Leaves have features that prevent uncontrolled water loss such as cuticles and wax layers. Because these impermeable layers also prevent diffusion of carbondioxide into the leaf air space plants have developed structures that act like valves and control the flow of water vapour and carbondioxide: stomata. The central opening, called pore or aperture, is surrounded by two guard cells that contain chloroplasts. The guard cells can swell (increase in turgor) and shrink (decrease in turgor), thereby regulating the size of the stomatal aperture. In this lecture we have a look at the principles that determine the pore size, and the mechanics behind it.