Stoma are tiny pores found on the underside of most leaves, though species such as grasses have them on both leaf surfaces. They are used for absorbing carbon dioxide from the atmosphere, so that the plant can photosynthesise, but an unfortunate consequence is that the plant loses a lot of water through stomata. To control this water loss the plant has 'guard cells' beside the stomata which can open and close the pores in response to the state of the plant.
