In vascular plants, the major conducting tissues are xylem and phloem. The xylem tissues may be classified as primary or secondary depending on where they are derived. The primary xylem is the type of xylem produced from the primary growth. It differentiates into two parts: (1) protoxylem and (2) metaxylem. The protoxylem is the first one to form followed by the metaxylem (thus, the name). Both protoxylem and metaxylem are found in the primary vascular bundle, and involved in the conduction of water and minerals towards the apical shoot. They are produced by the procambium. Both of them contain tracheids, vessels, and parenchyma. However, the metaxylem has more tracheary elements than the protoxylem. Furthermore, the lumen of tracheary elements of the metaxylem is larger than that of the protoxylem. This makes the metaxylem more efficient in conduction than the protoxylem. The metaxylem also has few fibers, which are lacking in protoxylem. It is not capable of stretch (unlike protoxylem). However, it is not subjected to strain or stress.
Word origin: meta- (“later”) + xylem, from Greek xúlon (“wood”)