Here xylem and phloem occur in separate bundles and lie on different radii alternating with each other. These are characteristics of roots.
When xylem lies inwards facing pith and phloem lies outwards facing the cortex, the vascular bundle is collateral. In gymnosperms and dicot stem cambium is present in between xylem and phloem such a vascular bundles is called open. The vascular bundles of monocots do not have cambium and are called closed.
These have phloem on both sides of xylem. Usually a strip of cambium is present outside and inside of the xylem. They are characteristic of family Cucurbitaceae and Solanaceae.
Amphivasal. Here phloem is surrounded by xylem. It is seen in some monocots. Although collateral bundles are most common in seed plants, a number of monocot species have amphivasal bundles instead, or have amphicribral ones (in which phloem surrounds xylem).