Wood forms in trees during five distinct and successive biological processes: cell division, cell elongation, cell wall thickening, cell death and heartwood production. The key to wood formation lies in the different vascular systems in plants - harder xylem cells replicate faster than softer phloem cells - which is why trees have more wood than other materials.
The formation of wood starts in what is called the vascular cambium, the part of the tree that moves nutrients toward the inner part of the plant. The vascular cambium is part of what forms rings on trees. The outer part of the vascular cambium is softer phloem, whereas the inner part is xylem, better known as wood. When nutrients cross from phloem to xylem, the cells become elongated and thicker. These longer, thicker cells become wood.
When elongated cells reach a certain point, the organism programs these cells to die by releasing substances called hydrolases. This process intentionally kills the cells while keeping their walls intact. These walls give the wood its three-dimensional structure.
As of August 2014, the overall process of wood formation was not fully understood. However, scientists have begun isolating genes and enzymes that lead to the conversion of live plant cells into wood.