Types of vascular bundles

According to relative position of xylem and phloem, the vascular bun­dles are classified variously.

(A) Radial vascular bundle

In these types of vascular bundles, xylem and phloem tissues occur in separate groups on alternate radial positions. This is seen in roots.


(B) Conjoint vascular bundles

The xylem and phloem tissues are present on the same radius and just opposed to each other in conjoint vascu­lar bundles. It is a common occurrence in dicot stems. Depending on the number and position of phloem group, conjoint vascular bundles are of two types: collateral type and bi-collat­eral type.

Collateral vascular bundles are very common type and seen in stems of dicotyledons except the members of Cucurbitaceae and some members of Convolvulaceae. Cambium may be present or absent in between xylem and phloem patches making the vascular bundle open or closed respectively.

Bi-collateral vascular bundles con­tain two patches of phloem on either sides of the xylem on the same radius. The outer phloem or external phloem remains towards the periphery of the central cylinder and inner or internal phloem remains towards the centre.


However, there are two patches of cambium found in these vascular bundles. The outer cambium separates outer phloem and xylem, where as the inner cambium separates the xylem and the inner phloem. The outer cambium is concave in shape and is more active than the inner planoconcave cambium strip which is inactive or less active

(C) Concentric vascular bundles

Sometimes, the either xylem sur­rounds the phloem tissue or vice versa. Such vascular bundles are called con­centric vascular bundles.

When xylem surrounds the phloem tissue from all sides the vascular bundle is called amphivasal vascular bun­dle or leptocentric type. Such bundles are seen in monocot plant like Dracaena after secondary growth. When phloem surrounds the xylem tissue entirely, the vascular bundle is called amphicribal vascu­lar bundle or hadrocentric type. Such vascular bundles are seen in pteridophytes like Lycopodium, Selaginella.