Tracheids are long cells in the xylem of vascular plants. They transport water and mineral salts. Tracheids are one of two types of elements in the xylem, vessel elements being the other. Tracheids do not have perforation plates; vessel elements do. These define vascular plants, as opposed to non-vascular plants.
All tracheary elements get a thick lignified cell wall. When the plant is mature, the protoplast has broken down and disappeared. Tracheids have two functions. They transport material and provide structural support.
The secondary walls have thickenings in various forms: rings, spirals, networks or as extensive thickenings except where there are pits. Tracheids provide most of the structural support in softwoods, where they are the major cell type.
Because tracheids have a much larger surface area to volume ratio compared to vessel elements, they hold water against gravity (by adhesion) when transpiration is not occurring. This mechanism helps plants prevent air embolisms.
The term "tracheid" was introduced by Carl Sanio in 1863, originally as Tracheide, in German.
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- Sanio, C. (1863). Vergleichende Untersuchungen über die Elementarorgane des Holzkörpers. Bot Ztg 21:85–91; 93–98; 101–111.