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<!DOCTYPE html>
<html>
<head>
<meta charset='utf-8'>
<meta http-equiv='X-UA-Compatible' content='IE=edge'>
<title>Webtest</title>
<meta name='viewport' content='width=device-width, initial-scale=1'>
<link rel='stylesheet' type='text/css' media='screen' href='https://peteole.github.io/slideNavigation/design.css'>
<script src='https://peteole.github.io/touchmove/touchmove.js'></script>
<script src='https://peteole.github.io/slideNavigation/main.js'></script>
</head>
<body>
<h1 id="firstHeading" class="firstHeading" lang="en">Tree</h1>
<p> Hi! This is an example Wikipedia page copied from <a href="https://en.wikipedia.org/wiki/Tree">https://en.wikipedia.org/wiki/Tree</a> in order to demonstrate a new navigation system! Enjoy!</p>
<div id="siteSub" class="noprint">From Wikipedia, the free encyclopedia</div>
<div id="contentSub"></div>
<div id="jump-to-nav"></div>
<a class="mw-jump-link" href="#mw-head">Jump to navigation</a>
<a class="mw-jump-link" href="#p-search">Jump to search</a>
<div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Perennial woody
plant with elongated trunk</div>
<div role="note" class="hatnote navigation-not-searchable">For other uses, see <a href="/wiki/Tree_(disambiguation)"
class="mw-disambig" title="Tree (disambiguation)">Tree
(disambiguation)</a>.</div>
<p class="mw-empty-elt">
</p>
<div class="thumb tright">
<div class="thumbinner" style="width:222px;"><a href="/wiki/File:Ash_Tree_-_geograph.org.uk_-_590710.jpg"
class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/e/eb/Ash_Tree_-_geograph.org.uk_-_590710.jpg/220px-Ash_Tree_-_geograph.org.uk_-_590710.jpg"
decoding="async" width="220" height="293" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/e/eb/Ash_Tree_-_geograph.org.uk_-_590710.jpg/330px-Ash_Tree_-_geograph.org.uk_-_590710.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/e/eb/Ash_Tree_-_geograph.org.uk_-_590710.jpg/440px-Ash_Tree_-_geograph.org.uk_-_590710.jpg 2x"
data-file-width="480" data-file-height="640" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Ash_Tree_-_geograph.org.uk_-_590710.jpg" class="internal"
title="Enlarge"></a></div>Common ash (<i><a href="/wiki/Fraxinus_excelsior"
title="Fraxinus excelsior">Fraxinus excelsior</a></i>),
a <a href="/wiki/Deciduous" title="Deciduous">deciduous</a> broad-leaved (<a href="/wiki/Angiosperm"
class="mw-redirect" title="Angiosperm">angiosperm</a>) tree
</div>
</div>
</div>
<div class="thumb tright">
<div class="thumbinner" style="width:222px;"><a href="/wiki/File:Larix_decidua_Aletschwald.jpg"
class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Larix_decidua_Aletschwald.jpg/220px-Larix_decidua_Aletschwald.jpg"
decoding="async" width="220" height="293" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Larix_decidua_Aletschwald.jpg/330px-Larix_decidua_Aletschwald.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Larix_decidua_Aletschwald.jpg/440px-Larix_decidua_Aletschwald.jpg 2x"
data-file-width="3040" data-file-height="4048" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Larix_decidua_Aletschwald.jpg" class="internal"
title="Enlarge"></a></div>European larch (<i><a href="/wiki/Larix_decidua"
title="Larix decidua">Larix decidua</a></i>), a <a href="/wiki/Conifer" class="mw-redirect"
title="Conifer">coniferous</a> tree which is also deciduous
</div>
</div>
</div>
<p>In <a href="/wiki/Botany" title="Botany">botany</a>, a <b>tree</b> is a <a href="/wiki/Perennial_plant"
title="Perennial plant">perennial plant</a> with an elongated <a href="/wiki/Plant_stem"
title="Plant stem">stem</a>, or <a href="/wiki/Trunk_(botany)" title="Trunk (botany)">trunk</a>,
supporting branches and leaves in most species. In some usages, the definition of a tree may be
narrower, including only woody plants with <a href="/wiki/Secondary_growth" title="Secondary growth">secondary
growth</a>, plants that are usable as <a href="/wiki/Lumber" title="Lumber">lumber</a> or plants above a
specified height. In wider definitions, the taller <a href="/wiki/Arecaceae" title="Arecaceae">palms</a>, <a
href="/wiki/Cyatheales" title="Cyatheales">tree ferns</a>, <a href="/wiki/Musa_(genus)"
title="Musa (genus)">bananas</a>,
and <a href="/wiki/Bamboo" title="Bamboo">bamboos</a> are also trees. Trees are not a <a href="/wiki/Taxon"
title="Taxon">taxonomic group</a> but include a variety of plant species that <a
href="/wiki/Convergent_evolution" title="Convergent evolution">have independently evolved</a> a
trunk and branches as a way to tower above other plants to compete for sunlight. Trees tend to be
long-lived, some reaching several thousand years old. Trees have been in existence for 370 million
years. It is estimated that there are just over 3 trillion mature trees in the world.<sup
id="cite_ref-Crowther_1-0" class="reference"><a href="#cite_note-Crowther-1">[1]</a></sup>
</p>
<p>A tree typically has many secondary branches supported clear of the ground by the trunk. This trunk
typically contains <a href="/wiki/Woody_tissue" class="mw-redirect" title="Woody tissue">woody
tissue</a> for strength, and <a href="/wiki/Vascular_tissue" title="Vascular tissue">vascular
tissue</a> to carry materials from one part of the tree to another. For most trees it is surrounded
by a layer of <a href="/wiki/Bark_(botany)" title="Bark (botany)">bark</a> which serves as a protective
barrier. Below the ground, the <a href="/wiki/Tree#Roots" title="Tree">roots</a> branch and spread out
widely; they serve to anchor the tree and extract moisture and nutrients from the soil. Above ground,
the branches divide into smaller branches and shoots. The shoots typically bear leaves, which capture
light energy and convert it into sugars by <a href="/wiki/Photosynthesis"
title="Photosynthesis">photosynthesis</a>, providing the food for the tree's growth and development.
</p>
<p>Trees usually reproduce using seeds. Flowers and fruit may be present, but some trees, such as conifers,
instead have pollen cones and seed cones. Palms, bananas, and bamboos also produce seeds, but tree ferns
produce <a href="/wiki/Spore" title="Spore">spores</a> instead.
</p>
<p>Trees play a significant role in reducing <a href="/wiki/Soil_erosion" title="Soil erosion">erosion</a>
and moderating the <a href="/wiki/Climate" title="Climate">climate</a>. They remove <a
href="/wiki/Carbon_dioxide" title="Carbon dioxide">carbon dioxide</a> from the <a
href="/wiki/Atmosphere_of_Earth" title="Atmosphere of Earth">atmosphere</a> and store large
quantities of <a href="/wiki/Carbon" title="Carbon">carbon</a> in their tissues. Trees and forests
provide a habitat for many species of animals and plants. <a href="/wiki/Tropical_rainforest"
title="Tropical rainforest">Tropical rainforests</a> are among the most <a href="/wiki/Biodiversity"
title="Biodiversity">biodiverse</a> habitats in the world. Trees provide <a href="/wiki/Shade_tree"
title="Shade tree">shade and shelter</a>, timber for construction, fuel for cooking and heating, and
fruit for food as well as having many other uses. In parts of the world, forests are shrinking as trees
are cleared to increase the amount of land available for agriculture. Because of their longevity and
usefulness, trees have always been revered, with <a href="/wiki/Sacred_grove" title="Sacred grove">sacred
groves</a> in various cultures, and they play a role in many of the
world's <a href="/wiki/Mythology" class="mw-redirect" title="Mythology">mythologies</a>.
