The some times hazardous "knees"
that project up out of the waters around several species of trees that are
found in swamps or wetlands are called pneumatophors. These striking
features can grow to several feet in length in some
tropical species.
Mangrove and cypress trees very commonly have them growing out beyond the
limbs of the tree and in mangrove swamps the can form a nearly impenetrable
barrier. The exact function of these growths is not firmly established but
some research indicates the are involved in gas exchange for submereged roots.
The mechanism that causes trees to grow pneumatophores
is not understood and trees that can grow "knees" don't always grow them..
And so some trees that are subjected to flooding do develop knees and
some trees don't and sometimes trees that are not flooded grow them. So sometimes
trees that are growing close together may have one growing them and the other
one not. Such as these trees in the picture
below.
Phot by Dennis Olson