A water-conducting cell in the xylem, i.e. a *vessel element
. The secondary cell walls of the tracheary elements show various types of thickening (see illustration). Annular and spiral patterns of thickening allow further extension of the tracheary element and tend to be found in protoxylem. The other forms of thickening are seen in metaxylem. Although bryophytes do not posses xylem, some do possess water-conducting cells known as *hydroids
. Some consider these are also tracheary elements.
Types of secondary wall thickening in tracheary elements.