This is the growing tip of a thick, aerial stilt root of a Rhizophora mangrove (Family Rhizophoraceae) at Port Douglas, Queensland, Australia.
The aerial stilt roots of red mangrove, Rhizophora mangal (family Rhizophoraceae), originate from the lower branches and grow downward into the mud or sand of open seawater, to be submerged at high tide.
Seedlings of Rhizophora mangal (Family Rhizophoraceae), the common New World red mangrove, have taken hold in mud and lava of an Hawaiian mangrove swamp near Hilo.