Closed vascular bundles lack

In dicot stems, cambium is present between phloem and xylem. Such vascular bundles, because of the presence of cambium, posses the ability to form secondary xylem and phloem tissues and hence, are called open vascular bundles. On the contrary, vascular bundles in monocots have no cambium. Hence, they do not form secondary tissues, and are referred to as closed.