</p>
<div id="toc" class="toc"><input type="checkbox" role="button" id="toctogglecheckbox" class="toctogglecheckbox"
style="display:none" />
<div class="toctitle" lang="en" dir="ltr">
<h2>Contents</h2><span class="toctogglespan"><label class="toctogglelabel"
for="toctogglecheckbox"></label></span>
</div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="#Definition"><span class="tocnumber">1</span> <span
class="toctext">Definition</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="#Overview"><span class="tocnumber">2</span> <span
class="toctext">Overview</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="#Distribution"><span class="tocnumber">3</span> <span
class="toctext">Distribution</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="#Parts_and_function"><span class="tocnumber">4</span>
<span class="toctext">Parts and function</span></a>
<ul>
<li class="toclevel-2 tocsection-5"><a href="#Roots"><span class="tocnumber">4.1</span>
<span class="toctext">Roots</span></a></li>
<li class="toclevel-2 tocsection-6"><a href="#Trunk"><span class="tocnumber">4.2</span>
<span class="toctext">Trunk</span></a></li>
<li class="toclevel-2 tocsection-7"><a href="#Buds_and_growth"><span class="tocnumber">4.3</span>
<span class="toctext">Buds and growth</span></a>
</li>
<li class="toclevel-2 tocsection-8"><a href="#Leaves"><span class="tocnumber">4.4</span>
<span class="toctext">Leaves</span></a></li>
<li class="toclevel-2 tocsection-9"><a href="#Reproduction"><span class="tocnumber">4.5</span> <span
class="toctext">Reproduction</span></a></li>
<li class="toclevel-2 tocsection-10"><a href="#Seeds"><span class="tocnumber">4.6</span>
<span class="toctext">Seeds</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-11"><a href="#Evolutionary_history"><span class="tocnumber">5</span> <span
class="toctext">Evolutionary history</span></a></li>
<li class="toclevel-1 tocsection-12"><a href="#Tree_ecology"><span class="tocnumber">6</span> <span
class="toctext">Tree ecology</span></a></li>
<li class="toclevel-1 tocsection-13"><a href="#Uses"><span class="tocnumber">7</span> <span
class="toctext">Uses</span></a>
<ul>
<li class="toclevel-2 tocsection-14"><a href="#Food"><span class="tocnumber">7.1</span>
<span class="toctext">Food</span></a></li>
<li class="toclevel-2 tocsection-15"><a href="#Fuel"><span class="tocnumber">7.2</span>
<span class="toctext">Fuel</span></a></li>
<li class="toclevel-2 tocsection-16"><a href="#Timber"><span class="tocnumber">7.3</span>
<span class="toctext">Timber</span></a></li>
<li class="toclevel-2 tocsection-17"><a href="#Art"><span class="tocnumber">7.4</span> <span
class="toctext">Art</span></a>
<ul>
<li class="toclevel-3 tocsection-18"><a href="#Bonsai"><span class="tocnumber">7.4.1</span>
<span class="toctext">Bonsai</span></a>
</li>
<li class="toclevel-3 tocsection-19"><a href="#Tree_shaping"><span
class="tocnumber">7.4.2</span> <span class="toctext">Tree
shaping</span></a></li>
</ul>
</li>
<li class="toclevel-2 tocsection-20"><a href="#Bark"><span class="tocnumber">7.5</span>
<span class="toctext">Bark</span></a></li>
<li class="toclevel-2 tocsection-21"><a href="#Ornamental_trees"><span class="tocnumber">7.6</span>
<span class="toctext">Ornamental trees</span></a>
</li>
<li class="toclevel-2 tocsection-22"><a href="#Other_uses"><span class="tocnumber">7.7</span> <span
class="toctext">Other uses</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-23"><a href="#Care"><span class="tocnumber">8</span> <span
class="toctext">Care</span></a></li>
<li class="toclevel-1 tocsection-24"><a href="#Mythology"><span class="tocnumber">9</span> <span
class="toctext">Mythology</span></a></li>
<li class="toclevel-1 tocsection-25"><a href="#Superlative_trees"><span class="tocnumber">10</span>
<span class="toctext">Superlative trees</span></a></li>
<li class="toclevel-1 tocsection-26"><a href="#See_also"><span class="tocnumber">11</span> <span
class="toctext">See also</span></a></li>
<li class="toclevel-1 tocsection-27"><a href="#Notes"><span class="tocnumber">12</span> <span
class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-28"><a href="#References"><span class="tocnumber">13</span> <span
class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-29"><a href="#Further_reading"><span class="tocnumber">14</span>
<span class="toctext">Further reading</span></a></li>
</ul>
</div>
<h2><span class="mw-headline" id="Definition">Definition</span></h2>
<div class="thumb tright">
<div class="thumbinner" style="width:222px;"><a href="/wiki/File:Tree_secondary_growth_diagram.jpg"
class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Tree_secondary_growth_diagram.jpg/220px-Tree_secondary_growth_diagram.jpg"
decoding="async" width="220" height="253" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Tree_secondary_growth_diagram.jpg/330px-Tree_secondary_growth_diagram.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Tree_secondary_growth_diagram.jpg/440px-Tree_secondary_growth_diagram.jpg 2x"
data-file-width="2467" data-file-height="2841" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Tree_secondary_growth_diagram.jpg" class="internal"
title="Enlarge"></a></div>Diagram of <a href="/wiki/Secondary_growth"
title="Secondary growth">secondary growth</a> in a <a href="/wiki/Eudicot" class="mw-redirect"
title="Eudicot">eudicot</a> or <a href="/wiki/Coniferous" class="mw-redirect"
title="Coniferous">coniferous</a> tree showing idealised vertical and
horizontal sections. A new layer of wood is added in each growing season, thickening the stem,
existing branches and roots.
</div>
</div>
</div>
<p>Although "tree" is a term of common parlance, there is no universally recognised precise definition of
what a tree is, either <a href="/wiki/Botany" title="Botany">botanically</a> or in common language.<sup
id="cite_ref-as.miami.edu_2-0" class="reference"><a href="#cite_note-as.miami.edu-2">[2]</a></sup>
In its broadest sense, a tree is any
plant with the general form of an elongated stem, or trunk, which supports the <a href="/wiki/Photosynthesis"
title="Photosynthesis">photosynthetic</a> leaves or branches at some
distance above the ground.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3">[3]</a></sup>
Trees are also typically defined by height,<sup id="cite_ref-Gschwantner,_Thomas_2009_4-0" class="reference"><a
href="#cite_note-Gschwantner,_Thomas_2009-4">[4]</a></sup> with smaller plants from 0.5
to 10 m (1.6 to 32.8 ft) being called shrubs,<sup id="cite_ref-treedictionary_5-0"
class="reference"><a href="#cite_note-treedictionary-5">[5]</a></sup> so the minimum height
of a tree is only loosely defined.<sup id="cite_ref-Gschwantner,_Thomas_2009_4-1" class="reference"><a
href="#cite_note-Gschwantner,_Thomas_2009-4">[4]</a></sup> Large <a href="/wiki/Herbaceous"
class="mw-redirect" title="Herbaceous">herbaceous</a> plants such as <a href="/wiki/Papaya"
title="Papaya">papaya</a> and bananas are trees in this broad sense.<sup id="cite_ref-as.miami.edu_2-1"
class="reference"><a href="#cite_note-as.miami.edu-2">[2]</a></sup><sup id="cite_ref-6"
class="reference"><a href="#cite_note-6">[6]</a></sup>
</p>
<p>A commonly applied narrower definition is that a tree has a woody trunk formed by <a
href="/wiki/Secondary_growth" title="Secondary growth">secondary growth</a>, meaning that the trunk
thickens each year by growing outwards, in addition to the primary upwards growth from the <a
href="/wiki/Apical_meristem" class="mw-redirect" title="Apical meristem">growing tip</a>.<sup
id="cite_ref-Gschwantner,_Thomas_2009_4-2" class="reference"><a
href="#cite_note-Gschwantner,_Thomas_2009-4">[4]</a></sup><sup id="cite_ref-7"
class="reference"><a href="#cite_note-7">[7]</a></sup> Under such a definition, herbaceous
plants such as <a href="/wiki/Arecaceae" title="Arecaceae">palms</a>, bananas and papayas are not
considered trees regardless of their height, growth form or stem girth. Certain <a href="/wiki/Monocotyledon"
title="Monocotyledon">monocots</a> may be considered trees under a
slightly looser definition;<sup id="cite_ref-8" class="reference"><a href="#cite_note-8">[8]</a></sup>
while the <a href="/wiki/Yucca_brevifolia" title="Yucca brevifolia">Joshua tree</a>, bamboos and palms do not
have secondary growth and never
produce true wood with growth rings,<sup id="cite_ref-botany.wisc.edu_9-0" class="reference"><a
href="#cite_note-botany.wisc.edu-9">[9]</a></sup><sup id="cite_ref-RoddStackhouse2008_10-0"
class="reference"><a href="#cite_note-RoddStackhouse2008-10">[10]</a></sup> they may produce
