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.