Stomata are anatomical features that are located on the underside of the leaves of plants that live on land. These structures, which are the 'pores' of the plant's skin, provide openings for the exchange of oxygen and carbon dioxide. Water is also released through the stomata in a process called transpiration. Stomata are opened and closed with cells called guard cells. These cells swell by the process of osmosis when there is an excess of water in the plant. This swelling causes the stomata to open, allowing water to evaporate. When the amount of water within the plant begins to lower below the point necessary for photosynthesis, the guard cells shrink and the stomata close to conserve water.
In addition to the correct level of water, a plant requires carbon dioxide for the process of photosynthesis. The plant also requires a means to release the oxygen that is a byproduct of the photosynthesis process. Stomata offer the plant both a means of entry and exit as the gas transfer takes place. The plant's stomata allow carbon dioxide to enter the plant's leaves to be used in the photosynthesis process. The stomata also provide a means for the elimination of oxygen from the plant after photosynthesis has taken place. When there is ample carbon dioxide in the atmosphere, and abundant light, the stomata open to bring in the carbon dioxide required for the photosynthesis process.
In order for photosynthesis to take place, the plant requires six molecules of water for every six molecules of carbon dioxide. The stomata control both the amount of water present in the plant, and the amount of carbon dioxide that enters. In order to conserve water during dry times, the stomata remain closed to reduce the loss of water vapor. Due to the requirement for carbon dioxide, it is possible for the lack of moisture that forces the stomata to stay closed to prevent the process of photosynthesis from occurring.