"pseudo-wood" by
<a href="/wiki/Lignin" title="Lignin">lignifying</a> cells formed by primary growth.<sup id="cite_ref-11"
class="reference"><a href="#cite_note-11">[11]</a></sup> Tree species in
the genus <a href="/wiki/Dracaena_(plant)" title="Dracaena (plant)"><i>Dracaena</i></a>, despite also
being monocots, do have secondary growth caused by meristem in their trunk, but it is different from the
thickening meristem found in dicotyledonous trees.<sup class="noprint Inline-Template Template-Fact"
style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed"
title="Wikipedia:Citation needed"><span
title="This claim needs references to reliable sources. (September 2019)">citation
needed</span></a></i>]</sup>
</p>
<p>Aside from structural definitions, trees are commonly defined by use; for instance, as those plants which
yield lumber.<sup id="cite_ref-fao.org_12-0" class="reference"><a
href="#cite_note-fao.org-12">[12]</a></sup>
</p>
<h2><span class="mw-headline" id="Overview">Overview</span></h2>
<p>The tree growth habit is an <a href="/wiki/Evolution" title="Evolution">evolutionary</a> <a
href="/wiki/Adaptation" title="Adaptation">adaptation</a> found in different groups of plants: by
growing taller, trees are able to compete better for sunlight.<sup id="cite_ref-LowmanRinker2004_13-0"
class="reference"><a href="#cite_note-LowmanRinker2004-13">[13]</a></sup> Trees tend to be
tall and long-lived,<sup id="cite_ref-Rémy_14-0" class="reference"><a
href="#cite_note-Rémy-14">[14]</a></sup> some reaching several thousand years old.<sup
id="cite_ref-Koch_15-0" class="reference"><a href="#cite_note-Koch-15">[15]</a></sup>
Several trees are among the oldest organisms now living.<sup id="cite_ref-16" class="reference"><a
href="#cite_note-16">[16]</a></sup> 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 <a href="/wiki/Shrub"
title="Shrub">shrubs</a>, which have a similar growth form, by usually growing larger and having a
single main stem;<sup id="cite_ref-treedictionary_5-1" class="reference"><a
href="#cite_note-treedictionary-5">[5]</a></sup> but there is no consistent distinction
between a tree and a shrub,<sup id="cite_ref-HawthorneLawrence2012_17-0" class="reference"><a
href="#cite_note-HawthorneLawrence2012-17">[17]</a></sup> made more confusing by the
fact that trees may be reduced in size under harsher environmental conditions such as on mountains and
<a href="/wiki/Subarctic" title="Subarctic">subarctic</a> areas. The tree form has evolved separately in
unrelated classes of plants in response to similar environmental challenges, making it a classic example
of <a href="/wiki/Parallel_evolution" title="Parallel evolution">parallel evolution</a>. With an
estimated 60,000-100,000 species, the number of trees worldwide might total twenty-five per cent of all
living plant species.<sup id="cite_ref-18" class="reference"><a href="#cite_note-18">[18]</a></sup><sup
id="cite_ref-19" class="reference"><a href="#cite_note-19">[19]</a></sup> The greatest number of
these grow in tropical
regions and many of these areas have not yet been fully surveyed by <a href="/wiki/Botanist" class="mw-redirect"
title="Botanist">botanists</a>, making tree diversity and ranges poorly
known.<sup id="cite_ref-20" class="reference"><a href="#cite_note-20">[20]</a></sup>
</p>
<div class="thumb tleft">
<div class="thumbinner" style="width:222px;"><a href="/wiki/File:Nedravazhakola.jpg" class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/2/2d/Nedravazhakola.jpg/220px-Nedravazhakola.jpg"
decoding="async" width="220" height="165" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/2/2d/Nedravazhakola.jpg/330px-Nedravazhakola.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/2/2d/Nedravazhakola.jpg/440px-Nedravazhakola.jpg 2x"
data-file-width="2048" data-file-height="1536" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Nedravazhakola.jpg" class="internal" title="Enlarge"></a></div>
Tall <a href="/wiki/Herbaceous" class="mw-redirect" title="Herbaceous">herbaceous</a> <a
href="/wiki/Monocotyledon" title="Monocotyledon">monocotyledonous</a> plants such as banana lack
secondary growth, but
are trees under the broadest definition.
</div>
</div>
</div>
<p>The majority of tree species are <a href="/wiki/Angiosperm" class="mw-redirect"
title="Angiosperm">angiosperms</a>. There are about 1000 species of <a href="/wiki/Gymnosperm"
title="Gymnosperm">gymnosperm</a> trees,<sup id="cite_ref-21" class="reference"><a
href="#cite_note-21">[21]</a></sup> including <a href="/wiki/Conifer" class="mw-redirect"
title="Conifer">conifers</a>, <a href="/wiki/Cycad" title="Cycad">cycads</a>, <a href="/wiki/Ginkgoales"
title="Ginkgoales">ginkgophytes</a> and <a href="/wiki/Gnetophyta" title="Gnetophyta">gnetales</a>; they
produce seeds which are not enclosed in fruits, but in open
structures such as pine <a href="/wiki/Conifer_cone" title="Conifer cone">cones</a>, and many have tough
waxy leaves, such as pine needles.<sup id="cite_ref-22" class="reference"><a
href="#cite_note-22">[22]</a></sup> Most angiosperm trees are <a href="/wiki/Eudicots"
title="Eudicots">eudicots</a>, the "true dicotyledons", so named because the seeds contain two <a
href="/wiki/Cotyledon" title="Cotyledon">cotyledons</a> or seed leaves. There are also some trees
among the old lineages of flowering plants called <a href="/wiki/Basal_angiosperms"
title="Basal angiosperms">basal angiosperms or paleodicots</a>; these include <i><a href="/wiki/Amborella"
title="Amborella">Amborella</a></i>, <i><a href="/wiki/Magnolia" title="Magnolia">Magnolia</a></i>,
nutmeg and avocado,<sup id="cite_ref-Hodson2012_23-0" class="reference"><a
href="#cite_note-Hodson2012-23">[23]</a></sup> while trees such as
bamboo, palms and bananas are monocots.
</p>
<p>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.<sup id="cite_ref-Transport_24-0" class="reference"><a href="#cite_note-Transport-24">[24]</a></sup>
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 <a href="/wiki/Vascular_cambium"
title="Vascular cambium">vascular cambium</a> allows the expansion of vascular tissue that produces
woody growth. Because this growth ruptures the epidermis of the stem, woody plants also have a <a
href="/wiki/Cork_cambium" title="Cork cambium">cork cambium</a> 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.<sup
id="cite_ref-Coder_25-0" class="reference"><a href="#cite_note-Coder-25">[25]</a></sup>
</p>
<p>Trees are either <a href="/wiki/Evergreen" title="Evergreen">evergreen</a>, having foliage that persists
and remains green throughout the year,<sup id="cite_ref-26" class="reference"><a
href="#cite_note-26">[26]</a></sup> or <a href="/wiki/Deciduous"
title="Deciduous">deciduous</a>, shedding their leaves at the end of the growing season and then
having a dormant period without foliage.<sup id="cite_ref-27" class="reference"><a
href="#cite_note-27">[27]</a></sup> Most conifers are evergreens, but larches (<i><a
href="/wiki/Larix" class="mw-redirect" title="Larix">Larix</a></i> and <i><a href="/wiki/Pseudolarix"
title="Pseudolarix">Pseudolarix</a></i>) are deciduous, dropping their
needles each autumn, and some species of cypress (<i><a href="/wiki/Glyptostrobus"
title="Glyptostrobus">Glyptostrobus</a></i>, <i><a href="/wiki/Metasequoia"
title="Metasequoia">Metasequoia</a></i> and <i><a href="/wiki/Taxodium"
title="Taxodium">Taxodium</a></i>) shed small leafy shoots annually in a process known as <a
href="/wiki/Cladoptosis" title="Cladoptosis">cladoptosis</a>.<sup id="cite_ref-treedictionary_5-2"
class="reference"><a href="#cite_note-treedictionary-5">[5]</a></sup> The <a
href="/wiki/Crown_(botany)" title="Crown (botany)">crown</a> is the spreading top of a tree
including the branches and leaves,<sup id="cite_ref-28" class="reference"><a
href="#cite_note-28">[28]</a></sup> while the uppermost layer in a forest, formed by the
crowns of the trees, is known as the <a href="/wiki/Canopy_(biology)" title="Canopy (biology)">canopy</a>.<sup
id="cite_ref-29" class="reference"><a href="#cite_note-29">[29]</a></sup> A sapling is a young
tree.<sup id="cite_ref-30" class="reference"><a href="#cite_note-30">[30]</a></sup>
</p>
<p>Many tall palms are herbaceous<sup id="cite_ref-eeb.ucla.edu_31-0" class="reference"><a
href="#cite_note-eeb.ucla.edu-31">[31]</a></sup> monocots; these do not undergo
secondary growth and never produce wood.<sup id="cite_ref-botany.wisc.edu_9-1" class="reference"><a
href="#cite_note-botany.wisc.edu-9">[9]</a></sup><sup id="cite_ref-RoddStackhouse2008_10-1"
