Within each bundle, the phloem is generally located toward the periphery of the stem surface. The xylem occupies a more interior position.

Within each bundle, the phloem is generally located toward the periphery of the stem surface. The xylem occupies a more interior position. Within the xylem there are usually two big thick-walled vessel elements (a cell type), and also often an irregular shaped hole, which can be seen particularly well in corn stems. How does this hole arise, i.e., where does it come from?