Douglas Packard, age 12, of Niagara Falls,Ont., Canada, for his question:

DOES A TREE TRUNK GROW FROM THE INSIDE OR OUTSIDE?

A seedling tree builds the very first cells of its woody trunk. Its baby twigs contain the first woody cells for its future branches. Later growth is added in rings around rims, and twiggy new growth is added at the tips. But no new growth is added to the oldest wood in the middle.

The special cells that produce new growth are called the cambium, which forms a layer just below the bark. During the growing season, the cambium adds cells to the bark and a new layer of tough boxy cells to the wood. Each year a slightly larger ridge of new wood is added around the outside of the trunk  and by counting the rings we can tell the age of the tree.