Stomata on a rose leaf. Coloured scanning electron micrograph (SEM) of the lower leaf surface of a Garden Rose Rosa sp., showing stomata. Epidermal cells (green) cover the leaf surface. Scattered on the epidermis are stomata (singular: stoma), tiny pores each of which is bordered by two kidney- shaped guard cells. Some of the stomata are open. Guard cells serve to open and close each stoma: opening the pore allows gases to be exchanged by leaf tissues during photosynthesis; and closing the pore at night or during dry periods prevents water loss. In the rose, more stomata occur on the underside of leaves. Magnification: x500 at 6x9cm size. Magnification: x1,520 at 9.5x5inch size.