Air Cells, hollow spaces within the cellular tissue of the stems, leaves, and other parts of plants, containing air only, the sap and other matters being contained in different receptacles. They most frequently occur in water plants, and very conspicuously in the splendid Victoria regia of the lakes of South America, enabling its rosy leaves to float; and in the Val-lisneria spiralis, of which the male specimens, immersed in the water, rise from the bottom to meet the long-stalked females which stand over the surface. Other receptacles of air are to be found in the cambium (the layer of gelatinous cellular tissue between the wood and the bark) of trees. Here the longitudinal rows of cells become broader, and exhibit in the progress of growth small flat air bubbles between the walls of the contiguous cells; gradually the bubbles become globular or oval, and after the cell walls have increased in thickness, a small canal is formed within the new mass, giving rise to porous vessels. This is readily observable in limes and willows. The air bubbles obstruct the passage of the sap, and thus cause the consolidation of the wood.

The difference between the wood of needle-leafed trees (such as the pine, fir, spruce, larch, etc.) and of broad-leafed trees chiefly depends upon the number of the cells that are converted into porous vessels.