Air Cells, small cavities containing air only, found in the stems and leaves of plants. They are largest and most numerous in water plants such as the lily, the leaves of which are buoyed up by their means. The minute cells in the lungs of animals are also called air cells, and there are curious air cells in the bodies of birds. These are connected with the lungs and are situated in the chest cavity and in the abdomen, and sometimes extend even into the bones. They are most fully developed in birds that have strong, powerful flight, such as the albatross. See AIR.