in which the central core of xylem consists mainly of tracheids interspersed with numerous parenchyma cells. It thus resembles the medulla (*pith
) of a *siphonostele
(hence the name). The medullated protostele shows how the siphonostele may have evolved from a protostele, with the pith originating in the xylem itself. Medullated protosteles are exhibited in some primitive ferns, e.g. Gleichenia.