The air space in a leaf allows communication between the interior and exterior environment of the plant. The air spaces in a plant are called the stomata and the boundary layer.
Stomata are cells on the skin, or epidermis, of the above-ground portion of a plant. These cells allow carbon dioxide to enter the plant and water and oxygen to exit. Stomata are found on soft stems, flower pedals and leaves. Stomata are the pores of the leaf and aid in the process called transpiration, which is defined as the loss of water vapor through a plant's surface. The opening of each stomata is guarded by specialized cells called stoma. When the stoma allow the stomata to open, transpiration increases and when the stomata close, transpiration decreases. Further, stomata help the leaves produce food for the plant via the process of photosynthesis.
In addition to the stomata, leaves maintain a boundary layer of air on their surface. This boundary layer also influences the speed of transpiration since the water vapor that leaves the stomata must pass through the boundary layer to escape into the atmosphere. The boundary layer may differ in depth depending on the plant and the location of the stomata on the leaf. Various plants use other parts of their structure to alter the size of the boundary layer. For example, pubescent leaves tend to have larger boundary layers since the hairs increase the layer of still air in the boundary.