Stomata on the underside of an apple tree leaf. Enhanced scanning electron micrograph (SEM) of the lower leaf surface of an apple tree leaf, showing the stomata. The leaf's surface is covered with epidermal cells. Scattered among the epidermis are stomata (singular: stoma), tiny pores each of which is bordered by two kidney-shaped guard cells. Guard cells open and close each stoma. Opening the stoma allows gases to be exchanged by leaf tissues during photosynthesis while closing the pore at prevents water loss. Magnification: w=140.0 micrometers.