Polarised light micrograph of stomata (singular stoma) on the epidermis of a leaf of Araucaria. The opening and closing of each stoma is controlled by a pair of crescent shaped guard cells which control the intake of carbon dioxide and the expulsion of oxygen and water, thus facilitating photosynthesis. The guard cells are flaccid when closed and turgid when open. Magnification: x16 at 10cm wide.