Beech stoma. Coloured scanning electron micrograph (SEM). of a stoma on the underside of a beech (Fagus sylvatica) leaf. 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: x2667 when printed 10cm wide.