Roots are organs that provide structural stability for trees.  Roots also absorb water and minerals.

Root Structure

Roots are made up of a number of specialized components.  The root hairs, tiny structures extending from the main root stems, have very thin  walls which absorb water and minerals.  This mineral solution is passed into the vascular core of the root from where it is transported throughout the tree.  At the tip of the root, there exists a protective structure called the root cap.  These loose cells are shed as the root grows into the soil.

Different trees have slightly different root systems.  Some trees, such as the pine, have a strong central root called the taproot.  This is usually larger than any other roots and often extends deep into the ground.  Because substantial damage to this root can be fatal to the tree, trees with taproots are generally difficult to transplant.

Other trees, such as the elm or maple, do not have a dominant taproot.  Their root systems are characterized by a large number of roots often closer to the surface.

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Root Growth

Generally, root growth is influenced by moisture and gravity.  In other words, unless there are substantial amounts of moisture near the surface, roots tend to grow downwards through the soil.  

Roots are always growing and, like a tree's trunk, they grow both longer and wider.  At the tip of the roots, the growing region is called the meristem.   This is where most of the lengthwise growth takes place.  In addition to this, wood is added to the inside of the root and phloem is added towards the outside.  

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Source: MS Encarta online encyclopedia

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