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tracheid
Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. Tracheids, unlike vessel elements, do not have perforation plates.
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).
Tracheids provide most of the structural support in softwoods, where they are the major cell type.
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.
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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