If most of the parts of Xylem tissue are dead, how do they function?
The function of xylem can be carried out efficiently since most elements of xylem are dead. Xylem transports water from roots to leaves. The tracheary elements of xylem are completely dead at maturity, and act like pipes (by having hollow, dead cells with no end walls) to allow water and dissolved minerals to flow through them. The development of a xylem tracheary element consists of a thickening reinforcement of the cells walls, followed by a controlled death of the cell. The reinforced, empty cell wall then acts like a pipe for water to flow through.
Because xylem is under tension, rather than under pressure, there is no tendency for xylem contents to move out of the xylem. Therefore, there is no need for a membrane to keep the xylem contents in. And being dead, there is no cytoplasm to interfere with transport.