class="reference"><a href="#cite_note-RoddStackhouse2008-10">[10]</a></sup> 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, <a href="/wiki/Order_(biology)" title="Order (biology)">order</a> <a
href="/wiki/Cyatheales" title="Cyatheales">Cyatheales</a>, have
tall straight trunks, growing up to 20 metres (66 ft), but these are composed not of wood but of <a
href="/wiki/Rhizome" title="Rhizome">rhizomes</a> which grow vertically and are covered by numerous
<a href="/wiki/Adventitiousness" class="mw-redirect" title="Adventitiousness">adventitious
roots</a>.<sup id="cite_ref-32" class="reference"><a href="#cite_note-32">[32]</a></sup>
</p>
<h2><span class="mw-headline" id="Distribution">Distribution</span></h2>
<div role="note" class="hatnote navigation-not-searchable">Further information: <a href="/wiki/Forest"
title="Forest">Forest</a></div>
<div class="thumb tleft">
<div class="thumbinner" style="width:172px;"><a href="/wiki/File:Daintree_Rainforest_4.jpg" class="image"><img
alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/3/31/Daintree_Rainforest_4.jpg/170px-Daintree_Rainforest_4.jpg"
decoding="async" width="170" height="227" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/3/31/Daintree_Rainforest_4.jpg/255px-Daintree_Rainforest_4.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/3/31/Daintree_Rainforest_4.jpg/340px-Daintree_Rainforest_4.jpg 2x"
data-file-width="2448" data-file-height="3264" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Daintree_Rainforest_4.jpg" class="internal"
title="Enlarge"></a></div>The <a href="/wiki/Daintree_Rainforest"
title="Daintree Rainforest">Daintree Rainforest</a>
</div>
</div>
</div>
<p>The number of trees in the world, according to a 2015 estimate, is 3.04 trillion, of which 1.39 trillion
(46%) are in the tropics or <a href="/wiki/Subtropics" title="Subtropics">sub-tropics</a>, 0.61 trillion
(20%) in the <a href="/wiki/Temperate_climate" title="Temperate climate">temperate zones</a>, and 0.74
trillion (24%) in the <a href="/wiki/Forest" title="Forest">coniferous</a> <a href="/wiki/Taiga"
title="Taiga">boreal forests</a>. The estimate is about eight times higher than previous estimates,
and is based on tree densities measured on over 400,000 plots. It remains subject to a wide margin of
error, not least because the samples are mainly from Europe and North America. The estimate suggests
that about 15 billion trees are cut down annually and about 5 billion are planted. In the 12,000 years
since the start of human agriculture, the number of trees worldwide has decreased by 46%.<sup
id="cite_ref-Crowther_1-1" class="reference"><a href="#cite_note-Crowther-1">[1]</a></sup><sup
id="cite_ref-33" class="reference"><a href="#cite_note-33">[33]</a></sup><sup id="cite_ref-34"
class="reference"><a href="#cite_note-34">[34]</a></sup><sup id="cite_ref-35" class="reference"><a
href="#cite_note-35">[35]</a></sup>
</p>
<p>In suitable environments, such as the <a href="/wiki/Daintree_Rainforest" title="Daintree Rainforest">Daintree
Rainforest</a> in <a href="/wiki/Queensland" title="Queensland">Queensland</a>, or the mixed <a
href="/wiki/Podocarpus" title="Podocarpus">podocarp</a> and <a
href="/wiki/Temperate_broadleaf_and_mixed_forest" title="Temperate broadleaf and mixed forest">broadleaf
forest</a> of <a href="/wiki/Ulva_Island,_New_Zealand" class="mw-redirect"
title="Ulva Island, New Zealand">Ulva
Island, New Zealand</a>, forest is the more-or-less stable <a href="/wiki/Climax_community"
title="Climax community">climatic climax community</a> at the end of a plant succession, where open
areas such as grassland are colonised by taller plants, which in turn give way to trees that eventually
form a forest canopy.<sup id="cite_ref-36" class="reference"><a href="#cite_note-36">[36]</a></sup><sup
id="cite_ref-37" class="reference"><a href="#cite_note-37">[37]</a></sup>
</p>
<div class="thumb tright">
<div class="thumbinner" style="width:222px;"><a href="/wiki/File:New_snow,_Swabian_Alps_(2019).jpg"
class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/8/82/New_snow%2C_Swabian_Alps_%282019%29.jpg/220px-New_snow%2C_Swabian_Alps_%282019%29.jpg"
decoding="async" width="220" height="146" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/8/82/New_snow%2C_Swabian_Alps_%282019%29.jpg/330px-New_snow%2C_Swabian_Alps_%282019%29.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/8/82/New_snow%2C_Swabian_Alps_%282019%29.jpg/440px-New_snow%2C_Swabian_Alps_%282019%29.jpg 2x"
data-file-width="4928" data-file-height="3264" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:New_snow,_Swabian_Alps_(2019).jpg" class="internal"
title="Enlarge"></a></div>Conifers in the <a href="/wiki/Swabian_Jura"
title="Swabian Jura">Swabian alps</a>
</div>
</div>
</div>
<p>In <a href="/wiki/Temperateness" class="mw-redirect" title="Temperateness">cool temperate</a> regions,
conifers often predominate; a widely distributed climax community in the far north of the northern
hemisphere is moist <a href="/wiki/Taiga" title="Taiga">taiga</a> or northern coniferous forest (also
called boreal forest).<sup id="cite_ref-38" class="reference"><a href="#cite_note-38">[38]</a></sup><sup
id="cite_ref-39" class="reference"><a href="#cite_note-39">[39]</a></sup> Taiga is the world's
largest land <a href="/wiki/Biome" title="Biome">biome</a>, forming 29% of the world's forest cover.<sup
id="cite_ref-40" class="reference"><a href="#cite_note-40">[40]</a></sup> The long cold
winter of the far north is unsuitable for plant growth and trees must grow rapidly in the short summer
season when the temperature rises and the days are long. Light is very limited under their dense cover
and there may be little plant life on the forest floor, although fungi may abound.<sup id="cite_ref-41"
class="reference"><a href="#cite_note-41">[41]</a></sup> Similar woodland is found on
mountains where the altitude causes the average temperature to be lower thus reducing the length of the
growing season.<sup id="cite_ref-42" class="reference"><a href="#cite_note-42">[42]</a></sup>
</p>
<p>Where rainfall is relatively evenly spread across the seasons in temperate regions, <a
href="/wiki/Temperate_broadleaf_and_mixed_forest" title="Temperate broadleaf and mixed forest">temperate
broadleaf and mixed forest</a> typified by
species like oak, beech, birch and maple is found.<sup id="cite_ref-43" class="reference"><a
href="#cite_note-43">[43]</a></sup> Temperate forest is also found in the southern
hemisphere, as for example in the Eastern Australia temperate forest, characterised by <i><a
href="/wiki/Eucalyptus" title="Eucalyptus">Eucalyptus</a></i> forest and open acacia
woodland.<sup id="cite_ref-44" class="reference"><a href="#cite_note-44">[44]</a></sup>
</p>
<p>In tropical regions with a <a href="/wiki/Monsoon_climate" class="mw-redirect"
title="Monsoon climate">monsoon</a> or monsoon-like climate, where a drier part of the year
alternates with a wet period as in the <a href="/wiki/Amazon_rainforest" title="Amazon rainforest">Amazon
rainforest</a>, different species of broad-leaved trees dominate
the forest, some of them being deciduous.<sup id="cite_ref-45" class="reference"><a
href="#cite_note-45">[45]</a></sup> In tropical regions with a drier <a
href="/wiki/Savanna_climate" class="mw-redirect" title="Savanna climate">savanna climate</a> and
insufficient rainfall to support dense forests, the canopy is not closed, and plenty of sunshine reaches
the ground which is covered with grass and scrub. <i><a href="/wiki/Acacia" title="Acacia">Acacia</a></i> and <a
href="/wiki/Baobab" class="mw-redirect" title="Baobab">baobab</a> are well adapted to living in such
areas.<sup id="cite_ref-46" class="reference"><a href="#cite_note-46">[46]</a></sup>
</p>
<h2><span class="mw-headline" id="Parts_and_function">Parts and function</span></h2>
<div class="thumb tright">
<div class="thumbinner" style="width:222px;"><a href="/wiki/File:WisconsinScenery.jpg" class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/0/07/WisconsinScenery.jpg/220px-WisconsinScenery.jpg"
decoding="async" width="220" height="183" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/0/07/WisconsinScenery.jpg/330px-WisconsinScenery.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/0/07/WisconsinScenery.jpg/440px-WisconsinScenery.jpg 2x"
data-file-width="1920" data-file-height="1600" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:WisconsinScenery.jpg" class="internal" title="Enlarge"></a>
