Stomata. Coloured scanning electron micrograph (SEM) of the underside surface of a leaf from an Elder tree, Sambucus nigra, showing an open stomata. Stomata (stoma) are breathing pores which regulate the exchange of gases between the leaf's interior and the atmosphere. Stomatal closure is a natural response to darkness or drought. Each stomata is controlled by the water content within a pair of guard cells flanking its central pore. When these guard cells increase in water content and turgidity their inner walls bow away from each other and the stomata widens. Conversely when the guard cells lose water the stomata closes. Magnification: x715 at 6x4.5cm size.