Corks are made from the bark of oak trees.
lemme google...
here you go..
From
http://www.thewinedoctor.com/advisor...rvecorks.shtml
Corks and Cork Alternatives
The vast majority of corks are made from the bark of the oak tree Quercus suber, and come from the oak forests of Portugal. There are alternatives to the standard cork, however, the motives behind these alternatives being in some cases financial, and in some cases a quest for quality and consistency.