Tree

==Overview==
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The tree growth habit is an [[evolution]]ary [[adaptation]] found in different groups of plants: by growing taller, trees are able to compete better for sunlight.<ref name="LowmanRinker2004">{{cite book |last1=Lowman |first1=V. |last2=Rinker |first2=H. Bruce |title=Forest Canopies |url=https://books.google.com/books?id=T9qY2AxnUowC&pg=PA119 |year=2004 |publisher=Academic Press |isbn=978-0-08-049134-9 |page=119 |access-date=28 February 2016 |archive-date=4 March 2017 |archive-url=https://web.archive.org/web/20170304124617/https://books.google.com/books?id=T9qY2AxnUowC&pg=PA119 |url-status=live }}</ref> Trees tend to be tall and long-lived,<ref name="Rémy">{{cite journal |last1=Petit |first1=Rémy J. |last2=Hampe |first2=Arndt |year=2006 |title=Some Evolutionary Consequences of Being a Tree |url=http://www.ecologicalevolution.org/content/Petit_How2BeATree.pdf |url-status=dead |journal=Annual Review of Ecology, Evolution, and Systematics |volume=37 |pages=187–214 |doi=10.1146/annurev.ecolsys.37.091305.110215 |archive-url=https://web.archive.org/web/20140116084547/http://www.ecologicalevolution.org/content/Petit_How2BeATree.pdf |archive-date=16 January 2014 |hdl=10261/64097}}</ref> some reaching several thousand years old.<ref name=Koch>{{cite journal |url=http://www.planta.cn/forum/files_planta/the_limits_to_tree_height_995.pdf |archive-url=https://web.archive.org/web/20121021080125/http://www.planta.cn/forum/files_planta/the_limits_to_tree_height_995.pdf |archive-date=21 October 2012 |title=The limits to tree height |last1=Koch |first1=George W. |author2=Sillett, Stephen C. |author3=Jennings, Gregory M. |author4=Davis, Stephen D. |year=2004 |journal=Letters to Nature |volume=428 |issue=6985 |pages=851–4 |publisher=Nature Publishing Group |doi=10.1038/nature02417 |pmid=15103376 |bibcode=2004Natur.428..851K |s2cid=11846291 |access-date=18 December 2021 |url-status=dead }}</ref> Several trees are among the oldest organisms now living.<ref>{{cite magazine|title=These are 11 of the Oldest Things in the World|url=http://time.com/10688/worlds-oldest-things/|magazine=Time|access-date=11 October 2017|archive-date=17 October 2017|archive-url=https://web.archive.org/web/20171017052036/http://time.com/10688/worlds-oldest-things/|url-status=live}}</ref> Trees have modified structures such as thicker stems composed of specialised cells that add structural strength and durability, allowing them to grow taller than many other plants and to spread out their foliage. They differ from [[shrub]]s, which have a similar growth form, by usually growing larger and having a single main stem;<ref name="treedictionary"/> but there is no consistent distinction between a tree and a shrub,<ref name="HawthorneLawrence2012">{{cite book |last1=Hawthorne |first1=William |author2=Lawrence, Anna |title=Plant Identification: Creating User-Friendly Field Guides for Biodiversity Management |url=https://books.google.com/books?id=rHEpVBM5-eIC&pg=PA138 |year=2012 |publisher=Routledge |isbn=978-1-84407-079-4 |page=138 |access-date=28 February 2016 |archive-date=7 February 2017 |archive-url=https://web.archive.org/web/20170207094210/https://books.google.com/books?id=rHEpVBM5-eIC&pg=PA138 |url-status=live }}</ref> made more confusing by the fact that trees may be reduced in size under harsher environmental conditions such as on mountains and [[subarctic]] areas. The tree form has evolved separately in unrelated classes of plants in response to similar environmental challenges, making it a classic example of [[parallel evolution]]. With an estimated 60,000-100,000 species, the number of trees worldwide might total twenty-five per cent of all living plant species.<ref>{{cite news |url=http://usatoday30.usatoday.com/tech/science/2008-05-02-890171068_x.htm |title=Scientists to capture DNA of trees worldwide for database |last=Hajela |first=Deepti |access-date=18 December 2021 |work=USA Today |date=2 May 2008 |archive-date=19 March 2022 |archive-url=https://web.archive.org/web/20220319090451/http://usatoday30.usatoday.com/tech/science/2008-05-02-890171068_x.htm |url-status=live }}</ref><ref>{{cite news |url=https://www.bbc.com/news/science-environment-39492977 |title=World is home to '60,000 tree species' |last=Kinver |first=Mark |access-date=18 December 2021 |work=BBC Science and Environment News |date=5 April 2017 |archive-date=18 December 2021 |archive-url=https://web.archive.org/web/20211218151721/https://www.bbc.com/news/science-environment-39492977 |url-status=live }}</ref> The greatest number of these grow in tropical regions; many of these areas have not yet been fully surveyed by [[botanist]]s, making tree diversity and ranges poorly known.<ref>{{cite book |title=Plant diversity and complexity patterns: local, regional, and global dimensions: proceedings of an international symposium held at the Royal Danish Academy of Sciences and Letters in Copenhagen, Denmark, 25–28 May 2003 |last1=Friis |first1=Ib |last2=Balslev |first2=Henrik |last3=Kongelige |first3=Danske |editor-first4=Videnskabernes |editor-last4=Selskab (eds.) |year=2005 |publisher=Kgl. Danske Videnskabernes Selskab |isbn=978-87-7304-304-2 |pages=57–59}}</ref>

