Sugar produced at a source must be loaded into sieve-tube members before it can be translocated to a sink. Porous cross walls called sieve plates are in between the members and allow phloem to move freely along the sieve tubes. The cells are dead at functional maturity. Tracheids are long, thin, tapered cells having lignin-hardened secondary walls with pits. Water flows from cell to cell through the pits, and they also function in support.
Vessel elements are wider, shorter, thinner-walled, and less tapered than tracheids.
Bulk-flow, as a result of transpiration, pulls water through these structures.