Tracheae, a term applied, strictly in vegetable histology to true vessels or cell-fusions in which the transverse walls have been absorbed. They occur in the wood of many spermaphytes. Elongated cells with lignified walls, losing their cell-contents (cytoplasm) at an early stage, and otherwise resembling tracheae are termed tracheids, the two being classed together as tracheal tissue. Tracheids are especially characteristic of the wood of Pteridophyta and Coniferae. Tracheal tissue serves chiefly for the conduction of water.