The vascular bundles are classified accordingly as per the relative location of xylem and phloem.
(1) Radial vascular bundle: In these types of vascular bundles xylem and phloem tissues occur at alternate radial locations in different groups. This can be seen at the roots.
(2) Conjoint vascular bundles: In conjoint vascular bundles, the xylem and phloem tissues are present on the same radius and only opposed to each other. In dicot stems this is a common phenomenon. Conjoint vascular bundles are of two forms, depending on the number and the location of the phloem group.
(i) Collateral type – Except for the members of Cucurbitaceae and certain members of Convolvulaceae, vascular bundles are of a very specific nature and are found in dicotyledon stems. Cambium can be present or absent in between xylem and phloem patches that open or close the vascular bundle, accordingly.
(ii) Bi-collateral type – Ascular bundles comprise two patches of phloem in the same area on each side of the xylem. The outer phloem remains to the central cylinder periphery and the inner phloem remains toward the middle. Those vascular bundles consist of two patches of cambium. The external cambium distinguishes the outer phloem from the xylem, where the internal cambium distinguishes the xylem from the internal phloem. The outer cambium is concave in shape and much more active than the internal planoconcave cambium strips which are less active.
(3) Concentric vascular bundles: The either xylem surrounds the tissue of the phloem occasionally or vice versa. Such bundles of vascular substances are called concentered vascular bundles. When xylem surrounds the phloem tissue from all sides, the vascular bundle is called amphivasal vascular bundle (leptocentric type). Such bundles are seen after secondary growth in monocot plants like Dracaena. The vascular bundle is called amphicribal vascular bundle (hadrocentric type) when phloem covers the xylem tissue altogether. These vascular bundles occur in pteridophytes such as Selaginella etc.