Guard cells open and close the stomata in a leaf. When the plant has lots of water the guard cells fill with it and become plump and turgid. This makes the stomata open so gases can be exchanged for photosynthesisWhen the plant is short of water, the guard cells lose water and become placid, making the stomata closeThis helps to minimise water loss by preventing water vapour from escaping during gas exchangeThin outer walls and thickened inner walls make the opening and closing workThey're also sensitive to light and close at night to save water without losing out on photosynthesis