</div>A young <a href="/wiki/Red_pine" class="mw-redirect" title="Red pine">red pine</a> (<i>Pinus
resinosa</i>) with spread of roots visible, as a
result of soil erosion
</div>
</div>
</div>
<h3><span class="mw-headline" id="Roots">Roots</span></h3>
<div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Root" title="Root">Root</a>
</div>
<p>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 <a href="/wiki/Radicle" title="Radicle">radicle</a> or embryonic root is the first part of
a <a href="/wiki/Seedling" title="Seedling">seedling</a> to emerge from the seed during the process of
<a href="/wiki/Germination" title="Germination">germination</a>. This develops into a <a href="/wiki/Taproot"
title="Taproot">taproot</a> which goes straight downwards. Within a few weeks
<a href="/wiki/Lateral_root" title="Lateral root">lateral roots</a> 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 <a
href="/wiki/Root_hair" title="Root hair">root hairs</a>. These are in immediate contact with the
soil particles and can absorb water and nutrients such as <a href="/wiki/Potassium"
title="Potassium">potassium</a> in solution. The roots require oxygen to <a
href="/wiki/Respiration_(physiology)" title="Respiration (physiology)">respire</a> and only a few
species such as <a href="/wiki/Mangrove" title="Mangrove">mangroves</a> and the <a
href="/wiki/Taxodium_ascendens" title="Taxodium ascendens">pond cypress</a> (<i>Taxodium
ascendens</i>) can live in permanently waterlogged soil.<sup id="cite_ref-47" class="reference"><a
href="#cite_note-47">[47]</a></sup>
</p>
<p>In the soil, the roots encounter the <a href="/wiki/Hyphae" class="mw-redirect" title="Hyphae">hyphae</a>
of fungi. Many of these are known as <a href="/wiki/Mycorrhiza" title="Mycorrhiza">mycorrhiza</a> and
form a <a href="/wiki/Mutualism_(biology)" title="Mutualism (biology)">mutualistic</a> 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 <a href="/wiki/Phosphorus" title="Phosphorus">phosphorus</a> from the fungus, while the
fungus obtains the <a href="/wiki/Carbohydrate" title="Carbohydrate">carbohydrate</a> products of
photosynthesis from the tree.<sup id="cite_ref-48" class="reference"><a
href="#cite_note-48">[48]</a></sup> The hyphae of the fungus can link different trees
and a network is formed, transferring nutrients and signals from one place to another.<sup id="cite_ref-49"
class="reference"><a href="#cite_note-49">[49]</a></sup> 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 <a href="/wiki/Heavy_metal_(chemistry)"
class="mw-redirect" title="Heavy metal (chemistry)">heavy
metals</a> within its tissues.<sup id="cite_ref-50" class="reference"><a
href="#cite_note-50">[50]</a></sup> Fossil evidence shows that roots have been
associated with mycorrhizal fungi since the early <a href="/wiki/Paleozoic" title="Paleozoic">Paleozoic</a>,
four hundred million years ago, when the first <a href="/wiki/Vascular_plant" title="Vascular plant">vascular
plants</a> colonised dry land.<sup id="cite_ref-51" class="reference"><a
href="#cite_note-51">[51]</a></sup>
</p>
<div class="thumb tleft">
<div class="thumbinner" style="width:222px;"><a href="/wiki/File:Bombax_LalBagh.JPG" class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/7/7b/Bombax_LalBagh.JPG/220px-Bombax_LalBagh.JPG"
decoding="async" width="220" height="165" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/7/7b/Bombax_LalBagh.JPG/330px-Bombax_LalBagh.JPG 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/7/7b/Bombax_LalBagh.JPG/440px-Bombax_LalBagh.JPG 2x"
data-file-width="2560" data-file-height="1920" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Bombax_LalBagh.JPG" class="internal" title="Enlarge"></a></div>
Buttress roots of the kapok tree (<i><a href="/wiki/Ceiba_pentandra" title="Ceiba pentandra">Ceiba
pentandra</a></i>)
</div>
</div>
</div>
<p>Some trees such as the <a href="/wiki/Alders" title="Alders">alders</a> (<i>Alnus</i> species) have a <a
href="/wiki/Symbiosis" title="Symbiosis">symbiotic</a> relationship with <i><a href="/wiki/Frankia"
title="Frankia">Frankia</a></i> species, a filamentous bacterium that can fix nitrogen from the
air, converting it into <a href="/wiki/Ammonia" title="Ammonia">ammonia</a>. They have <a
href="/wiki/Actinorhizal_plant" title="Actinorhizal plant">actinorhizal</a> 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.<sup id="cite_ref-52" class="reference"><a
href="#cite_note-52">[52]</a></sup> The plant hormones called <a href="/wiki/Cytokinin"
title="Cytokinin">cytokinins</a> initiate root nodule formation, in a process closely related to
mycorrhizal association.<sup id="cite_ref-53" class="reference"><a href="#cite_note-53">[53]</a></sup>
</p>
<p>It has been demonstrated that some trees are interconnected through their root system, forming a colony.
The interconnections are made by the <a href="/wiki/Inosculation" title="Inosculation">inosculation</a>
process, a kind of natural <a href="/wiki/Grafting" title="Grafting">grafting</a> or welding of vegetal
tissues. The tests to demonstrate this networking are performed by injecting chemicals, sometimes <a
href="/wiki/Radioactive_decay" title="Radioactive decay">radioactive</a>, into a tree, and then
checking for its presence in neighbouring trees.<sup id="cite_ref-54" class="reference"><a
href="#cite_note-54">[54]</a></sup>
</p>
<p>The roots are, generally, an underground part of the tree, but some tree species have evolved roots that
are <a href="/wiki/Aerial_roots" class="mw-redirect" title="Aerial roots">aerial</a>. 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 <a
href="/wiki/Rhizophora_mangle" title="Rhizophora mangle">red mangrove</a> that develops <a
href="/wiki/Buttress_roots" class="mw-redirect" title="Buttress roots">prop roots</a> that loop out
of the trunk and branches and descend vertically into the mud.<sup id="cite_ref-Singapore_55-0"
class="reference"><a href="#cite_note-Singapore-55">[55]</a></sup> A similar structure is
developed by the <a href="/wiki/Ficus_benghalensis" title="Ficus benghalensis">Indian banyan</a>.<sup
id="cite_ref-Thomas_56-0" class="reference"><a href="#cite_note-Thomas-56">[56]</a></sup>
Many large trees have <a href="/wiki/Buttress_root" title="Buttress root">buttress roots</a> 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.<sup id="cite_ref-57" class="reference"><a
href="#cite_note-57">[57]</a></sup>
</p>
<p>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 <a
href="/wiki/Pneumatophores" class="mw-redirect" title="Pneumatophores">pneumatophores</a>, and are
present, among others, in <a href="/wiki/Avicennia_germinans" title="Avicennia germinans">black
mangrove</a> and pond cypress.<sup id="cite_ref-Singapore_55-1" class="reference"><a
href="#cite_note-Singapore-55">[55]</a></sup>
</p>
<h3><span class="mw-headline" id="Trunk">Trunk</span></h3>
<div class="thumb tright">
<div class="thumbinner" style="width:172px;"><a href="/wiki/File:Buk1.JPG" class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/9/9d/Buk1.JPG/170px-Buk1.JPG" decoding="async"
width="170" height="227" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/9/9d/Buk1.JPG/255px-Buk1.JPG 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/9/9d/Buk1.JPG/340px-Buk1.JPG 2x"
data-file-width="768" data-file-height="1024" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Buk1.JPG" class="internal" title="Enlarge"></a></div>
Northern beech (<i><a href="/wiki/Fagus_sylvatica" title="Fagus sylvatica">Fagus
sylvatica</a></i>) trunk in autumn
</div>
</div>
</div>
<div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Trunk_(botany)"
title="Trunk (botany)">Trunk (botany)</a></div>
<p>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.<sup id="cite_ref-King1990_58-0" class="reference"><a
href="#cite_note-King1990-58">[58]</a></sup> 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.<sup id="cite_ref-Russelltrunk_59-0" class="reference"><a
href="#cite_note-Russelltrunk-59">[59]</a></sup>
</p>
<p>In the case of angiosperms and gymnosperms, the outermost layer of the trunk is the <a href="/wiki/Bark_(botany)"
title="Bark (botany)">bark</a>, mostly composed of dead cells of <a href="/wiki/Phellem" class="mw-redirect"
title="Phellem">phellem</a> (cork).<sup id="cite_ref-Junikka1994_60-0" class="reference"><a
href="#cite_note-Junikka1994-60">[60]</a></sup> 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 <a href="/wiki/Lenticel"
title="Lenticel">lenticels</a>, through which oxygen diffuses. Bark is continually replaced by a
