The vascular bundles consist of complex tissues, the phloem and the xylem. In dicotyledonous stems, cambium is present between phloem and xylem. Such vascular bundles, because of the presence of cambium, possess the ability to form secondary xylem and phloem tissues, i.e are open to growth and hence are called open vascular bundles. In the monocotyledons, the vascular bundles have no cambium present in them. Hence, since they do not form secondary tissues they are referred to as closed.