Protoxylem

Protoxylem is the first-formed portion of the primary xylem in vascular plants, produced during early primary growth from the procambium. It develops in the elongating regions of the axis and represents the portion of xylem that differentiates closest to the tips of the plant.

Protoxylem tracheary elements are typically small in diameter and show simple secondary wall thickening, often annular or helical. These features allow flexibility as the stem or root elongates. In many species, protoxylem becomes crushed or displaced as growth continues, and it is progressively replaced by the later-forming metaxylem elements which differentiate behind it.

Functionally, protoxylem supplies initial water transport in regions that are still expanding. However, as the plant matures and continuous secondary growth proceeds, protoxylem's role diminishes and the larger metaxylem elements assume the major function of long-distance water transport.

Protoxylem is part of the primary xylem and is produced before metaxylem. Its position and structure can vary among species, but it is generally distinguished from metaxylem by its smaller size, earlier development, and thinner, more flexible cell walls. The transition from protoxylem to metaxylem marks the shift from the earliest, elongation-driven phase of xylem differentiation to the formation of a more robust, later-forming water-conducting system.