living layer of cells called the <a href="/wiki/Cork_cambium" title="Cork cambium">cork cambium</a> or
phellogen.<sup id="cite_ref-Junikka1994_60-1" class="reference"><a
href="#cite_note-Junikka1994-60">[60]</a></sup> The <a href="/wiki/Platanus_%C3%97_acerifolia"
title="Platanus × acerifolia">London plane</a> (<i>Platanus
× acerifolia</i>) periodically sheds its bark in large flakes. Similarly, the bark of the <a
href="/wiki/Betula_pendula" title="Betula pendula">silver birch</a> (<i>Betula pendula</i>) 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 <a href="/wiki/Pine"
title="Pine">pine</a> (<i>Pinus</i> species) the bark exudes sticky <a href="/wiki/Resin"
title="Resin">resin</a> which deters attackers whereas in <a href="/wiki/Hevea_brasiliensis"
title="Hevea brasiliensis">rubber trees</a> (<i>Hevea brasiliensis</i>) it is a milky <a href="/wiki/Latex"
title="Latex">latex</a> that oozes out. The <a href="/wiki/Cinchona_officinalis"
title="Cinchona officinalis">quinine bark tree</a> (<i>Cinchona officinalis</i>) contains bitter
substances to make the bark unpalatable.<sup id="cite_ref-Russelltrunk_59-1" class="reference"><a
href="#cite_note-Russelltrunk-59">[59]</a></sup> Large tree-like plants with lignified
trunks in the <a href="/wiki/Pteridophyta" class="mw-redirect" title="Pteridophyta">Pteridophyta</a>, <a
href="/wiki/Arecales" title="Arecales">Arecales</a>, <a href="/wiki/Cycadophyta" class="mw-redirect"
title="Cycadophyta">Cycadophyta</a> and <a href="/wiki/Poales" title="Poales">Poales</a> such as the
tree ferns, palms, cycads and bamboos have different structures and outer coverings.<sup id="cite_ref-61"
class="reference"><a href="#cite_note-61">[61]</a></sup>
</p>
<div class="thumb tleft">
<div class="thumbinner" style="width:222px;"><a href="/wiki/File:Taxus_wood.jpg" class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/0/0b/Taxus_wood.jpg/220px-Taxus_wood.jpg"
decoding="async" width="220" height="223" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/0/0b/Taxus_wood.jpg/330px-Taxus_wood.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/0/0b/Taxus_wood.jpg 2x"
data-file-width="421" data-file-height="427" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Taxus_wood.jpg" class="internal" title="Enlarge"></a>
</div>A section of <a href="/wiki/Taxus_baccata" title="Taxus baccata">yew</a> (<i>Taxus
baccata</i>) showing 27 annual growth rings, pale <a href="/wiki/Sapwood_(wood)" class="mw-redirect"
title="Sapwood (wood)">sapwood</a> and dark <a href="/wiki/Heartwood" class="mw-redirect"
title="Heartwood">heartwood</a>
</div>
</div>
</div>
<p>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. <a
href="/wiki/Dutch_elm_disease" title="Dutch elm disease">Dutch elm disease</a> is caused by a fungus
(<i><a href="/wiki/Ophiostoma" title="Ophiostoma">Ophiostoma</a></i> species) carried from one <a
href="/wiki/Elm" title="Elm">elm</a> 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.<sup id="cite_ref-62" class="reference"><a
href="#cite_note-62">[62]</a></sup>
</p>
<p>The innermost layer of bark is known as the <a href="/wiki/Phloem" title="Phloem">phloem</a> and this is
involved in the transport of the <a href="/wiki/Plant_sap" class="mw-redirect" title="Plant sap">sap</a>
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 <a
href="/wiki/Parenchyma" title="Parenchyma">parenchyma</a> cells which provide padding and include
fibres for strengthening the tissue.<sup id="cite_ref-Lalonde_63-0" class="reference"><a
href="#cite_note-Lalonde-63">[63]</a></sup> 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 <a href="/wiki/Xylem"
title="Xylem">xylem</a> on the inside.<sup id="cite_ref-64" class="reference"><a
href="#cite_note-64">[64]</a></sup>
</p>
<p>The newly created xylem is the <a href="/wiki/Sapwood_(wood)" class="mw-redirect"
title="Sapwood (wood)">sapwood</a>. 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 <a
href="/wiki/Heartwood" class="mw-redirect" title="Heartwood">heartwood</a> 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 <a href="/wiki/Cellulose" title="Cellulose">cellulose</a>, a <a
href="/wiki/Polysaccharide" title="Polysaccharide">polysaccharide</a>, and most of the remainder is
lignin, a complex <a href="/wiki/Polymer" title="Polymer">polymer</a>. A transverse section through a
tree trunk or a horizontal core will show concentric circles or lighter or darker wood – tree rings.<sup
id="cite_ref-woodanatomy_65-0" class="reference"><a href="#cite_note-woodanatomy-65">[65]</a></sup>
These rings are the <a href="/wiki/Growth_rings" class="mw-redirect" title="Growth rings">annual growth
rings</a><sup id="cite_ref-66" class="reference"><a href="#cite_note-66">[66]</a></sup><sup
id="cite_ref-67" class="reference"><a href="#cite_note-67">[67]</a></sup> There may also be
rays running at right angles to growth rings. These are <a href="/wiki/Medullary_ray_(botany)"
title="Medullary ray (botany)">vascular rays</a> which are thin sheets of living tissue permeating
the wood.<sup id="cite_ref-woodanatomy_65-1" class="reference"><a
href="#cite_note-woodanatomy-65">[65]</a></sup> Many older trees may become hollow but
may still stand upright for many years.<sup id="cite_ref-68" class="reference"><a
href="#cite_note-68">[68]</a></sup>
</p>
<h3><span class="mw-headline" id="Buds_and_growth">Buds and growth</span></h3>
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<div class="mw-stack stack-container stack-right mobile-float-reset">
<div class="stack-object">
<div class="thumb tright">
<div class="thumbinner" style="width:172px;"><a href="/wiki/File:Illustration_Quercus_robur0.jpg"
class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/e/ef/Illustration_Quercus_robur0.jpg/170px-Illustration_Quercus_robur0.jpg"
decoding="async" width="170" height="273" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/e/ef/Illustration_Quercus_robur0.jpg/255px-Illustration_Quercus_robur0.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/e/ef/Illustration_Quercus_robur0.jpg/340px-Illustration_Quercus_robur0.jpg 2x"
data-file-width="1462" data-file-height="2346" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Illustration_Quercus_robur0.jpg" class="internal"
title="Enlarge"></a></div>Buds, leaves, flowers and fruit of
oak (<i><a href="/wiki/Quercus_robur" title="Quercus robur">Quercus robur</a></i>)
</div>
</div>
</div>
<div class="thumb tright">
<div class="thumbinner" style="width:172px;"><a href="/wiki/File:Illustration_Abies_alba0.jpg"
class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Illustration_Abies_alba0.jpg/170px-Illustration_Abies_alba0.jpg"
decoding="async" width="170" height="260" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Illustration_Abies_alba0.jpg/255px-Illustration_Abies_alba0.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Illustration_Abies_alba0.jpg/340px-Illustration_Abies_alba0.jpg 2x"
data-file-width="1559" data-file-height="2381" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Illustration_Abies_alba0.jpg" class="internal"
title="Enlarge"></a></div>Buds, leaves and reproductive structures of white fir
(<i><a href="/wiki/Abies_alba" title="Abies alba">Abies alba</a></i>)
</div>
</div>
</div>
<div class="thumb tright">
<div class="thumbinner" style="width:172px;"><a href="/wiki/File:Cycas_circinalis(draw).jpg"
class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/7/7f/Cycas_circinalis%28draw%29.jpg/170px-Cycas_circinalis%28draw%29.jpg"
decoding="async" width="170" height="219" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/7/7f/Cycas_circinalis%28draw%29.jpg/255px-Cycas_circinalis%28draw%29.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/7/7f/Cycas_circinalis%28draw%29.jpg/340px-Cycas_circinalis%28draw%29.jpg 2x"
data-file-width="476" data-file-height="613" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Cycas_circinalis(draw).jpg" class="internal"
title="Enlarge"></a></div>Form, leaves and reproductive structures of queen sago
(<i><a href="/wiki/Cycas_circinalis" title="Cycas circinalis">Cycas circinalis</a></i>)
</div>
</div>
</div>
</div>
</div>
<div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Bud" title="Bud">Bud</a>
</div>
<div class="thumb tleft">
<div class="thumbinner" style="width:222px;"><a href="/wiki/File:Magnolia_Bud.jpg" class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/f/f6/Magnolia_Bud.jpg/220px-Magnolia_Bud.jpg"
