Leaves, outgrowths of the shoot apex, are found in a variety of shapes, sizes, and arrangements. Most leaves have a blade, a petiole, stipules, and veins. The blade is the flattened portion of the leaf. The petiole is the slender stalk of the leaf. The stipules, found on some but not all leaves, are located at the base of the petiole where it joins the stem. Stipules may be leaflike and show considerable variation in size. Veins, xylem, and phloem run through the leaf. Leaves are the main photosynthetic organ. However, they are also important in gas exchange and water movement throughout the whole plant.