Editing Ulmus americana (section)

==Description==
The American elm is a [[deciduous]] tree which, under ideal conditions, can grow to heights of {{convert|21 to 35|m|ft|abbr=off|sp=us}}.<ref name="FNA">{{cite web |last1=Sherman-Broyles |first1=Susan L. |title=''Ulmus americana'' - FNA |date=5 November 2020 |url=http://floranorthamerica.org/Ulmus_americana |website=Flora of North America |access-date=10 March 2024 }}</ref> The trunk may have a [[diameter at breast height]] (dbh) of more than {{convert|4|ft|m|abbr=on|order=flip}}, supporting a high, spreading umbrella-like canopy. The [[leaves]] are alternate, {{Convert|7–20|cm|abbr=off|sp=us|frac=2}} long, with double-serrate margins and an oblique base. The leaves turn yellow in the fall. The [[perfect flower]]s are small, purple-brown and, being wind-pollinated, [[apetalous]]. The flowers are also [[protogynous]], the female parts maturing before the male, thus reducing, but not eliminating, self-fertilization,<ref name="Hans">{{cite journal|last=Hans|first=A. S.|year=1981|title=Compatibility and Crossability Studies in Ulmus|journal=Silvae Genetica|volume= 30|pages=4–5}}</ref> and emerge in early spring before the leaves. The [[fruit]] is a flat [[Samara (fruit)|samara]] {{Convert|2|cm|abbr=on|frac=4}} long by 1.5&nbsp;cm broad, with a circular papery wing surrounding the single {{Convert|4.5|mm|abbr=off|sp=us|frac=8}} [[seed]]. As in the closely related ''[[Ulmus laevis]]'' (European white elm), the flowers and seeds are borne on 1&ndash;3&nbsp;cm long stems. American elm is wholly insensitive to daylight length ([[photoperiod]]), and will continue to grow well into autumn until injured by frost.<ref name="Downs">{{cite journal|last1=Downs|first1=R. J.|last2=Borthwick|first2=H. A. |year=1956|title=Effects of Photoperiod on Growth of Trees|journal=Botanical Gazette|volume=117|issue=4|pages=310–326|doi=10.1086/335918|s2cid=83948676}}</ref> [[Ploidy]] is ''2n'' = 56, or more rarely, ''2n'' = 28.<ref>{{cite journal|last1=Whittemore|first1=A. T.|last2=Olsen|first2=R. T.|year=2011|title=''Ulmus americana'' (Ulmaceae) is a Polyploid Complex|journal=American Journal of Botany|volume=98|issue=4|pages=754–760|doi=10.3732/ajb.1000372|pmid=21613171|url=https://naldc-legacy.nal.usda.gov/naldc/download.xhtml?id=50198&content=PDF|access-date=2018-04-29|archive-date=October 28, 2020|archive-url=https://web.archive.org/web/20201028052850/https://naldc-legacy.nal.usda.gov/naldc/download.xhtml?id=50198&content=PDF|url-status=dead}}</ref>

For over 80 years, ''U. americana'' had been identified as a [[tetraploid]], i.e. having double the usual number of chromosomes, making it unique within the genus. However, a study published in 2011 by the [[Agricultural Research Service]] of the  [[United States Department of Agriculture]] revealed that about 20% of wild American elms are [[diploid]] and may even constitute another species. Moreover, several triploid trees known only in cultivation, such as [[Ulmus americana 'Jefferson'|'Jefferson']], are possessed of a high degree of resistance to DED, which ravaged American elms in the 20th century. This suggests that the diploid parent trees, which have markedly smaller cells than the tetraploid, may too be highly resistant to the disease.<ref name="Whittemore">Whittemore, A. & Olsen, R. (2011). Ulmus americana (Ulmaceae) is a polyploidy complex. ''American Journal of Botany'' 98(4): 754–760. 2011. Botanical Society of America.</ref><ref name="Kaplan">{{cite web |last=Kaplan |first=K. |date=30 March 2011 |title=Hidden elm population may hold genes to combat Dutch elm disease |url=http://www.ars.usda.gov/is/pr/2011/110330.htm |url-status=live |archive-url=https://web.archive.org/web/20150402144640/http://www.ars.usda.gov/is/pr/2011/110330.htm |archive-date=April 2, 2015 |work=ARS News |publisher=[[United States Department of Agriculture]]: Agricultural Research Service}}</ref> 
<gallery>
File:Lovers' Elm, Gwynne estate, Dufferin Street.jpg|''U. americana'', Toronto, c. 1914
File:ElmBark.jpg|Bark
File:Jardin jeanne arc.jpg|Boles
File:American Elm - Ulmus americana, Occoquan Regional Park, Lorton, Virginia.jpg|Flowers
File:Ulmus americana (5102579562).jpg|Leaf
File:Ulmus americana (5101983801).jpg|Foliage
File:Ulmus americana seedlings.jpg|Seedlings
</gallery>