When you look at an oak tree, or any tree in a forest, you see only part of it. An oak tree's roots, reaching down into the soil from the base of the trunk, number many more than its branchesperhaps five or six times more. The very foundations of a forest floor are its roots.
Scientists don't know the size of an average tree's root system because no one has ever been able to dig up an entire system. Not only do the roots go deep and far, there are millions of them.
If the roots of a large oak tree could be placed end to end, they would stretch for hundreds of miles!
The thick, brown roots you may have seen on or near the surface of the soil are old and mostly provide supportone of two important jobs performed by roots. Trees are naturally top-heavy and, like tall buildings, they need a strong foundation to anchor them and keep them from toppling over.
Thin roots branch off from thick ones, and still thinner rootlets branch off from them. The rootletsno thicker than string at their growing tipsare covered with thousands of tiny hairs, each one the width of a single cell. These root hairs absorb water and minerals. That's the second important job performed by roots.
Water goes up through the trunk to the leaves, where some of it is used to make food for the tree. Much of the rest of the water evaporates. Hundreds of gallons of water may evaporate from a single tree on one warm, sunny day. To make food and to keep from drying up, a tree needs a constant supply of water. Since young roots and root hairs are the only part of a tree that can absorb water, they have to do a very good job, and there have to be a lot of them.
You can see a root system for yourself by digging up a small weed. Dig in a circle at least a few inches from the weed's stem and down as deep as your trowel will go. Put the weed and all the soil you dug out with it in a shallow pan filled with water. As the water loosens the soil from the roots, you'll be able to see what a large root system even a little weed has. You could put the weed in a glass of water and watch to see if the roots grow during the next few days.
If you want to see root hairs, place some radish seeds on a damp paper towel in a bowl. Cover the bowl with plastic wrap to keep the towel moist. You'll probably need to sprinkle the towel with a little water every day. In a few days, the root should start to grow. It will look fuzzy because of all its root hairs.

BONUS: Color the Roots

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