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

Answer 1

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.

Answer 2

In monocots the vascular bundles are without a fascicular cambium, hence these are called closed vascular bundles. In dicots, the vascular bundles, invariably have a layer of cambium between xylem and phloem and are called open vascular bundles.