Vascular bundles

In monocots the vascular bundles are conjoint and collateral.
There is no cambium between xylem and phloem, so there is no scope for secondary growth; such bundles are called closed.

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.