Tree

===Leaves===
{{main|Leaf}}

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.<ref name=Pessarakli/> They are an important investment by the tree and may be thorny or contain [[phytolith]]s, [[lignin]]s, [[tannin]]s or [[poison]]s 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 [[chlorophyll]] and the red and yellow pigments already present in the blades become apparent.<ref name=Pessarakli>{{cite book |author=Pessarakli, Mohammad |title=Handbook of Photosynthesis |url=https://books.google.com/books?id=0_3XqlcKPpwC&pg=PA725 |year=2005 |publisher=CRC Press |isbn=978-0-8247-5839-4 |pages=717–739 |access-date=28 February 2016 |archive-date=21 May 2016 |archive-url=https://web.archive.org/web/20160521021252/https://books.google.com/books?id=0_3XqlcKPpwC&pg=PA725 |url-status=live }}</ref> Synthesis in the leaf of a [[plant hormone]] called [[auxin]] also ceases. This causes the cells at the junction of the [[Petiole (botany)|petiole]] 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.<ref>{{cite book |url=https://books.google.com/books?id=_16xbB2Py_UC&pg=PA734 |title=Biology: Concepts and Applications |author1=Starr, Cecie |author2=Evers, Christine |author3=Starr, Lisa |publisher=Cengage Learning |date=2010 |page=734 |isbn=978-1-4390-4673-9 |access-date=28 February 2016 |archive-date=19 March 2022 |archive-url=https://web.archive.org/web/20220319090500/https://books.google.com/books?id=_16xbB2Py_UC&pg=PA734 |url-status=live }}</ref> Many deciduous trees flower before the new leaves emerge.<ref>{{cite journal |first=Stephen H. |last=Bullock |author2=Solis-Magallanes, J. Arturo |title=Phenology of canopy trees of a tropical deciduous forest in Mexico |journal=Biotropica |volume=22 |issue=1 |date=March 1990 |pages=22–35 |doi=10.2307/2388716 |jstor=2388716|bibcode=1990Biotr..22...22B }}</ref> A few trees do not have true leaves but instead have structures with similar external appearance such as [[Phylloclade]]s – [[Aerial stem modification|modified stem]] structures<ref>{{Cite book |last=Beentje |first=Henk |year=2010 |title=The Kew Plant Glossary |location=Richmond, Surrey |publisher=[[Royal Botanic Gardens, Kew]] |isbn=978-1-84246-422-9}} p. 87.</ref> – as seen in the genus ''[[Phyllocladus]]''.<ref name="pagephyllocladaceae">Page, Christopher N. (1990). "Phyllocladaceae" pp. 317–319. In: Klaus Kubitzki (general editor); Karl U. Kramer and Peter S. Green (volume editors) ''The Families and Genera of Vascular Plants'' volume I. Springer-Verlag: Berlin; Heidelberg, Germany. {{ISBN|978-0-387-51794-0}}</ref>