bio

Tracheid

A dead, hollow water-conducting cell with a lignified secondary wall — the basic unit of xylem in most vascular plants. The dead part is essential: a living cell resists flow, an empty tube does not. The lignified wall keeps the tube from collapsing under the negative tension a plant uses to haul water upward. The appearance of tracheids in the late Silurian and early Devonian is what unlocked height, because a single tissue handled both transport and support.