The vascular or fascicular tissue system is derived from procambium and serves as the conducting system of plants.
The vascular tissue system consists of varying number of strands or bundles of tissues embedded in the ground tissue known as vascular bundles. Each vascular bundle is constituted by xylem and phloem.
A strip of meristematic tissue called cambium may or may not be present in between xylem and phloem. When cambium is present the vascular bundle is said to be open vascular bundle and when cambium is absent it is said to be closed.
Open vascular bundles are associated with dicot stem where as closed ones are seen in monocot stem. Further, the vascular bundles are arranged in a ring in the ground tissue of dicot stem, whereas they are scattered in the ground tissue of monocot stem.
Depending upon the organ of plants and their function in the plant body, the arrangement of xylem and phloem differs in vascular bundles. Accordingly vascular bundles are of the following types.
(i) Radial: Vascular bundles have xylem and phloem patches arranged separately on alternate radi. This is seen in roots.
(ii) Conjoint: Vascular bundles have xylem and phloem arranged on the same radius situated opposite to each other. This is seen usually in stems and leaves. Conjoint vascular bundles can again be of two types depending upon the number and position of phloem patches.
(a) Collateral vascular bundles have one patch of phloem and one patch of xylem. They can be either open or closed type. It is a very common type of vascular bundle seen in stems of most dicot except the members of family cucurbitaceous and some members of family Convolvulaceae.
(b) Bicollateral vascular bundle consists of two patches of phloem on either side of a single patch of xylem. There are also two patches of cambium.
The outer patch of phloem lying towards the periphery is separated from the xylem by outer cambium strip. The inner patch of phloem lying towards the centre is separated from xylem by inner strip of cambium.
These vascular bundles are always open. The outer combium is concave and the inner is Plano-concave in appearance. These vascular bundles are found in the stems of the members of family Cucurbitaceous and some members of Convolvulaceae.
(iii) Concentric: These vascular bundles have either phloem surrounding the xylem or xylem surrounding the phloem. This is again of two types.
(a) Amphicribral: – Xylem lies at the centre and is surrounded by a ring of phloem. It is also known as hadrocentric and in common in pteridophytes like Lycopidium, Selaginella etc.
(b) Amphivasal: – Phloem lies at the centre and is surrounded by xylem. It is; known as leptocentric type. Such bundles are seen in certain monocot stem Dracaena.