Transpiration is the botanical term for this exchange of gases. Air, containing the carbon dioxide that plants need for photosynthesis, enters the plant in this process by way of small holes on the undersides of the leaves, called 'stomata'. This same process in reverse is how the plant moves oxygen, the by product of photosynthesis, out to release it into the atmosphere. In the transpiration process, plants use a water layer at the cell level to dissolve the carbon dioxide to be carried into the cell, since air can not move through the cell wall directly. Oxygen is moved out through the cell wall dissolved in water in the same manner.