There are several levels of structure in studying a leaf. A leaf attached to the tree at a node. The stalk that attaches the leaf to the tree is the petiole which extend into the midrib of the leaf called the rachis. There may be a pair of extensions from the base of the petiole called stiplules. Many veins branch off the rachis. The flat part of the leaf is called the blade. There are many shapes of leaves and different margins.
If you take a section of the leaf, you would find the following tissue layers from top to the bottom of the leaf: upper epidermis covered with a waxy cuticle, palisade mesophyll and spongy mesophyll in which the photosynthesis occurs, a lower epidermis perforated with numerous holes called stomata. The stomata are surrounded by guard cells that can open and close. The stomata allow gas exchange (carbon dioxide in and oxygen and water vapor out). There will be cross sections of veins as well. In the vein, there will be xylem carrying water and minerals at the top of the vein with phloem on the bottom of the vein. The structure of the leaf allows for maximum absorption of sunlight for photosynthesis.
This website does an excellent job explaining the leaf layers:
http://www.purchon.com/biology/flash/leaf.swfhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html#Leaves%20and%20Leaf%20StructureBio_man 8)
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