According to the arrangment of xylem and phloem, the vascular bundles are of following types :
(a) 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.
(b) 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:
(i) Collateral : A conjoint bundle with xylem to-wards the inner side facing the pith and phloem towards the outside 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 with¬out a strip of cambium, it is said to be closed, e.g, monocot stems.
(ii) Bicollateral: When a conjoint bundle has phlo-em both on the outer and inner side of xylem, usually a strip of cambium is present on both outer and inner sides of xylem. e.g, family Cu- curbitaceae.
(iii) Concentric : When one kind of vascular tissue (xylem or phloem) forms a solid core, while the other surrounds it completely on all sides, the vascular bundle is called concentric. Concentric bundles are of two types :
(i) Amphivasal or Leptocentric : Phloem lie in the centre of the vascular bundle and is completely surrounded by xylem e.g., Dracaena, Yucca etc.
(ii) Amphicribal or Hadrocentric : Xylem forms
are central core, while phloem surrounds it on all sides. It occurs in some aquatic angiosperms and staminal bundles of anthers.