slow growth, large seed size, late reproductive maturity) are the opposite of those exhibited by modern plants that thrive in disturbed settings.〔Royer ''et al''. (2003), p. 92.〕 Given the slow rate of evolution of the genus, it is possible that ''Ginkgo'' represents a preangiosperm strategy for survival in disturbed streamside environments. ''Ginkgo'' evolved in an era before flowering plants, when ferns, cycads, and cycadeoids dominated disturbed streamside environments, forming a low, open, shrubby canopy. The large seeds of ''Ginkgo'' and its habit of "bolting"—growing to a height of before elongating its side branches—may be adaptations to such an environment. Because diversity in the genus ''Ginkgo'' drops through the Cretaceous (along with that of ferns, cycads, and cycadeoids) at the same time the flowering plants were on the rise, supports the notion that flowering plants, with their better adaptations to disturbance, displaced ''Ginkgo'' and its associates over time.〔Royer ''et al''. (2003), p. 93.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ginkgo」の詳細全文を読む スポンサード リンク
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