Dear Student, Xylem tissue is also called as wood because in the growth of a plant several factors are involved one of them is when the conducting tissue formed by the lateral cambium which is called “secondary” xylem and phloem to differentiate them from the primary xylem and phloem derived directly from cell divisions of apical meristems. The cells of secondary xylem are shorter than those of primary xylem. As they mature, the walls of these cells become hard and impregnated with lignin. The conducting xylem vessels are not fully functional until they are dead and their cytoplasm has disintegrated, these dead xylem tissues are called wood. That is the reason why xylem is also called wood. Regards