Tree

===Trunk===
[[File:Buk1.JPG|thumb|upright|Northern beech (''[[Fagus sylvatica]]'') trunk in autumn]]
{{main|Trunk (botany)}}

The main purpose of the trunk is to raise the leaves above the ground, enabling the tree to overtop other plants and outcompete them for light.<ref name="King1990">{{cite journal |last1=King |first1=David A. |title=The Adaptive Significance of Tree Height |journal=The American Naturalist |volume=135 |issue=6 |year=1990 |pages=809–828 |doi=10.1086/285075 |s2cid=85160969 |quote=competition for light is the primary factor responsible for the evolution and maintenance of the arboreal life form. The resulting evolutionarily stable growth pattern maximizes the competitive ability of the individual }}</ref> It also transports water and nutrients from the roots to the aerial parts of the tree, and distributes the food produced by the leaves to all other parts, including the roots.<ref name=Russelltrunk>{{cite book |title=The World Encyclopedia of Trees |last1=Russell |first1=Tony |author2=Cutler, Catherine |year=2003 |publisher=Lorenz Books |isbn=978-0-7548-1292-0 |pages=[https://archive.org/details/isbn_9780754812920/page/16 16–17] |url=https://archive.org/details/isbn_9780754812920/page/16 }}</ref>

In the case of angiosperms and gymnosperms, the outermost layer of the trunk is the [[bark (botany)|bark]], mostly composed of dead cells of [[phellem]] (cork).<ref name="Junikka1994">{{cite journal |last1=Junikka |first1=Leo |title=Survey of English Macroscopic Bark Terminology |journal=IAWA Journal |volume=15 |issue=1 |year=1994 |pages=3–45 |doi=10.1163/22941932-90001338 |quote=phellem{{!}}a secondarily formed protective tissue in stems and roots consisting of dead cells with chiefly suberised<!--made cork-like--> walls: developed outward from the phellogen and forming a part of periderm|doi-access=free }}</ref> It provides a thick, waterproof covering to the living inner tissue. It protects the trunk against the elements, disease, animal attack and fire. It is perforated by a large number of fine breathing pores called [[lenticel]]s, through which oxygen diffuses. Bark is continually replaced by a living layer of cells called the [[cork cambium]] or phellogen.<ref name="Junikka1994"/> The [[Platanus × acerifolia|London plane]] (''Platanus × acerifolia'') periodically sheds its bark in large flakes. Similarly, the bark of the [[Betula pendula|silver birch]] (''Betula pendula'') peels off in strips. As the tree's girth expands, newer layers of bark are larger in circumference, and the older layers develop fissures in many species. In some trees such as the [[pine]] (''Pinus'' species) the bark exudes sticky [[resin]] which deters attackers whereas in [[Hevea brasiliensis|rubber trees]] (''Hevea brasiliensis'') it is a milky [[latex]] that oozes out. The [[Cinchona officinalis|quinine bark tree]] (''Cinchona officinalis'') contains bitter substances to make the bark unpalatable.<ref name=Russelltrunk/> Large tree-like plants with lignified trunks in the [[Pteridophyta]], [[Arecales]], [[Cycadophyta]] and [[Poales]] such as the tree ferns, palms, cycads and bamboos have different structures and outer coverings.<ref>{{cite book |title=Biology |last=Campbell |first=Neil A. |author2=Reece, Jane B. |year=2002 |publisher=Pearson Education |isbn=978-0-201-75054-6 |page=725|edition=6th }}</ref>

[[File:Taxus wood.jpg|thumb|left|A section of [[Taxus baccata|yew]] (''Taxus baccata'') showing 27 annual growth rings, pale [[Sapwood (wood)|sapwood]] and dark [[heartwood]]]]

Although the bark functions as a protective barrier, it is itself attacked by boring insects such as beetles. These lay their eggs in crevices and the larvae chew their way through the cellulose tissues leaving a gallery of tunnels. This may allow fungal spores to gain admittance and attack the tree. [[Dutch elm disease]] is caused by a fungus (''[[Ophiostoma]]'' species) carried from one [[elm]] tree to another by various beetles. The tree reacts to the growth of the fungus by blocking off the xylem tissue carrying sap upwards and the branch above, and eventually the whole tree, is deprived of nourishment and dies. In Britain in the 1990s, 25 million elm trees were killed by this disease.<ref>{{cite web |url=http://www.forestry.gov.uk/fr/HCOU-4U4JCL |title=Dutch elm disease in Britain |author=Webber, Joan |publisher=Forest Research |access-date=16 July 2012 |archive-date=9 March 2018 |archive-url=https://web.archive.org/web/20180309165010/https://www.forestry.gov.uk/fr/hcou-4u4jcl |url-status=dead }}</ref>

