The leaves of a plant are the true organs of photosynthesis.  Most often dicot leaves (such as Syringa vulgaris) can be divided into two main mesophyll regions.  The upper layer is called the palisade parenchyma (or palisade mesophyll) and is comprised mainly of cylindrical cells lined up in a fairly organized pattern.  The lower layer is called the spongy parenchyma (or spongy mesophyll) and is comprised of oddly shaped cells which are surrounded by a great deal of air space.  This is particularly helpful because it allows carbon dioxide to diffuse through the tissue so that it can surround photosynthesizing cells.  Leaves are highly vascularized and the xylem in the veins supply the cells of the mesophyll layers with water.  The products of photosynthesis are moved via the phloem to the rest of the plant.  Guard cells and stomata provide the leaf with a complex mechanism for water loss reduction and gas exchange.