Brian Bridson, age 10, of Lansing, Mich.i for his questions

What is the xylem of a tree, trunk?

Xylem is a word which comes right out of the world of science. In everyday language, we call the xylem of a tree trunk just plain wood. When we look closely at a piece of wood, we see that it is marked with graceful lines, grooves and sometimes whorls. These patterns are called the grain of the wood and fine furniture wood is treated so that the grain shows up to best advantage.

With a hand lens or a microscope we get a closer look at the grain in a piece of wood. We see that the ridges and grooves are made of tiny boxlike sections all fitted together in a perfect packing job. Every part of a plant is made of cells of some sort, just as every part of your body is made of tiny individual cells all fitted together. The cells which make the wood of a tree trunk are called xylem cells. This word was coined from an older Greek word for wood.

The xylem cells in a tree trunk come in different sizes and some have thicker walls than others. If we look closely., we see that the assorted sizes are arranged in order. A cross section of the trunk shows that they are arranged in circles, one inside another, The circles form bands of lighter and darksr wood. They form the tree rings by which we can tell the arse of a tree.

In a normal year, a tree adds a new circle of xylem cells clear around its trunk. Growth begins in the spring when water and sunshine are plentiful. The tree takes in plenty of nourishment and this is used to grow xylem cells at a great rate. Growth takes place in a layer of cells called the cambium, just under the bark. The cambium cells grow new xylem cells on the inside and new bark cells on the outside of the trunk.

The xylem cells which grow in the spring are large and thin walled. These new cells help carry moisture up and down the tree. When spring is ended they have formed a new ribbon of cells clear around the tree trunk. They will continue to add to their numbers throughout the summer. But growth slows down after the spring rains are over. The xylem cells which form 1 ater are smaller and they have thicker walls e

All xylem cells have sturdy woody walls4 But those that grow later in the year are the sturdiest. The z ibbon of spring cells tends to be softer than/ring of cells formed later in the year. They are also denser, which makes the later ring of wood look darker.

In every growing season, the tree adds a ring of lighter xylem and a ring of darker xylem around its trunk. Together, these bands form a tree ring. By counting the rings of spring and summer xylem, we can tell how long the tree has been living.