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.