| A tracheid is a single elongated cell with pointed ends and a secondary, cellulosic wall thickened with lignin inside the xylem of vascular plants and having it makes them different from non-vascular plants.
Tracheids are one of two types tracheary elements, vessel elements are the other.
In living cells, simple pits are holes in the secondary wall and it is to let tracheids "communicate" plant signals via plasmodesmata. Pits between tracheids have no plasmodesmata (the cell are dead and have no cytoplasm), but they do contain a pressure valve, a ball of lignified tissue (the torus). If the pressure in an element becomes too high, it will force the torus trapped between the two rings of thickened tissue into the hole, blocking it.
Pits between tracheids and parenchyma cells in the medullary rays are termed window pits, and allow water to enter the living storage parenchyma. |