<P> A vascular bundle is a part of the transport system in vascular plants . The transport itself happens in vascular tissue, which exists in two forms: xylem and phloem . Both these tissues are present in a vascular bundle, which in addition will include supporting and protective tissues . </P> <P> The xylem typically lies adaxial with phloem positioned abaxial . In a stem or root this means that the xylem is closer to the centre of the stem or root while the phloem is closer to the exterior . In a leaf, the adaxial surface of the leaf will usually be the upper side, with the abaxial surface the lower side . This is why aphids are typically found on the underside of a leaf rather than on the top, since the sugars manufactured by the plant are transported by the phloem, which is closer to the lower surface . </P>

Difference between open and closed type of vascular bundles