The cork oak, is a medium-sized, evergreen oak tree. It is the primary source of cork for wine bottle stoppers and other uses, such as cork flooring. It is native to southwest Europe and northwest Africa. For use in wine, the cork stopper has unique qualities, which interact beneficially with wine. The secret is its complex structure. Each cork stopper is a sophisticated natural valve, made up of around 800 million tiny cells of suberin (almost 40 million cells per cubic centimeter), filled with an air-like gas. When compressed, the corks behave like a cushion and constantly try to return to their original shape and size, thus adapting to the bottleneck and maintaining a tight seal. It is the air cells that allow a tiny amount of oxygen to permeate the bottle after it has been sealed, which has a beneficial impact on the development of the wine.