decoding="async" width="220" height="147" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/f6/Magnolia_Bud.jpg/330px-Magnolia_Bud.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/f6/Magnolia_Bud.jpg/440px-Magnolia_Bud.jpg 2x"
data-file-width="2216" data-file-height="1476" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Magnolia_Bud.jpg" class="internal" title="Enlarge"></a>
</div>Dormant <i><a href="/wiki/Magnolia" title="Magnolia">Magnolia</a></i> bud
</div>
</div>
</div>
<p>Trees do not usually grow continuously throughout the year but mostly have spurts of active expansion
followed by periods of rest. This pattern of growth is related to climatic conditions; growth normally
ceases when conditions are either too cold or too dry. In readiness for the inactive period, trees form
<a href="/wiki/Bud" title="Bud">buds</a> to protect the <a href="/wiki/Meristem" title="Meristem">meristem</a>,
the zone of active growth. Before the period of dormancy, the last
few leaves produced at the tip of a twig form scales. These are thick, small and closely wrapped and
enclose the growing point in a waterproof sheath. Inside this bud there is a rudimentary stalk and
neatly folded miniature leaves, ready to expand when the next growing season arrives. Buds also form in
the <a href="/wiki/Axil" class="mw-redirect" title="Axil">axils</a> of the leaves ready to produce new
side shoots. A few trees, such as the <a href="/wiki/Eucalyptus" title="Eucalyptus">eucalyptus</a>, have
"naked buds" with no protective scales and some conifers, such as the <a href="/wiki/Chamaecyparis_lawsoniana"
title="Chamaecyparis lawsoniana">Lawson's cypress</a>, have no
buds but instead have little pockets of meristem concealed among the scale-like leaves.<sup
id="cite_ref-buds_69-0" class="reference"><a href="#cite_note-buds-69">[69]</a></sup>
</p>
<p>When growing conditions improve, such as the arrival of warmer weather and the longer days associated
with spring in temperate regions, growth starts again. The expanding shoot pushes its way out, shedding
the scales in the process. These leave behind scars on the surface of the twig. The whole year's growth
may take place in just a few weeks. The new stem is unlignified at first and may be green and downy. The
Arecaceae (palms) have their leaves spirally arranged on an unbranched trunk.<sup id="cite_ref-buds_69-1"
class="reference"><a href="#cite_note-buds-69">[69]</a></sup> In
some tree species in temperate climates, a second spurt of growth, a <a href="/wiki/Lammas_growth"
title="Lammas growth">Lammas growth</a> may occur which is believed to be a strategy to compensate
for loss of early foliage to insect predators.<sup id="cite_ref-70" class="reference"><a
href="#cite_note-70">[70]</a></sup>
</p>
<p>Primary growth is the elongation of the stems and roots. Secondary growth consists of a progressive
thickening and strengthening of the tissues as the outer layer of the epidermis is converted into bark
and the cambium layer creates new phloem and xylem cells. The bark is inelastic.<sup id="cite_ref-71"
class="reference"><a href="#cite_note-71">[71]</a></sup> Eventually the growth of a tree
slows down and stops and it gets no taller. If damage occurs the tree may in time become hollow.<sup
id="cite_ref-72" class="reference"><a href="#cite_note-72">[72]</a></sup>
</p>
<h3><span class="mw-headline" id="Leaves">Leaves</span></h3>
<div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Leaf" title="Leaf">Leaf</a>
</div>
<p>Leaves are structures specialised for photosynthesis and are arranged on the tree in such a way as to
maximise their exposure to light without shading each other.<sup id="cite_ref-Pessarakli_73-0"
class="reference"><a href="#cite_note-Pessarakli-73">[73]</a></sup> They are an important
investment by the tree and may be thorny or contain <a href="/wiki/Phytolith" title="Phytolith">phytoliths</a>,
<a href="/wiki/Lignin" title="Lignin">lignins</a>, <a href="/wiki/Tannin" title="Tannin">tannins</a> or <a
href="/wiki/Poison" title="Poison">poisons</a>
to discourage herbivory. Trees have evolved leaves in a wide range of shapes and sizes, in response to
environmental pressures including climate and predation. They can be broad or needle-like, simple or
compound, lobed or entire, smooth or hairy, delicate or tough, deciduous or evergreen. The needles of
coniferous trees are compact but are structurally similar to those of broad-leaved trees. They are
adapted for life in environments where resources are low or water is scarce. Frozen ground may limit
water availability and conifers are often found in colder places at higher altitudes and higher
latitudes than broad leaved trees. In conifers such as fir trees, the branches hang down at an angle to
the trunk, enabling them to shed snow. In contrast, broad leaved trees in temperate regions deal with
winter weather by shedding their leaves. When the days get shorter and the temperature begins to
decrease, the leaves no longer make new <a href="/wiki/Chlorophyll" title="Chlorophyll">chlorophyll</a>
and the red and yellow pigments already present in the blades become apparent.<sup id="cite_ref-Pessarakli_73-1"
class="reference"><a href="#cite_note-Pessarakli-73">[73]</a></sup> Synthesis in the leaf of a <a
href="/wiki/Plant_hormone" title="Plant hormone">plant hormone</a> called <a href="/wiki/Auxin"
title="Auxin">auxin</a> also ceases. This causes the cells at the junction of the <a
href="/wiki/Petiole_(botany)" title="Petiole (botany)">petiole</a> and the twig to weaken until the
joint breaks and the leaf floats to the ground. In tropical and subtropical regions, many trees keep
their leaves all year round. Individual leaves may fall intermittently and be replaced by new growth but
most leaves remain intact for some time. Other tropical species and those in arid regions may shed all
their leaves annually, such as at the start of the dry season.<sup id="cite_ref-74" class="reference"><a
href="#cite_note-74">[74]</a></sup> Many deciduous trees flower before the new leaves
emerge.<sup id="cite_ref-75" class="reference"><a href="#cite_note-75">[75]</a></sup> A few
trees do not have true leaves but instead have structures with similar external appearance such as <a
href="/wiki/Phylloclade" title="Phylloclade">Phylloclades</a> – <a href="/wiki/Aerial_stem_modification"
title="Aerial stem modification">modified stem</a>
structures<sup id="cite_ref-76" class="reference"><a href="#cite_note-76">[76]</a></sup> – as
seen in the genus <i><a href="/wiki/Phyllocladus" title="Phyllocladus">Phyllocladus</a></i>.<sup
id="cite_ref-pagephyllocladaceae_77-0" class="reference"><a
href="#cite_note-pagephyllocladaceae-77">[77]</a></sup>
</p>
<h3><span class="mw-headline" id="Reproduction">Reproduction</span></h3>
<div role="note" class="hatnote navigation-not-searchable">Further information: <a href="/wiki/Plant_reproduction"
title="Plant reproduction">Plant reproduction</a>, <a href="/wiki/Pollination"
title="Pollination">Pollination</a>, and <a href="/wiki/Seed_dispersal" title="Seed dispersal">Seed
dispersal</a></div>
<p>Trees can be <a href="/wiki/Pollination" title="Pollination">pollinated</a> either by wind or by animals,
mostly insects. Many angiosperm trees are insect pollinated. Wind pollination may take advantage of
increased wind speeds high above the ground.<sup id="cite_ref-78" class="reference"><a
href="#cite_note-78">[78]</a></sup> Trees use a variety of methods of <a
href="/wiki/Seed_dispersal" title="Seed dispersal">seed dispersal</a>. Some rely on wind, with
winged or plumed seeds. Others rely on animals, for example with edible fruits. Others again eject their
seeds (ballistic dispersal), or use gravity so that seeds fall and sometimes roll.<sup
id="cite_ref-NathanSeidler2006_79-0" class="reference"><a
href="#cite_note-NathanSeidler2006-79">[79]</a></sup>
</p>
<h3><span class="mw-headline" id="Seeds">Seeds</span></h3>
<div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Seed" title="Seed">Seed</a>
</div>
<p>Seeds are the primary way that trees reproduce and their seeds vary greatly in size and shape. Some of
the largest seeds come from trees, but the largest tree, <i><a href="/wiki/Sequoiadendron_giganteum"
title="Sequoiadendron giganteum">Sequoiadendron giganteum</a></i>, produces one of the smallest
tree seeds.<sup id="cite_ref-Walker1997_80-0" class="reference"><a
href="#cite_note-Walker1997-80">[80]</a></sup> The great diversity in tree fruits and
seeds reflects the many different ways that tree species have evolved to <a href="/wiki/Seed_dispersal"
title="Seed dispersal">disperse</a> their offspring.
