Why do the cells of phloem need to be living whereas the cells of xylem do not?

There is a layer of living Phloem and there is a layer of living xylem. Both these layer produce dead tissue, the Phloem produces bark (dead protective outer layer), the xylem produces wood (dead supportive inner layer). Both the wood and the bark are dead. It is only the two thin layers of Phloem and Xylem that are alive. ... more