False-colour scanning electron micrograph (SEM) of open stomata on the surface of a tobacco leaf, Nicotiana tabacum. Stomata are breathing pores scattered over the surfaces of leaves, & sometimes stems; they regulate the exchange of gases between the leaf's interior & the atmosphere. Stomatal closure is a natural response to darkness & to drought (when it is a means of conserving water). Each pore is controlled by two guard cells on either side; when the guard cells are full of water the pore is open; when they lose turgidity, the pore closes. The jigsaw puzzle pattern around the stomata corresponds to the cells of the leaf epidermis. Magnification: x450 at 6x4.5 cm size.