</p>
<div class="thumb tleft">
<div class="thumbinner" style="width:222px;"><a href="/wiki/File:Samara_olmo_frassino_acero.png"
class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/6/60/Samara_olmo_frassino_acero.png/220px-Samara_olmo_frassino_acero.png"
decoding="async" width="220" height="181" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/60/Samara_olmo_frassino_acero.png/330px-Samara_olmo_frassino_acero.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/60/Samara_olmo_frassino_acero.png/440px-Samara_olmo_frassino_acero.png 2x"
data-file-width="1520" data-file-height="1250" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:Samara_olmo_frassino_acero.png" class="internal"
title="Enlarge"></a></div>Wind dispersed seed of elm (<i><a href="/wiki/Ulmus"
class="mw-redirect" title="Ulmus">Ulmus</a></i>), ash (<i><a href="/wiki/Fraxinus"
title="Fraxinus">Fraxinus</a></i>) and maple (<i><a href="/wiki/Acer_(plant)"
class="mw-redirect" title="Acer (plant)">Acer</a></i>)
</div>
</div>
</div>
<p>For a tree seedling to grow into an adult tree it needs light. If seeds only fell straight to the ground,
competition among the concentrated saplings and the shade of the parent would likely prevent it from
flourishing. Many seeds such as <a href="/wiki/Birch" title="Birch">birch</a> are small and have papery
wings to aid dispersal by the wind. <a href="/wiki/Ash_(Fraxinus)" class="mw-redirect"
title="Ash (Fraxinus)">Ash</a> trees and <a href="/wiki/Maple" title="Maple">maples</a> have larger
seeds with blade shaped wings which spiral down to the ground when released. The <a href="/wiki/Ceiba_pentandra"
title="Ceiba pentandra">kapok</a> tree has cottony threads to catch the
breeze.<sup id="cite_ref-Dispersal_81-0" class="reference"><a
href="#cite_note-Dispersal-81">[81]</a></sup>
</p>
<p>The seeds of conifers, the largest group of gymnosperms, are enclosed in a cone and most species have
seeds that are light and papery that can be blown considerable distances once free from the cone.<sup
id="cite_ref-EvertEichhorn2004_82-0" class="reference"><a
href="#cite_note-EvertEichhorn2004-82">[82]</a></sup> Sometimes the seed remains in the
cone for years waiting for a trigger event to liberate it. Fire stimulates release and germination of
seeds of the <a href="/wiki/Jack_pine" title="Jack pine">jack pine</a>, and also enriches the forest
floor with wood ash and removes competing vegetation.<sup id="cite_ref-83" class="reference"><a
href="#cite_note-83">[83]</a></sup> Similarly, a number of angiosperms including <i><a
href="/wiki/Acacia_cyclops" title="Acacia cyclops">Acacia cyclops</a></i> and <i><a
href="/wiki/Acacia_mangium" title="Acacia mangium">Acacia mangium</a></i> have seeds that
germinate better after exposure to high temperatures.<sup id="cite_ref-BaskinBaskin2001_84-0"
class="reference"><a href="#cite_note-BaskinBaskin2001-84">[84]</a></sup>
</p>
<p>The <a href="/wiki/Delonix_regia" title="Delonix regia">flame tree</a> <i>Delonix regia</i> does not rely
on fire but shoots its seeds through the air when the two sides of its long pods crack apart explosively
on drying.<sup id="cite_ref-Dispersal_81-1" class="reference"><a
href="#cite_note-Dispersal-81">[81]</a></sup> The miniature cone-like catkins of <a
href="/wiki/Alder" title="Alder">alder</a> trees produce seeds that contain small droplets of oil
that help disperse the seeds on the surface of water. Mangroves often grow in water and some species
have <a href="/wiki/Propagule" title="Propagule">propagules</a>, which are buoyant fruits with seeds
that start germinating before becoming detached from the parent tree.<sup id="cite_ref-BarbourBillings1999_85-0"
class="reference"><a href="#cite_note-BarbourBillings1999-85">[85]</a></sup><sup id="cite_ref-86"
class="reference"><a href="#cite_note-86">[86]</a></sup> These float on the water and may
become lodged on emerging mudbanks and successfully take root.<sup id="cite_ref-Dispersal_81-2"
class="reference"><a href="#cite_note-Dispersal-81">[81]</a></sup>
</p>
<p>Other seeds, such as apple pips and plum stones, have fleshy receptacles and smaller fruits like <a
href="/wiki/Crataegus" title="Crataegus">hawthorns</a> have seeds enclosed in edible tissue; animals
including mammals and birds eat the fruits and either discard the seeds, or swallow them so they pass
through the gut to be deposited in the animal's droppings well away from the parent tree. The
germination of some seeds is improved when they are processed in this way.<sup id="cite_ref-87"
class="reference"><a href="#cite_note-87">[87]</a></sup> <a href="/wiki/Nut_(fruit)"
title="Nut (fruit)">Nuts</a> may be gathered by animals such as squirrels that <a
href="/wiki/Hoarding_(animal_behavior)" title="Hoarding (animal behavior)">cache</a> any not
immediately consumed.<sup id="cite_ref-LeveySilva2002_88-0" class="reference"><a
href="#cite_note-LeveySilva2002-88">[88]</a></sup> Many of these caches are never
revisited, the nut-casing softens with rain and frost, and the seed germinates in the spring.<sup
id="cite_ref-89" class="reference"><a href="#cite_note-89">[89]</a></sup> Pine cones may
similarly be hoarded by <a href="/wiki/Red_squirrel" title="Red squirrel">red squirrels</a>, and <a
href="/wiki/Grizzly_bear" title="Grizzly bear">grizzly bears</a> may help to disperse the seed by
raiding squirrel caches.<sup id="cite_ref-90" class="reference"><a href="#cite_note-90">[90]</a></sup>
</p>
<p>The single extant species of Ginkgophyta (<i>Ginkgo biloba</i>) has fleshy seeds produced at the ends of
short branches on female trees,<sup id="cite_ref-91" class="reference"><a
href="#cite_note-91">[91]</a></sup> and <i><a href="/wiki/Gnetum" title="Gnetum">Gnetum</a></i>,
a tropical and subtropical group of gymnosperms produce seeds at
the tip of a shoot axis.<sup id="cite_ref-BhatnagarMoitra1996_92-0" class="reference"><a
href="#cite_note-BhatnagarMoitra1996-92">[92]</a></sup>
</p>
<h2><span class="mw-headline" id="Evolutionary_history">Evolutionary history</span></h2>
<div class="thumb tright">
<div class="thumbinner" style="width:172px;"><a
href="/wiki/File:PSM_V18_D630_Restoration_of_a_lepidodendron.jpg" class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/c/c4/PSM_V18_D630_Restoration_of_a_lepidodendron.jpg/170px-PSM_V18_D630_Restoration_of_a_lepidodendron.jpg"
decoding="async" width="170" height="325" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/c/c4/PSM_V18_D630_Restoration_of_a_lepidodendron.jpg/255px-PSM_V18_D630_Restoration_of_a_lepidodendron.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/c/c4/PSM_V18_D630_Restoration_of_a_lepidodendron.jpg/340px-PSM_V18_D630_Restoration_of_a_lepidodendron.jpg 2x"
data-file-width="580" data-file-height="1108" /></a>
<div class="thumbcaption">
<div class="magnify"><a href="/wiki/File:PSM_V18_D630_Restoration_of_a_lepidodendron.jpg"
class="internal" title="Enlarge"></a></div><i><a href="/wiki/Lepidodendron"
title="Lepidodendron">Lepidodendron</a></i>, an extinct <a href="/wiki/Lycopodiophyta"
class="mw-redirect" title="Lycopodiophyta">lycophyte</a> tree
</div>
</div>
</div>
<div class="thumb tright">
<div class="thumbinner" style="width:172px;"><a
href="/wiki/File:PSM_V16_D476_Principal_palms_and_cycadeae_of_middle_tertiary_europe.jpg"
class="image"><img alt=""
src="//upload.wikimedia.org/wikipedia/commons/thumb/8/8c/PSM_V16_D476_Principal_palms_and_cycadeae_of_middle_tertiary_europe.jpg/170px-PSM_V16_D476_Principal_palms_and_cycadeae_of_middle_tertiary_europe.jpg"
decoding="async" width="170" height="272" class="thumbimage"
srcset="//upload.wikimedia.org/wikipedia/commons/thumb/8/8c/PSM_V16_D476_Principal_palms_and_cycadeae_of_middle_tertiary_europe.jpg/255px-PSM_V16_D476_Principal_palms_and_cycadeae_of_middle_tertiary_europe.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/8/8c/PSM_V16_D476_Principal_palms_and_cycadeae_of_middle_tertiary_europe.jpg/340px-PSM_V16_D476_Principal_palms_and_cycadeae_of_middle_tertiary_europe.jpg 2x"
data-file-width="1395" data-file-height="2236" /></a>
<div class="thumbcaption">
<div class="magnify"><a
href="/wiki/File:PSM_V16_D476_Principal_palms_and_cycadeae_of_middle_tertiary_europe.jpg"
class="internal" title="Enlarge"></a></div>Palms and cycads as they might have appeared
in the middle <a href="/wiki/Tertiary_era" class="mw-redirect" title="Tertiary era">Tertiary</a>
</div>
</div>
</div>
<div role="note" class="hatnote navigation-not-searchable">Further information: <a
href="/wiki/Evolutionary_history_of_plants" title="Evolutionary history of plants">Evolutionary
history of plants</a></div>
<p>The earliest trees were <a href="/wiki/Tree_fern" title="Tree fern">tree ferns</a>, <a href="/wiki/Horsetail"
class="mw-redirect" title="Horsetail">horsetails</a> and <a href="/wiki/Lycophytes" class="mw-redirect"
title="Lycophytes">lycophytes</a>, which grew in forests
in the <a href="/wiki/Carboniferous" title="Carboniferous">Carboniferous</a> period. The first tree may
have been <i><a href="/wiki/Wattieza" title="Wattieza">Wattieza</a></i>, fossils of which have been
found in New York State in 2007 dating back to the <a href="/wiki/Middle_Devonian" class="mw-redirect"
title="Middle Devonian">Middle Devonian</a> (about 385 million years ago). Prior to this discovery,
<i><a href="/wiki/Archaeopteris" title="Archaeopteris">Archaeopteris</a></i> was the earliest known