Vascular bundles in monocotyledons are considered closed because:
There are no vessels with perforations
Xylem is surrounded all around by phloem
A bundle sheath surrounds each bundle
Cambium is absent
Vascular Bundles -
Xylem, phloem and cambium are part of vascular bundles and may be following type:
Radial- When xylem and phloem are arranged on different radii alternating with each other. Example: Roots The development of xylem is centripetal or exarch.
Conjoint- Xylem and phloem are present on the same radius. It can be collateral or biocollateral.If cambium is present between xylem and phloem it is called open. It cambium is absent it is called closed.