<P> All tracheary elements 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 contributing to the transport system and providing structural support . The secondary walls have thickenings in various forms--as annular rings; as continuous helices (called helical or spiral); as a network (called reticulate); as transverse nets (called scalariform); or, as extensive thickenings except in the region of pits (called pitted). </P> <P> Tracheids provide most of the structural support in softwoods, where they are the major cell type . </P> <P> 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 embolisms . </P> <P> The term "tracheid" was introduced by Carl Sanio in 1863, originally as Tracheide, in German . </P>

Where do you find tracheids and vessels how do they help the plant