13. What types of cell form xylem? What are the main features of those cells?

The main cells of xylem are tracheids and vessel elements (these are only in found in angiosperms). Tracheids and vessel elements are dead cells that have lost their cytoplasm. Only their cell wall impregnated with lignin (an impermeable biopolymer) remains. Tracheids form tubes that connect to neighboring tubes through pores. Vessel elements do not contain pores but instead they connect to the following vessel element through perforations at their extremities.