The principal site of photosynthesis and the site of transpiration. Leaves are covered with a waxy layer, the cuticle , which prevents water loss. Small microscopic openings, the stomata , are usually found on the undersides of a leaf. These openings regulate the flow of gases and water vapor in and out of a leaf. The conversion of light energy, water and carbon dioxide in the presence of chlorophyll to carbohydrates, with oxygen being released as a by-product.
Some leaves are sessile , lacking a petiole.
At the base of every leaf, there is always an axillary bud. Some leaves are identified by their characteristic leaf margin, the edge of the leaf. A leaf with several blades or leaflets is a compound leaf . Leaves can have different arrangements on a stem. Plants with alternate leaves have one leaf at each node.