Although air-filled intercellular spaces are necessary and ubiquitous in higher plants, little attention has been paid to the possible mechanisms by which these spaces are kept from being flooded. The most likely mechanism is that the living plant cell may maintain a hydrophobic monolayer on the surfaces of adjacent intercellular spaces. The existence of ;apparent free space' in cell walls and the fact that detergent solutions do not enter the intercellular spaces argue against this hypothesis. It is concluded that the mechanism by which these important air spaces are maintained is still unknown.