Leaves are numerous and vary in shape and size based on which vascular plant they are on. Leaves have mesophyll tissue in their middle layer made of parenchyma cells. The parenchyma cells in leaves are:Palisade parenchyma cells and,Spongy parenchyma cells.The palisade parenchyma is tightly packed, and the spongy parenchyma is loosely packed. Both have chloroplasts, the photosynthetic organelle of plants. The epidermis is made of a layer or layers of epidermal cells that secrete a waxy covering called a cuticle that helps prevent leaves from drying out. The epidermis also contains stomatal openings, which allow for gas exchange on the leaf surface. Stomata are controlled by the opening and closing of guard cells.