metaxylem

Metaxylem is a component of the primary xylem in vascular plants and forms after protoxylem during the differentiation of vascular tissue. In most angiosperms, metaxylem constitutes the inner portion of the primary xylem within the stele, located interior to the protoxylem.

Development and anatomy: Metaxylem originates from the procambium and differentiates as the organ lengthens after the protoxylem. The elements of metaxylem are relatively large-diameter tracheary elements—vessel elements in many angiosperms or tracheids in other groups—with thickened walls and bordered pits. They mature later than protoxylem elements and become the main conduits for water transport as the plant grows.

Function and significance: Metaxylem conducts water and minerals from roots to shoots and provides some mechanical support. As the inner, more mature xylem, it remains functional through much of the plant’s life, supporting continued transport after primary growth.

Distribution and variation: In roots, protoxylem develops near the tip and metaxylem differentiates interior to it; in stems, metaxylem lies interior to protoxylem within the primary xylem. The arrangement of protoxylem and metaxylem varies among plant taxa, reflecting different patterns of primary vascular development.