Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements will develop a thick lignified cell wall, and at maturity the protoplast has broken down and disappeared. The presence of tracheary elements is the defining characteristic of vascular plants to differentiate them from non-vascular plants.
The two major functions that tracheids may fulfill are
- as part of the transport system
- in structural support
In most cases, the prime function of tracheids is that of transporting water. They occur in vascular bundles throughout the non-woody parts of the vascular plant and provide water and minerals collected by the roots to leaves and other parts of the plant (stem, flowers, fruits etc).
A good example of structural support is in softwoods where tracheids are the major cell type. Tracheids give softwood its strength.
Because tracheids have a much higher surface to volume ratio compared to vessel elements, they serve to hold water against gravity (by adhesion) when transpiration is not occurring. This is likely one mechanism that helps plants prevent air emboli.
- Wilson, K. & D.J.B. White (1986). The Anatomy of Wood: its Diversity and variability. Stobart & Son Ltd, London