Stinging nettle leaf. Coloured scanning electron micrograph (SEM) of a section through a freeze-fractured nettle (Urtica dioica) leaf, showing its cell structure. One pair of guard cells (lower centre) have fractured showing the stoma (pore) in cross section at their centre. Stomata are opened and closed in order to control gas exchange. Magnification x720 when printed 10 centimetres wide.