What is the cavity found in vascular bundles of monocot stems?

The cavity found in vascular bundles of monocot stems is called a lysigenous cavity or protoxylem lacuna. This cavity forms in the position where the first-formed xylem elements (protoxylem) were located. As the stem matures, these early xylem vessels, which developed when the stem was still elongating, become stretched, crushed, and eventually disintegrate, leaving behind an air-filled space. This lysigenous cavity is created by the breakdown and dissolution of protoxylem cells rather than by the separation of cells (which would be schizogenous). The presence of this cavity is a diagnostic feature of many monocot stems and can be observed in cross-sections of stems from plants like corn, wheat, and many other grasses and monocotyledonous species.