Tree

===Roots===
[[File:WisconsinScenery.jpg|thumb|A young [[red pine]] (''Pinus resinosa'') with spread of roots visible, as a result of soil erosion]]
{{main|Root}}

The roots of a tree serve to anchor it to the ground and gather water and nutrients to transfer to all parts of the tree. They are also used for reproduction, defence, survival, energy storage and many other purposes. The [[radicle]] or embryonic root is the first part of a [[seedling]] to emerge from the seed during the process of [[germination]]. This develops into a [[taproot]] which goes straight downwards. Within a few weeks [[lateral root]]s branch out of the side of this and grow horizontally through the upper layers of the soil. In most trees, the taproot eventually withers away and the wide-spreading laterals remain. Near the tip of the finer roots are single cell [[root hair]]s. These are in immediate contact with the soil particles and can absorb water and nutrients such as [[potassium]] in solution. The roots require oxygen to [[Respiration (physiology)|respire]] and only a few species such as [[mangrove]]s and the [[Taxodium ascendens|pond cypress]] (''Taxodium ascendens'') can live in permanently waterlogged soil.<ref>{{cite book |title=The World Encyclopedia of Trees |last=Russell |first=Tony |author2=Cutler, Catherine |year=2003 |publisher=Lorenz Books |isbn=978-0-7548-1292-0 |pages=[https://archive.org/details/isbn_9780754812920/page/14 14–15] |url=https://archive.org/details/isbn_9780754812920/page/14}}</ref>

In the soil, the roots encounter the [[hyphae]] of fungi. Many of these are known as [[mycorrhiza]] and form a [[Mutualism (biology)|mutualistic]] relationship with the tree roots. Some are specific to a single tree species, which will not flourish in the absence of its mycorrhizal associate. Others are generalists and associate with many species. The tree acquires minerals such as [[phosphorus]] from the fungus, while the fungus obtains the [[carbohydrate]] products of photosynthesis from the tree.<ref>{{cite web |url=http://www.waldwissen.net/wald/baeume_waldpflanzen/oekologie/wsl_mykorrhiza_lebensgemeinschaft/index_EN |title=Mycorrhiza&nbsp;– a fascinating symbiosis in the forest |last1=Egli |first1=S. |author2=Brunner, I. |year=2011 |work=Forestknowledge |publisher=Swiss Federal Research Institute |access-date=15 July 2012 |archive-url=https://web.archive.org/web/20130509045021/https://www.waldwissen.net/wald/baeume_waldpflanzen/oekologie/wsl_mykorrhiza_lebensgemeinschaft/index_EN |archive-date=9 May 2013}}</ref> The hyphae of the fungus can link different trees and a network is formed, transferring nutrients and signals from one place to another.<ref>{{Cite journal |last=Heijden |first=Marcel G. A. van der |date=15 April 2016 |title=Underground networking |journal=Science |volume=352 |issue=6283 |pages=290–291 |doi=10.1126/science.aaf4694 |pmid=27081054|bibcode=2016Sci...352..290H |s2cid=133399719 }}</ref> The fungus promotes growth of the roots and helps protect the trees against predators and pathogens. It can also limit damage done to a tree by pollution as the fungus accumulate [[Heavy metal (chemistry)|heavy metals]] within its tissues.<ref>{{cite web |url=https://treesforlife.org.uk/into-the-forest/habitats-and-ecology/ecology/mycorrhizas/ |title=Mycorrhizas |last=Puplett |first=Dan |work=Trees for Life |access-date=15 July 2012 |archive-date=2 November 2019 |archive-url=https://web.archive.org/web/20191102063436/http://treesforlife.org.uk/into-the-forest/habitats-and-ecology/ecology/mycorrhizas/ |url-status=live }}</ref> Fossil evidence shows that roots have been associated with mycorrhizal fungi since the early [[Paleozoic]], four hundred million years ago, when the first [[vascular plant]]s colonised dry land.<ref>{{cite journal |last=Brundrett |first=Mark C. |year=2002 |title=Coevolution of roots and mycorrhizas of land plants |journal=New Phytologist |volume=154 |issue=2 |pages=275–304 |doi=10.1046/j.1469-8137.2002.00397.x|pmid=33873429 |doi-access=free }}</ref>

