what are protoxylem lacunae?

The primary xylem cells are known as protoxylem. It occurs at the innermost surface of the vascular bundle in stems and at the outer most surface in the roots. Protoxylem is composed of small parenchyma cells. The space surrounded by the protoxylem cells of the vascular bundle is known as the protoxylem lacunae. This space helps in long distance transport of water and mineral salts.