The innermost layer of bark is known as the [[phloem]] and this is involved in the transport of the [[Plant sap|sap]] containing the sugars made by photosynthesis to other parts of the tree. It is a soft spongy layer of living cells, some of which are arranged end to end to form tubes. These are supported by [[parenchyma]] cells which provide padding and include fibres for strengthening the tissue.<ref name=Lalonde>{{cite journal |author1=Lalonde, S. |author2=Wipf, D. |author3=Frommer, W. B. |year=2004 |title=Transport mechanisms for organic forms of carbon and nitrogen between source and sink |journal=Annual Review of Plant Biology |volume=55 |pages=341–372 |pmid=15377224 |doi=10.1146/annurev.arplant.55.031903.141758}}</ref> Inside the phloem is a layer of undifferentiated cells one cell thick called the vascular cambium layer. The cells are continually dividing, creating phloem cells on the outside and wood cells known as [[xylem]] on the inside.<ref>{{cite web |url=http://cronodon.com/BioTech/Plant_Bodies_Wood.html |title=Wood, tree trunks and branches |work=BioTech |publisher=Cronodon Museum |access-date=16 July 2012 |archive-date=2 May 2015 |archive-url=https://web.archive.org/web/20150502084356/http://cronodon.com/BioTech/Plant_Bodies_Wood.html |url-status=live }}</ref>

The newly created xylem is the [[sapwood (wood)|sapwood]]. It is composed of water-conducting cells and associated cells which are often living, and is usually pale in colour. It transports water and minerals from the roots to the upper parts of the tree. The oldest, inner part of the sapwood is progressively converted into [[heartwood]] as new sapwood is formed at the cambium. The conductive cells of the heartwood are blocked in some species. Heartwood is usually darker in colour than the sapwood. It is the dense central core of the trunk giving it rigidity. Three quarters of the dry mass of the xylem is [[cellulose]], a [[polysaccharide]], and most of the remainder is lignin, a complex [[polymer]]. A transverse section through a tree trunk or a horizontal core will show concentric circles of lighter or darker wood – tree rings.<ref name=woodanatomy/> These rings are the [[Growth rings|annual growth rings]]<ref>{{cite book |last=Fritts |first=H. C. |title=Tree Rings and Climate |publisher=Blackburn Press |year= 2001 |isbn=978-1-930665-39-2}}</ref><ref>{{cite web |last1=Helama |first1=Samuel |last2=Jalkanen |first2=Risto |title=Annual growth rings of trees |url=https://www.luke.fi/en/natural-resources/forest/forests-and-climate-change/annual-growth-rings-of-trees/ |publisher=Natural Resources Institute Finland (LUKE) |access-date=17 July 2019 |archive-date=6 August 2019 |archive-url=https://web.archive.org/web/20190806043449/https://www.luke.fi/en/natural-resources/forest/forests-and-climate-change/annual-growth-rings-of-trees/ |url-status=live }}</ref> There may also be rays running at right angles to growth rings. These are [[Medullary ray (botany)|vascular rays]] which are thin sheets of living tissue permeating the wood.<ref name=woodanatomy>{{cite web |url=http://www.botany.uwc.ac.za/ecotree/trunk/woodanatomy2.htm |title=The anatomy of a tree trunk – the wood 2 |url-status=bot: unknown |archive-url=https://web.archive.org/web/20120314190054/http://www.botany.uwc.ac.za/ecotree/trunk/woodanatomy2.htm |archive-date=14 March 2012}}</ref> Many older trees may become hollow but may still stand upright for many years.<ref>{{cite web |url=http://cronodon.com/BioTech/Plant_Bodies_Wood.html |title=Xylem and wood |work=BioTech |publisher=Cronodon Museum |access-date=16 July 2012 |archive-date=2 May 2015 |archive-url=https://web.archive.org/web/20150502084356/http://cronodon.com/BioTech/Plant_Bodies_Wood.html |url-status=live }}</ref>