Barbara Henry, age 12, of Williamsport, Pennsylvania, for her question:

What causes rings to form around a tree trunk?

Tree rings are alternating ridges of hard and softer wood. They are brought to light when we chop down a tree and slice its wood into strips of lumber. Strips of lumber usually are up and down slices of the trunk. Here we see the ridges in parallel lines, often wavy and sometimes gracefully curved around knotty wads of extra hard wood. They add the artistic grain to wood surfaces. When a trunk is sliced straight across, the ridges present a very different pattern. They are the tree rings, arranged one inside another from the small ones inside to the biggest ones just under the outer bark.

The wood is made from the stiff wails of boxy little plant cells, packed very closely together. The tree builds its woody walls from cellulose, using sugary ingredients manufactured with the help of summery sunlight. As it grows, it widens its trunk by adding rings of new cells around the outer rim. A wide ring of big boxy cells is added during a warm, moist growing season. A ring of small, tightly packed cells is added when growing conditions are less friendly. Counting from the center, you can figure the yearly seasons and the age of the tree.