Stomata (singular: stoma) are pores on the surface of the aerial parts of most higher plants. The size of the pore is controlled by changes in the turgor of a pair of specialized cells called guard cells that surround the pore. This enables CO2 entry into the leaf for photosynthesis to be optimized, while conserving as much water as possible. Guard cells integrate information from a wide range of environmental signals when formulating the optimal pore size. They have therefore been extensively used as a model cell type for studying how plant cells respond to changes in the environment providing us with a detailed understanding of the mechanisms underlying the control of guard cell turgor.