What is Leaf?
[[File:Leaves - Photo by Giovanni Ussi - Foglie 22.jpg|thumb|Leaves - Photo by Giovanni Ussi - Foglie 22]] [[File:Leaf epidermis w scale.jpg|thumb|230px|[[Scanning electron microscope|SEM]] image of ''Nicotiana alata'' leaf's epidermis, showing [[trichome]]s (hair-like appendages) and [[stoma]]ta (eye-shaped slits, visible at full resolution).]] [[File:Leaf Structure.svg|right|281px|Medium scale diagram of leaf internal anatomy]] [[File:Leaf Tissue Structure.svg|right|280px|Fine scale diagram of leaf structure]] [[File:Leaf morphology no title.svg|thumb|right|230px|Leaves can have different shapes. The part of [[biology]] that studies the shapes of things is called ''[[Morphology (biology)|Morphology]]'']] A '''leaf''' is a green above-ground [[plant]] [[Organ (anatomy)|organ]]. Its main functions are [[photosynthesis]] and gas exchange. A leaf is flat so it absorbs the most light, and thin, so that the sunlight can get to the [[chloroplast]]s in the cells. Most leaves have [[stomata]], which open and close. They [[wikt:regulation|regulate]] [[carbon dioxide]], [[oxygen]], and [[water vapour]] exchange with the [[atmosphere]]. Plants with leaves all year round are [[evergreen]]s, and those that shed their leaves are [[deciduous]]. Deciduous trees and shrubs generally lose their leaves in [[autumn]]. Before this happens, the leaves change colour. The leaves will grow back in [[Spring (season)|spring]]. Leaves are normally [[green]] in color, which comes from [[chlorophyll]] found in the chloroplasts. Plants that lack chlorophyll cannot [[photosynthesis|photosynthesize]]. Leaves come in many shapes and sizes. The biggest undivided leaf is that of a giant edible [[arum]]. This lives in [[marsh]]y parts of the [[tropical]] [[rain forest]] of [[Borneo]]. One of its leaves can be ten feet across and have a surface area of over 30 square feet (~2.8 sq. metres).<ref>Attenborough, David 1995. ''The private lives of plants: a natural history of plant behaviour''. London: BBC Books, p47. (includes photograph, but [[sp