^BOpen stoma. ^bColoured scanning electron micrograph (SEM) of an open stoma on the surface of a tobacco leaf (^INicotiana tabacum^i). Stomata are pores found on the leaf surface 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 (red); the pores open when they are full of water and close when they lose water. On the left is a closed stoma. Magnification: x880 at 10 centimetres wide.