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Leaf
A leaf is an organ of a vascular plant and is the principal lateral appendage of the stem. The leaves and stem together form the shoot. Foliage is a mass noun that refers to leaves collectively.〔Haupt, Arthur Wing (1953) (Plant morphology''. McGraw-Hill. )〕〔Mauseth, James D. (2008) ''Botany: An Introduction to Plant Biology''. Jones & Bartlett. ISBN 978-0-7637-5345-0〕
Typically a leaf is a thin, dorsiventrally flattened organ, borne above ground and specialized for photosynthesis. Most leaves have distinctive upper (adaxial) and lower (abaxial) surfaces that differ in colour, hairiness, the number of stomata (pores that intake and output gases) and other features. In most plant species, leaves are broad and flat. Such species are referred to as broad-leaved plants. Many gymnosperm species have thin needle-like leaves that can be advantageous in cold climates frequented by snow and frost. Leaves can also have other shapes and forms such as the scales in certain species of conifers. Some leaves are not above ground (such as bulb scales). Succulent plants often have thick juicy leaves, but some leaves are without major photosynthetic function and may be dead at maturity, as in some cataphylls, and spines). Furthermore, several kinds of leaf-like structures found in vascular plants are not totally homologous with them. Examples include flattened plant stems (called phylloclades and cladodes), and phyllodes (flattened leaf stems), both of which differ from leaves in their structure and origin.〔 Many structures of non-vascular plants, and even of some lichens, which are not plants at all (in the sense of being members of the kingdom Plantae), look and function much like leaves. The primary site of photosynthesis in most leaves (palisade mesophyll) almost always occurs on the upper side of the blade or lamina of the leaf〔 but in some species, including the mature foliage of ''Eucalyptus'' palisade occurs on both sides and the leaves are said to be isobilateral.
==Leaf development==
According to Agnes Arber's partial-shoot theory of the leaf, leaves are partial shoots.〔Arber, A. (1950). ''The Natural Philosophy of Plant Form''. Cambridge University Press.〕 Compound leaves are closer to shoots than simple leaves. Developmental studies have shown that compound leaves, like shoots, may branch in three dimensions.〔Rutishauser, R. and Sattler, R. 1997. Expression of shoot processes in leaf development of ''Polemonium caeruleum''. Botanische Jahrbücher für Systematik 119: 563-582.〕 On the basis of molecular genetics, Eckardt and Baum (2010) concluded that "it is now generally accepted that compound leaves express both leaf and shoot properties."
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