Forsythia stomata. Polarised light micrograph of a section through tissue from a lily of the valley (Convallaria majalis) plant, showing the stomata (round). Stomata are pores that open and close in order to regulate gas exchange in a plant. The aperture of each stoma is controlled by a pair of specialised cells known as guard cells (sausage-shaped). These cells swell to close the pore and shrink to open it. Stomata are found mainly on the underside of leaves. Magnification: x200 when printed 10 centimetres wide.