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[[File:Alnus glutinosa tervaleppä lehti.jpg|thumb|250px|A leaf.]]
The term '''leaf''' usually means an organ of a vascular plant. Many leaves are large, flat, green structures above ground, involved in photosynthesis. However many types of leaves lack one or more of these general characteristics. A leaf consists of a blade and a petiole, which sometimes have stipules at the base, and has a bud in the axil between the petiole and the stem unless there is no petiole as in the case of sessile leaves.<ref>{{cite book|author= Schooley, James|title=Introduction to Botany|page=307|publisher=Delmar Publishers|location=Albany|year=1997|isbn= 0-8273-7378-3}}</ref>
However many types of leaves lack one or more of these general characteristics. Some are not flat like the conifers, some are not above ground like the bulb scales and some lack major photosynthetic function like the spines. The leaves may have functions other than photosynthesis. Some leaves provide protection, support, storage and even acquisition of nitrogen.<ref>{{cite book|author=Mauseth, James D|title=Botany: An Introduction to Plant Biology|edition=2nd|publisher=Jones and Bartlett Publishers|location=Sudbury, Massachusetts|year=1998|page=144|isbn=0-7637-0746-5}}</ref>
Leaves are made of various types of plant tissues, some serving as support and other serving as filler material.<ref>{{cite book|editor=Michael Levy|publisher=Encyclopaedia Britannica|location=Chicago|year=2008||title=Plants, Algae and Fungi|page=16|isbn=978-1-59339-803-3}}</ref>