The simple answer - xylem cells. Recall that food and water are transported throughout a plant by the vascular system that is comprised of two major tissues, xylem and phloem.
Wood is comprised of the cells of the xylem.
The primary water transport cells are tracheids and vessels. In the final stage of their development, vessels, tracheids, and fiber cells die. The cell contents are reabsorbed and a hollow, rigid cell wall remains. Because xylem cells have thick, rigid cell walls, an additional function of the xylem is support. That's why redwood trees can get so big.