According to the arrangement of xylem and phloem, the vascular bundles are of following types:
(i) Radial: The xylem and phloem form separate bundles and lie on different radii, alternating with each other. Radial bundles are the most primitive type of vascular bundles and are characteristic of roots.
(ii) Conjoint: The vascular bundles which contain both xylem and phloem are called conjoint vascular bundles. The xylem and phloem are thus, arranged on the same radius.
Conjoint bundles are of following three types:
(a) Collateral: A conjoint bundle with xylem towards the inner side facing the pith and phloem towards the outer side facing the cortex, is called collateral bundle. When in a collateral bundle, a strip of cambium is present between xylem and phloem, the bundle is called open bundle e.g., dicot stems. When a collateral bundle is without a strip of cambium, it is said to be closed, e.g., monocot stems.
(b) Bicollateral: When a conjoint bundle has phloem both on the outer and inner side of xylem it is called bicollateral bundle. Usually a strip of cambium is present on both outer and inner sides of xylem. e.g., family Cucurbitaceae.
(c) Concentric: A type of conjoint vascular bundle with one vascular element completely surrounds the other. Either the phloem surrounds xylem or xylem surrounds the phloem.
