What's elastic and grows on trees? Bark, if it's a birch tree.

The bark stretches as the birch grows while the bark of other trees breaks into a rough texture, according to Georgia-Pacific Corp., a forest products firm.Birches are white, because their bark doesn't contain tannin - a compound that gives a dark coloring to most bark. The tree's wood, which is hard yet flexible, often is used for interior woodwork.