The primary function of leaves is to be the foremost location of photosynthesis in the plant, which is made possible by their physical structure. Leaves are covered in a thin epidermal layer of tissue that secretes a fatty substance called cutin, which forms a waterproof outer layer called the cuticle. Pores, or stomates, are present in the epidermis and are formed by specialized paired guard cells. The tissue between the upper and lower epidermis is known as the mesophyll and is lined with veins. The cells of the mesophyll contain photosynthetic pigments.