Welcome to You Ask Andy

Sharon Merritt, age 12, of newell, n.c., for her question:

  How does a tree make rings?

A tree trunk is built of woody circles, one outside another. When the tree is felled, you can count the tree rings on the top of the stump. When the trunk is sliced up and down into boards, the tree rings form the graceful grains of the wood.

As a tree grows, it adds layers of new wood around its trunk. These layers are tree rings, and a new ring is added every season. Every year a tree gets a little Wider around the waist. The newest wood is used by the tree to tote water up from the ground. The oldest wood in the heart of the trunk is no longer used. Your eyes are sharp enough to see the tree rings on a newly cut tree stump. But You need a hand lens or a magnifying glass to see that the rings are bundles of boxy cells stacked neatly together in an upright position. This woody tissue is called xylem. The outer rim of the tree is wrapped in a sleeve of rough bark. Between the bark and the xylem, there is a layer of cells which is much too fine For your eyes to see. It is called the cambium, and you need a microscope to see it.

The cambium layer is usually only one cell in thickness, and, like the bark, it covers the entire trunk in a closely fitting sleeve. The twigs are also made from xylem, cambium and bark cells.

The cambium cells are the growing cells. When conditions are right, they are always ready to divide and multiply. The new cells produced on the inside of the cambium are woody xylem cells. The new cells produced on the outer edge of the cambium are bark cells.

The rate at which the cambium produces new cells depends upon the growing conditions. In warm weather, with plenty of moisture and sunshine, the new cells are large and their walls are thin. When the weather is cold and dry, the cambium produces small, hard walled cells. Each year, with the changing seasons, the tree adds a ring of soft, paler xylem and another of hard, darker xylem. The different xylem cells form a tree ring  and a tree with a hundred tree rings has lived through a hundred years.

The first tree rings were made when the young tree was a shortie. As it grew taller, the new sleeve of xylem grew taller. The first rings could no longer tote the sap to the top of the tree, and the wood at the center of the trunk died. Sometimes the central heartwood rots away, and an old tree becomes hollow. But every year, a new ring of xylem is added around the trunk, and these living cells carry nourishment up and down the tree. 

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