[[File:Bombax LalBagh.JPG|thumb|left|Buttress roots of the kapok tree (''[[Ceiba pentandra]]'')]]
Some trees such as [[Alder]] (''Alnus'' species) have a [[Symbiosis|symbiotic]] relationship with ''[[Frankia]]'' species, a filamentous bacterium that can fix nitrogen from the air, converting it into [[ammonia]]. They have [[Actinorhizal plant|actinorhizal]] root nodules on their roots in which the bacteria live. This process enables the tree to live in low nitrogen habitats where they would otherwise be unable to thrive.<ref>{{cite web |url=http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm |title=Frankia and Actinorhizal Plants |last=Benson |first=David |publisher=University of Connecticut |access-date=15 July 2012 |archive-date=6 August 2018 |archive-url=https://web.archive.org/web/20180806064343/http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm |url-status=dead }}</ref> The plant hormones called [[cytokinin]]s initiate root nodule formation, in a process closely related to mycorrhizal association.<ref>{{cite book |title=Signaling in Plants |last=Baluška |first=František |author2=Mancuso, Stefano |year=2009 |publisher=Springer |isbn=978-3-540-89227-4 |pages=83–84 |url=https://books.google.com/books?id=RTFMl0cArgAC&pg=PA83 |access-date=29 May 2020 |archive-date=19 August 2020 |archive-url=https://web.archive.org/web/20200819211216/https://books.google.com/books?id=RTFMl0cArgAC&pg=PA83 |url-status=live }}</ref>

It has been demonstrated that some trees are interconnected through their root system, forming a colony. The interconnections are made by the [[inosculation]] process, a kind of natural [[grafting]] or welding of vegetal tissues. The tests to demonstrate this networking are performed by injecting chemicals, sometimes [[Radioactive decay|radioactive]], into a tree, and then checking for its presence in neighbouring trees.<ref>{{cite journal |last=Hough |first=Walter A. |title=Root extension of Individual trees in surface soils of a Natural Longleaf Pine-Turkey Oak Stand |journal=Forest Science |date=1 June 1965 |volume=11 |issue=2 |pages=223–242 |url=http://www.ingentaconnect.com/content/saf/fs/1965/00000011/00000002/art00020 |archive-url=https://web.archive.org/web/20131209062328/http://www.ingentaconnect.com/content/saf/fs/1965/00000011/00000002/art00020 |archive-date=9 December 2013 |url-status=dead }}</ref>

The roots are, generally, an underground part of the tree, but some tree species have evolved roots that are [[aerial roots|aerial]]. The common purposes for aerial roots may be of two kinds, to contribute to the mechanical stability of the tree, and to obtain oxygen from air. An instance of mechanical stability enhancement is the [[Rhizophora mangle|red mangrove]] that develops [[buttress roots|prop roots]] that loop out of the trunk and branches and descend vertically into the mud.<ref name=Singapore>{{cite web |url=http://mangrove.nus.edu.sg/guidebooks/text/1043.htm |title=How plants cope in the mangroves |editor=Ng, Peter K. L. |editor2=Sivasothi, N. |year=2001 |work=Mangroves of Singapore |access-date=15 July 2012 |archive-date=22 May 2012 |archive-url=https://web.archive.org/web/20120522185105/http://mangrove.nus.edu.sg/guidebooks/text/1043.htm |url-status=live }}</ref> A similar structure is developed by the [[Ficus benghalensis|Indian banyan]].<ref name=Thomas>{{cite book |title=Trees: Their Natural History |last=Thomas |first=Peter |year=2000 |publisher=Cambridge University Press |isbn=978-0-521-45963-1 |page=108 |url=https://books.google.com/books?id=w6bUF8bOfKEC&pg=PA108 |access-date=29 May 2020 |archive-date=20 August 2020 |archive-url=https://web.archive.org/web/20200820005841/https://books.google.com/books?id=w6bUF8bOfKEC&pg=PA108 |url-status=live }}</ref> Many large trees have [[buttress root]]s which flare out from the lower part of the trunk. These brace the tree rather like angle brackets and provide stability, reducing sway in high winds. They are particularly prevalent in tropical rainforests where the soil is poor and the roots are close to the surface.<ref>{{cite journal |last1=Crook |first1=M. J. |author2=Ennos, A. R. |author3=Banks, J. R. |year=1997 |title=The function of buttress roots: a comparative study of the anchorage systems of buttressed (''Aglaia'' and ''Nephelium ramboutan'' species) and non-buttressed (''Mallotus wrayi'') tropical trees |journal=Journal of Experimental Botany |volume=48 |issue=9 |pages=1703–1716 |doi=10.1093/jxb/48.9.1703|doi-access=free }}</ref>

Some tree species have developed root extensions that pop out of soil, in order to get oxygen, when it is not available in the soil because of excess water. These root extensions are called [[pneumatophores]], and are present, among others, in [[Avicennia germinans|black mangrove]] and pond cypress.<ref name=Singapore/>