- a specialized structure developed from the root in certain plants growing in swamps and marshes, serving as a respiratory organ.
- the air sac of a siphonophore, serving as a float.
- a specialized root of certain swamp plants, such as the mangrove, that branches upwards, rising above ground, and undergoes gaseous exchange with the atmosphere
- a polyp in coelenterates of the order Siphonophora, such as the Portuguese man-of-war, that is specialized as a float
