A long life multiplies a tree's reproductive opportunities, thus increasing its fitness. Therefore, characteristics that prolong life should be naturally selected. Longevity in trees is achieved by means of numerous behaviors and characteristics, some of which are unique to trees. These include the retention of stem-cell-like meristematic cells after each growth cycle; the ability to replace non-vigorous, lost, or damaged organs, both above and below ground, in the presence or absence of trauma; a sectored vascular system that allows part of a tree to survive where a whole one cannot; formation of clones; a mechanical structure that can react to forces tending to de-optimize it; a hormonal control system that coordinates the above behaviors; and synthesis of defensive compounds.