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.
