In the collateral vascular bundle, xylem and phloem lie together on the same radius, xylem being internal and phloem external in position. In this case, the phloem occurs on one side of the xylem strand. In a dicot stem, the collateral vascular bundle contains a strip of cambium between the xylem and phloem such vascular bundles are called open vascular bundles. However, the cambium strip is not found in the vascular bundle of the monocot stem. Such vascular bundles are called closed vascular bundles. Thus, the vascular bundle in the dicot stem is conjoint collateral and open and in the monocot stem, it is conjoint, collateral, and closed.
There are other three types of vascular bundles. They are:
- Bicollateral Bundles: Bicollateral bundle is a vascular bundle that has the phloem situated on the peripheral as well as the inner side of the xylem.
- Concentric Bundles: Concentric Bundle is a vascular bundle in which one type of vascular tissue surrounds the other type. In this, the phloem either encircles or is encircled by the xylem.
- Radial Vascular Bundle: Radial Vascular Bundle is a vascular bundle, in which the strands of the primary xylem and primary phloem, are separated from each other by tissues that are not vascular; and they occur at alternate radial locations. This can be significantly seen at the roots.