Trees are generation organisms, generating new tissue on the outside of the old tissue with each growth increment – Growth Rings.
The tree trunk, branch or twigs – the woody bits, are called Xylem tissue. All of these are built up around a pith, normally central. Each growth cycle lays down new wood on the outside of the previous years wood. At growth flush the cell division is rapid, producing large open vessels. The light rings in the picture. The slower growing tissue is the darker rings. The length of the growing season determines the width between each growth flush. The environmental conditions determine the length of the growth period – the better the conditions the longer and wider the ring.
When wounding occurs the tissue that has already been laid down reacts to the wounding. The Cambium – the layer of meristematic tissue between the inner bark and the wood, also responds by producing a different type of tissue, Barrier Zone. The Barrier Zone isolates the tissue present from tissue produced after the wounding event. It also produces a different type of wood at the wound site called Wound Wood. Wound wood is the tissue that grows over and may finally close the wound site.