Xylem and phloem use very different cells to transport their sap because of the different driving force used and how it is created. Xylem can use dead cells since the driving force (tension) is created by water loss from the living cells of the leaf while the pressure to drive phloem transport must be generated in the living phloem cells by their loading sugars, which pressurize when water diffuses in, following the sugars.
Work Step by Step
The key here is to review phloem and xylem transport and to compare the driving forces involved. The driving force is always generated in some way involving living cells, but the xylem cells can be dead since the force is generated not in them but in cells connected to them by the bulk flow of water.