Under a microscope, wood fibers resemble strands of hair. Wood fiber is the main component of paper.

Everyone loves trees. Their tall sturdy trunks, long graceful limbs, and lush canopy of leaves add beauty to our countryside. You can climb trees, build a play house in their branches, picnic in their cooling shade, or just enjoy walking among them. But trees are valued not only for their beauty; they are an essential part of our natural environment, and provide mankind with an important natural resource.

We all know that a tree has a trunk, branches, leaves, and roots. But what's inside a tree? Let's take a look.

The main building blocks of trees are cellulose fibers. Measuring between about 1/32 - 1/4 inch in length (depending upon the species of tree), these fibers are held together in parallel formation with a natural "glue" called lignin. Other natural chemicals, such as sugars, resins, and oils, are also contained within a tree.

Surprisingly, almost all of the material in a tree is nonliving. Only the leaves, the tips of the branches and roots, and a thin layer of cells just under the bark are actually alive. When a tree grows taller, the growth takes place only at the tips of the stems. For this reason, the birdhouse you've mounted on the trunk of a tree will always remain at the same height, and the swing you've hung from a lower limb will never grow too high to reach!

Trees grow in diameter, too, by the division of cells in the outermost layer. Called the cambium, this microscopic layer is only three to four cells wide! As cells in the cambium divide, the old outer bark splits, and cracks develop in the surface of the tree, giving bark its rough texture.

Cross-section of a tree trunk showing outer bark, inner bark, cambium, and annual growth rings. You can find a tree's age by counting the growth rings - one ring for every year.