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Ulmus americana 'Jefferson'
The American Elm cultivar ''Ulmus americana'' 'Jefferson' was cloned from a tree that grows along the National Mall in Washington D. C. (). Planted in the 1930s, it remains (2013) unscathed by Dutch elm disease, and was cloned (NPS 3487) by the U. S. National Park Service, which released it as 'Jefferson' in 2004. Early studies on this clone indicated triploid chromosome levels, suggesting it may be a hybrid between the tetraploid American Elm and a diploid species. A genetic study performed by the United States National Arboretum in 2004 confirmed the tree as ''Ulmus americana'', despite its atypical features.〔Pooler, M. R. & Townsend, A. M. (2005). DNA Fingerprinting of Clones and Hybrids of American Elm and Other Elm Species with AFLP Markers. ''Journal of Environmental Horticulture'', 23 (3): 113-117. September 2005.〕() A later study by the USDA's Agricultural Research Service confirmed the tree as a triploid, but derived from a crossing of two American Elms, one a tetraploid, the other a rarer diploid. 〔Kaplan, K. (2011). Hidden elm population may hold genes to combat Dutch elm disease. ARS News, 30 March 2011. USDA. () 〕
==Description==
'Jefferson' is distinguished by its low, spreading form with arching limbs and broad U-shaped crotches; the tree in the National Mall has attained a height of about 21 m (68 ft) after 80 years.〔(Photographs of mature and young 'Jefferson' elms on the National Mall, Washington DC: 'Elms of the Monumental Core', James L. Sherald, National Park Service (2009), p.38 )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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