Xylem and phloem together form the tissue system called vascular bundle. The tissue that transports water, mineral salts and food in the plant body is called vascular bundle or transport tissue system. The region of root and stem that contains vascular bundles is called stele. There are three types of vascular tissue. Connected, oriented and centered.
1. Conjoint vascular bundles: The vascular bundles in which xylem and phloem are connected at the same radius are called conjoint vascular bundles. The connected vascular bundle is divided into two parts. Equilateral and Equilateral.
(i) Collateral: A vascular bundle in which one segment of xylem and one segment of phloem lie side by side on the same radius is called collateral vascular bundle. In plant stem, phloem is at the top and xylem at the bottom. But in leaves, xylem is at the top and phloem at the bottom. On the basis of the presence or absence of cambium, lateral vascular bundles are divided into two. Open parallel and closed parallel.
Open Collateral: If there is dividing tissue called cambium between xylem and phloem of collateral vascular bundle, it is called open collateral vascular bundle. It is present in the stems of dicots (except pumpkins) and gymnosperms. It is present in plants like Cycus, Neetam, Suryamukhi, Shwetdrona, Raktadrona etc.
Closed Collateral: If there is no dividing tissue called cambium between the xylem and phloem of the collateral vascular bundle, it is called a closed collateral vascular bundle. E.g. Stems of monocots (Bhutra, Kachu).
(ii) Bicollateral: A vascular bundle that has xylem in the middle and phloem on both sides is called a bicollateral vascular bundle. These vascular bundles have xylem in the middle and exocambium on the outside and endocambium on the inside. Correspondingly, there is exophloem outside the exocambium and endophloem inside the endocambium. Dicotyledonous plants have equilateral vascular bundles in their stems. For example, gourd, pumpkin, cucumber etc.
2. Radial: The vascular bundle in which the xylem and phloem do not stay together and form different bundles are called radial vascular bundles. Roots of plants contain auric vascular bundles. The number of xylem or phloem bundles is more than five in the roots of monocotyledonous plants and less than five in the roots of dicotyledonous plants.
3. Centric: The vascular bundle in which the xylem is in the center and the phloem surrounds it or the phloem is in the center and the xylem surrounds it is called a centric vascular bundle. It is of two types.
(i) Hadrocentric : A vascular bundle in which xylem is in the center and phloem surrounds it is called hadrocentric or amphicribal. Like- Lycopodium, Selaginella, Pteris, Psilotum etc.
(ii) Leptocentric: A vascular bundle with phloem in the center and xylem surrounding it is called leptocentric or amphivascular. Such as Dracaena, Yucca etc.
Function of Central Vascular Bundle: Xylem transports water and mineral salts. Phloem transports food. It gives strength to the plant. Provides mechanical energy to plants.