The bulkheads structurally support the hull.
A keel
A keel
mast-a vertical spar to support sails on a ship
Mast .
The underside of a mushroom is called the "gill."
I
The underside of a table typically contains support structures such as legs, braces, and sometimes storage compartments.
These are called "stomata".
sorialso sporangiaSori are the structures on the underside of fronds in which the spores of ferns are produced.
plastron
Guiche
It is called a frog.