[[File:Nedravazhakola.jpg|thumb|left|Tall [[herbaceous]] [[monocotyledon]]ous plants such as banana lack secondary growth, but are trees under the broadest definition.]]

The majority of tree species are [[angiosperm]]s or hardwoods. Of the rest, many are [[gymnosperm]]s or softwood trees;<ref>{{cite web |title=The Gymnosperms (Conifers, cycads and allies) |url=http://www.theplantlist.org/browse/G/ |website=The Plant List |access-date=14 November 2017 |date=2010 |archive-date=24 August 2013 |archive-url=https://web.archive.org/web/20130824024109/http://www.theplantlist.org/browse/G/ |url-status=live }}</ref> these include [[conifer]]s, [[cycad]]s, [[Ginkgoales|ginkgophytes]] and [[Gnetophyta|gnetales]], which produce seeds which are not enclosed in fruits, but in open structures such as pine [[conifer cone|cones]], and many have tough waxy leaves, such as pine needles.<ref>{{cite book | title=The Gymnosperms |last1=Biswas |first1=Chhaya |author2=Johri, B.M. | publisher=Springer | date=1997 | isbn=978-3-662-13166-4}}</ref> Most angiosperm trees are [[eudicots]], the "true dicotyledons", so named because the seeds contain two [[cotyledon]]s or seed leaves. There are also some trees among the old lineages of flowering plants called [[Basal angiosperms|basal angiosperms or paleodicots]]; these include ''[[Amborella]]'', ''[[Magnolia]]'', [[Myristica|nutmeg]] and [[avocado]],<ref name=Hodson2012>{{cite book | url=https://books.google.com/books?id=_xl8MCXURoYC&pg=PA10 | title=Functional Biology of Plants | last1=Hodson | first1=Martin J. | author2=Bryant, John A. | publisher=John Wiley | date=2012 | pages=9–11 | isbn=978-1-119-96887-0 | access-date=28 February 2016 | archive-date=2 September 2021 | archive-url=https://web.archive.org/web/20210902214405/https://books.google.com/books?id=_xl8MCXURoYC&pg=PA10 | url-status=live }}</ref> while trees such as bamboo, palms and bananas are [[monocots]].

Wood gives structural strength to the trunk of most types of tree; this supports the plant as it grows larger. The vascular system of trees allows water, nutrients and other chemicals to be distributed around the plant, and without it trees would not be able to grow as large as they do. Trees, as relatively tall plants, need to draw water up the stem through the xylem from the roots by the suction produced as water evaporates from the leaves. If insufficient water is available the leaves will die.<ref name=Transport>{{cite web |url=http://cronodon.com/BioTech/Plant_Transport.html |title=Transport in plants |date=28 January 2007 |work=BioTech |publisher=Cronodon Museum |access-date=21 July 2012 |archive-date=14 October 2012 |archive-url=https://web.archive.org/web/20121014094306/http://cronodon.com/BioTech/Plant_Transport.html |url-status=live }}</ref> The three main parts of trees include the root, stem, and leaves; they are integral parts of the vascular system which interconnects all the living cells. In trees and other plants that develop wood, the [[vascular cambium]] allows the expansion of vascular tissue that produces woody growth. Because this growth ruptures the epidermis of the stem, woody plants also have a [[cork cambium]] that develops among the phloem. The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss. Both the production of wood and the production of cork are forms of secondary growth.<ref name=Coder>{{cite web |url=http://warnell.forestry.uga.edu/SERVICE/LIBRARY/index.php3?docID=171 |archive-url=https://web.archive.org/web/20140908112429/http://warnell.forestry.uga.edu/SERVICE/LIBRARY/index.php3?docID=171 |archive-date=8 September 2014 |title=Secondary Growth Anatomy and Tree Rings |last=Coder |first=Kim D. |publisher=Warnell School of Forest Resources, University of Georgia |date=1 August 1999 |access-date=8 September 2014 |url-status=dead }}</ref>

