Open stoma on the surface of a leaf, 3D computer reconstruction from electron microscope images. Stomata are pores found on the surfaces of plant leaves that regulate the exchange of gases between the leaf's interior and the atmosphere. Stomatal closure is a natural response to darkness or drought as a means of conserving water. Each pore is controlled by two guard cells; the pore (centre) opens when the guard cells are full of water and close when they lose water.