Why are the fibrovascular bundles of mono cots called closed bundles?

The collateral open vascular bundles of dicots display xylem and phloem that are separated from each other by the fascicular cambium. These vascular bundles are also often surrounded by abundle sheath and the whole structure is imbedded in a parenchyma tissue called pith. Another type of vascular bundles are the bicollateral bundles.