There are four types of primary xylem development: exarch, endarch, centrarch, and mesarch.
In endarch development, the protoxylem begins its development from the innermost procambial cells located adjacent to the pith and development progresses outward. Therefore, the protoxylem is found toward the inside and metaxylem toward the outside of the stem. Endarch development is considered the most highly advanced type of primary xylem development.