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Elms are deciduous and semi-deciduous trees comprising the genus ''Ulmus'' in the plant family Ulmaceae. The genus first appeared in the Miocene geological period about 20 million years ago, originating in what is now central Asia.〔Richens, R. H. (1983). ''Elm''. Cambridge University Press.〕 These trees flourished and spread over most of the Northern Hemisphere, inhabiting the temperate and tropical-montane regions of North America and Eurasia, presently ranging southward across the Equator into Indonesia. Elms are components of many kinds of natural forests. Moreover, during the 19th and early 20th centuries many species and cultivars were also planted as ornamental street, garden, and park trees in Europe, North America, and parts of the Southern Hemisphere, notably Australasia. Some individual elms reached great size and age. However, in recent decades, most mature elms of European or North American origin have died from Dutch elm disease, caused by a microfungus dispersed by bark beetles. In response, disease-resistant cultivars have been developed, capable of restoring the elm to forestry and landscaping. ==Taxonomy== There are about 30 to 40 species of ''Ulmus'' (elm); the ambiguity in number results from difficulty in delineating species, owing to the ease of hybridization between them and the development of local seed-sterile vegetatively propagated microspecies in some areas, mainly in the field elm (''Ulmus minor'') group. Rackham〔Rackham, O. (1980). ''Ancient woodland: its history, vegetation and uses''. Edward Arnold, London〕 describes ''Ulmus'' as the most difficult critical genus in the entire British flora, adding that 'species and varieties are a distinction in the human mind rather than a measured degree of genetic variation'. Eight species are endemic to North America, and a smaller number to Europe;〔Bean, W. J. (1981). ''Trees and shrubs hardy in Great Britain'', 7th edition. Murray, London〕 the greatest diversity is found in Asia.〔Fu, L., Xin, Y. & Whittemore, A. (2002). Ulmaceae, in Wu, Z. & Raven, P. (eds) ''(Flora of China )'', Vol. 5 (Ulmaceae through Basellaceae). Science Press, Beijing, and Missouri Botanical Garden Press, St. Louis, USA.〕 The classification adopted in the List of elm species, varieties, cultivars and hybrids is largely based on that established by Brummitt.〔Brummitt, R. K. (1992). ''Vascular Plant Families & Genera''. Royal Botanic Garden, Kew, London, UK.〕 A large number of synonyms have accumulated over the last three centuries; their currently accepted names can be found in the list List of elm Synonyms and Accepted Names. Botanists who study elms and argue over elm identification and classification are called ''pteleologists'', from the Greek πτελέα (:elm).〔Marren, Peter, ''Woodland Heritage'' (Newton Abbot, 1990).〕 As part of the sub-order urticalean rosids they are distant cousins of cannabis, hops, and nettles. 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「elm」の詳細全文を読む スポンサード リンク
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