Anatomy of Flowering Plants

In dicot stems, a strip of cambium (intrafascicular cambium) is present between the xylem and the phloem of each vascular bundle.
Because of this cambium the bundle can form secondary xylem and secondary phloem; such vascular bundles are called open vascular bundles.
In monocots the vascular bundles have no cambium and cannot form secondary tissues; they are called closed.
Conjunctive tissue is the parenchyma between xylem and phloem patches in roots, and the terms open/closed have nothing to do with the pericycle or endodermis.
Hence open vascular bundles are those capable of producing secondary xylem and phloem.