Xylem is vascular tissue found in vascular plants which transports water and minerals. Xylem transports water through tracheary elements, of which there are two kinds: tracheids and vessel elements. Both consist of dead cells once mature. Vessel elements are mostly only found in angiosperms, and so can be considered a newer innovation. It's believed that they allow for more efficient water transport than tracheids because they form an unobstructed tube, where as tracheids have minor obstructions between each dead cell. There are, however, advantages and disadvantages to both.
- ↑ Biology of Plants, 7th ed., Peter H. Raven, Ray F. Evert, Susan E. Eichhorn. W.H. Freeman, 2005