Stomata on the underside of an Ash (Fraxinus excelsior) leaf. 0.16 millimetres across in real life. Seven stomata are visible, pores in the surface of the leaf that allow gas exchange to occur. The rate of gas exchange is controlled by a pair of specialised guard cells that flank each pore, swelling or shrinking as required to alter the aperture. Carbon dioxide enters through the stomata and is used in photosynthesis to produce energy for the plant. Oxygen is released into the atmosphere as a by-product of this process, central to our very existence. Water vapour is released through the stomata, driving the process of transpiration in which water is drawn upwards through the plant’s tissues against gravity by capillary action. Peak District National Park, Derbyshire, UK. False-coloured scanning electron micrograph, produced at the University of Derby. 0.16 millimetres across in real life, x625 magnification when printed at 10cm across.