Pneumatophore

In botany, a pneumatophore is a specialised outgrowth of roots found in some wetland trees. Rising vertically above the ground and anoxic waters, this spongy tissue structure sends oxygen to the rest of the roots.

Pneumatophores on a black mangrove (Avicennia germinans) in a Brazilian marsh. © Ulf Mehlig, Wikimedia CC by-sa 2.5

In zoology, siphonophores, the colonial cnidaria that look like jellyfish, use pneumatophore floats.