Anatomy of Flowering Plants

In monocots the vascular bundles have no cambium between xylem and phloem.
They cannot form secondary tissues, so they are called closed.
Dicot stems have cambium between xylem and phloem and their bundles are open.

Phellogen (cork cambium) cuts off cells on its outer side that form cork (phellem).
Commercial bottle cork is obtained from the cork of Quercus suber.

In the vascular bundles of the monocot stem (e.g. maize) the lowermost protoxylem vessels and parenchyma break down and form a water-containing cavity.
It is called the protoxylem cavity (lysigenous cavity or lacuna).
Such cavities are not found in sunflower (dicot) or in the gymnosperms Pinus and Cycas.

Companion cells are specialised parenchyma cells closely associated with sieve tube elements in the phloem.
The two are connected by pit fields in their common walls, and the companion cells help maintain the pressure gradient in the sieve tubes.