Linked from
The 8 pages that link to Tracheid, each with the reason it gives.
XylemBroader topic: Tracheids conduct water and provide support in all vascular plants.
WoodBroader topic: Tracheids conduct water and provide support in most conifers.
Vascular tissueBroader topic: Tracheids are water-conducting cells found throughout vascular plants.
ZosterophyllBroader topic: Tracheids formed the conductive tissue in early vascular plant axes.