What transforms into xylem as a tree matures?

The correct answer is the pith. In a young tree, the pith is the central part of the stem, primarily composed of parenchyma cells that store nutrients and help in the initial growth of the tree. As the tree matures, the pith does not directly transform into xylem; however, it is involved in the tree's growth and development.

Xylem is the tissue responsible for water and nutrient transport upward through the tree, and it primarily forms from the vascular cambium, a layer of meristematic tissue that produces new xylem and phloem as the tree grows. While the pith itself is not the source of xylem, it is important to note that as a tree develops, the surrounding layers, including cambium, produce xylem. The dynamics of tree growth involve both new xylem production and the transformation and aging of existing cells; thus, the pith plays a role in the overall structure but is not converted into xylem itself.

The heartwood and sapwood comprise mature xylem, with heartwood being the inner, older wood that no longer conducts water, while sapwood is the younger, active xylem involved in water transport. Understanding