Marcescence

{{short description|Retention of dead plant organs that normally are shed}}

Image:Fagus grandifolia in winter.jpg) in winter]]

Image:oakmer.JPG) with marcescent foliage]]

File:Dead Leaf retention on Beech trees.JPG) tree]]

File:2022-04-25 14 03 38 Marcescent leaves and new leaves during spring on a Red Oak along White Barn Lane in the Franklin Farm section of Oak Hill, Fairfax County, Virginia.jpg) leafing out before dropping marcescent leaves]]

Marcescence is the withering and persistence of plant organs that normally are shed, and is a term most commonly applied to plant leaves.{{Cite book |last1=Hitchcock |first1=C. Leo |title=Flora of the Pacific Northwest |last2=Cronquist |first2=Arthur |date=2018 |publisher=University of Washington Press |isbn=978-0-29574-288-5 |edition=2nd |location=Seattle |page=xxxviii, xliii}}{{cite EB1911 |wstitle=Marcescent|volume=17|page=685}} The underlying physiological mechanism is that trees transfer water and sap from the roots to the leaves through their vascular cells, but in some trees as autumn begins, the veins carrying the sap slowly close until a layer of cells called the abscission layer completely closes off the vein allowing the tree to rid itself of the leaf.{{Cite web |title=Abscission and Marcescence in the Woods {{!}} Vermont Center for Ecostudies |url=https://vtecostudies.org/blog/abscission-and-marcescence-in-the-woods/ |access-date=2017-11-16 |website=vtecostudies.org|date=18 October 2013 }} Leaf marcescence is most often seen on juvenile plants and may disappear as the tree matures. It also may not affect the entire tree; sometimes leaves persist only on scattered branches.{{Cite news |date=2016-02-19 |title=When Leaves Don't Leave » Arnold Arboretum |url=https://www.arboretum.harvard.edu/leaves-dont-leave/ |access-date=2017-11-16 |work=Arnold Arboretum |language=en-US}} Marcescence is most obvious in deciduous trees that retain leaves through the winter. Several trees normally have marcescent leaves such as oak (Quercus),{{Cite journal |last=Berkley |first=Earl E. |date=1931 |title=Marcescent Leaves of Certain Species of Quercus |journal=Botanical Gazette |volume=92 |issue=1 |pages=85–93 |doi=10.1086/334178 |jstor=2471297 |s2cid=84953073}} beech (Fagus) and hornbeam (Carpinus), or marcescent stipules as in some but not all species of willows (Salix).{{Cite book |last=George W. Argus |title=Flora of North America |volume=7 |chapter=88. Salix planifolia Pursh |chapter-url=http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242445830}} All oak trees may display foliage marcescence, even species that are known to fully drop leaves when the tree is mature.{{Cite web |title=Species of Oaks That Retain Their Dead Leaves |url=http://www.gardenguides.com/info_8376489_species-oaks-retain-dead-leaves.html |access-date=2017-11-16 |website=Garden Guides |date=21 September 2017 |language=en}} Marcescent leaves of pin oak (Quercus palustris) complete development of their abscission layer in the spring.{{Cite journal |last1=Hoshaw |first1=Robert W. |last2=Guard |first2=Arthur T. |date=1949 |title=Abscission of Marcescent Leaves of Quercus palustris and Q. coccinea |journal=Botanical Gazette |volume=110 |issue=4 |pages=587–593 |doi=10.1086/335558 |jstor=2472663 |s2cid=83740327}} The base of the petiole remains alive over the winter. Many other trees may have marcescent leaves in seasons where an early freeze kills the leaves before the abscission layer develops or completes development. Diseases or pests can also kill leaves before they can develop an abscission layer.

Marcescent leaves may be retained indefinitely and do not break off until mechanical forces (wind for instance) cause the dry and brittle petioles to snap.{{Cite book |last=Addicott |first=Fredrick T. |url=https://books.google.com/books?id=4i87cALU8K0C&pg=PA51 |title=Abscission |publisher=University of California Press |year=1982 |isbn=978-0-520-04288-9 |page=51}} The evolutionary reasons for marcescence are not clear, theories include: protection of leaf buds from winter desiccation, and as a delayed source of nutrients or moisture-conserving mulch when the leaves finally fall and decompose in spring.{{Cite news |date=2016-02-19 |title=When Leaves Don't Leave » Arnold Arboretum |url=https://www.arboretum.harvard.edu/leaves-dont-leave/ |access-date=2017-11-16 |work=Arnold Arboretum |language=en-US}}

Many palms form a skirt-like or shuttlecock-like crown of marcescent leaves under new growth that may persist for years before being shed.{{Cite book |url=https://books.google.com/books?id=lFK-SY_mtKoC&pg=PA161 |title=Tropical Forests: Botanical Dynamics, Speciation & Diversity |publisher=Academic Press |year=1989 |isbn=978-0-08-098445-2 |editor-last=L.B. Holm-Nielsen |page=161}}{{Cite book |last=Dowe |first=John |url=https://books.google.com/books?id=Xw0FBcLkcbIC&pg=PA160 |title=Australian Palms: Biogeography, Ecology and Systematics |publisher=Csiro Publishing |year=2010 |isbn=978-0-643-10185-2 |page=160}} In some species only juveniles retain dead leaves{{Cite book |last1=Dransfield |first1=John |url=https://books.google.com/books?id=WEZFAQAAIAAJ |title=Genera Palmarum: the evolution and classification of palms |last2=Uhl |first2=Natalie W. |publisher=Kew Pub. |year=2008 |isbn=978-1-84246-182-2 |page=294 |quote=marcescent in immature [descriptive of several spp.]}} and marcescence in palms is considered a primitive trait.{{Cite book |last1=Moore |first1=Harold Emery |url=https://books.google.com/books?id=khyongEACAAJ |title=Major Trends of Evolution in Palms |last2=Uhl |first2=Natalie W. |publisher=New York Botanical Garden |year=1982 |page=69}}

The term marcescent is also used in mycology to describe a mushroom which (unlike most species, described as "putrescent") can dry out, but later revive and continue to disperse spores.See introduction to Roy E. Halling "[http://www.nybg.org/bsci/res/col/index.html A revision of Collybia s.l. in the northeastern United States & adjacent Canada]" Inst. of Syst. Botany, The New York Botanical Garden, Bronx, NY 10458-5126 Genus Marasmius is well known for this feature, which was considered taxonomically important by Elias Magnus Fries in his 1838 classification of the fungi.E. M. Fries Epicrisis systematis mycologici (1838) Uppsala: Typographia Academica