Trees are either [[evergreen]], having foliage that persists and remains green throughout the year,<ref>{{cite web |url=http://www.thefreedictionary.com/evergreen |title=Evergreen |work=TheFreeDictionary |access-date=7 August 2012 |archive-date=20 July 2012 |archive-url=https://web.archive.org/web/20120720092159/http://www.thefreedictionary.com/evergreen |url-status=live }}</ref> or [[deciduous]], shedding their leaves at the end of the growing season and then having a dormant period without foliage.<ref>{{cite web |url=http://www.thefreedictionary.com/deciduous |title=Deciduous |work=TheFreeDictionary |access-date=7 August 2012 |archive-date=21 July 2012 |archive-url=https://web.archive.org/web/20120721153143/http://www.thefreedictionary.com/deciduous |url-status=live }}</ref> Most conifers are evergreens, but larches (''[[Larix]]'' and ''[[Pseudolarix]]'') are deciduous, dropping their needles each autumn, and some species of cypress (''[[Glyptostrobus]]'', ''[[Metasequoia]]'' and ''[[Taxodium]]'') shed small leafy shoots annually in a process known as [[cladoptosis]].<ref name=treedictionary/> The [[Crown (botany)|crown]] is the spreading top of a tree including the branches and leaves,<ref>{{cite web |url=http://www.thefreedictionary.com/crown |title=Crown |work=TheFreeDictionary |access-date=7 August 2012 |archive-date=14 July 2012 |archive-url=https://web.archive.org/web/20120714195628/http://www.thefreedictionary.com/crown |url-status=live }}</ref> while the uppermost layer in a forest, formed by the crowns of the trees, is known as the [[Canopy (biology)|canopy]].<ref>{{cite web |url=http://www.thefreedictionary.com/canopy |title=Canopy |work=TheFreeDictionary |access-date=7 August 2012 |archive-date=12 July 2012 |archive-url=https://web.archive.org/web/20120712030046/http://www.thefreedictionary.com/canopy |url-status=live }}</ref> A sapling is a young tree.<ref>{{cite web |url=http://www.thefreedictionary.com/sapling |title=Sapling |work=TheFreeDictionary |access-date=7 August 2012 |archive-date=7 April 2014 |archive-url=https://web.archive.org/web/20140407194013/http://www.thefreedictionary.com/sapling |url-status=live }}</ref>

Many tall palms are herbaceous<ref name="eeb.ucla.edu">{{cite web |url=https://www.smmflowers.org/bloom/ANF-other/Glossary_UCLA_SantaMonicas_Web.htm |title=Detailed Scientific Descriptions, from A Naturalist's Flora of the Santa Monica Mountains and Simi Hills, California |publisher=National Park Service |access-date=6 May 2022 |quote=herbaceous monocotyledons lack cambial secondary growth but may have tough leaves and hard, fibrous stems (e.g., palms and Hesperoyucca whipplei) |archive-date=6 November 2022 |archive-url=https://web.archive.org/web/20221106131618/https://www.smmflowers.org/bloom/ANF-other/Glossary_UCLA_SantaMonicas_Web.htm |url-status=live }}</ref> monocots, which do not undergo secondary growth and never produce wood.<ref name="botany.wisc.edu"/><ref name="RoddStackhouse2008"/> In many tall palms, the terminal bud on the main stem is the only one to develop, so they have unbranched trunks with large spirally arranged leaves. Some of the tree ferns, [[Order (biology)|order]] [[Cyatheales]], have tall straight trunks, growing up to {{convert|20|m}}, but these are composed not of wood but of [[rhizome]]s which grow vertically and are covered by numerous [[Adventitiousness|adventitious roots]].<ref>{{cite encyclopedia |url=http://www.britannica.com/EBchecked/topic/603988/tree-fern |title=Tree fern |last=Yatskievych |first=George |encyclopedia=Encyclopædia Britannica |access-date=4 August 2012 |archive-date=10 June 2012 |archive-url=https://web.archive.org/web/20120610051907/http://www.britannica.com/EBchecked/topic/603988/tree-fern |url-status=live }}</ref>