sentences:  Xylem tracheids, wider cells with lignin-reinforced cell walls that were more resistant to collapse under the tension caused by water stress, occur  in more than one plant group by mid-Silurian, and may have a single evolutionary origin, possibly within the hornworts, uniting all tracheophytes.Alternatively,  they may have evolved more than once.Much later, in the Cretaceous, tracheids were followed by vessels in flowering plants.