" As the tree matures, the older vessels become plugged with crystalline structures called tyloses, and as a result, the center of the tree—the heartwood—doesn’t conduct water at all, making it well suited for use as a watertight barrel. American oaks are particularly rich in tylose as compared to European oaks. In fact, the European trees have to be carefully split along the grain, rather than cut, in order to avoid rupturing vessels and creating a leaky barrel. The trees also happen to produce an astonishing array of flavor compounds that break free from the wood in the presence of alcohol. European oak, Quercus robur in particular, is high in tannins, which give wine a certain roundness and full-bodied quality. American white oak, on the other hand, releases the same flavor molecules found in vanilla, coconut, peach, apricot, and cloves. "

― Amy Stewart , The Drunken Botanist: The Plants That Create the World's Great Drinks