In the primary structure of stem, root and leaves, the conductive tissue xylem and phloem are grouped together. A group of xylem and phloem forms a vascular bundle. The different types of vascular bundles met within plants are:
1. Radial bundles (Simple): Xylem and phloem are seen as patches and they alternate each other, and occupy the different radii on the axis separated by non conductive tissue.
example: Dicot and monocot roots
2. Concentric: One vascular element surrounds the other on all sides. It is of two types
a) Amphivasal or Leptocentric: In this xylem is seen towards outerside that surrounds the phloem.
Example: Dracena, Yucca
b) Amphicribal or Hydrocentric: Here phloem is seen towards the outer side and xylem towards the inner side. This condition is called amphicribal.
Example: Hydrilla, Selaginella, Pteris
3. Conjoint: Xylem and Phloem on same radii.
a) Collateral: in this type, xylem and phloem are arranged side by side on the same radius. This may be either open or closed. Usually xylem is seen towards the inner side and phloem towards outer side.
a 1)Collateral open: Xylem towards the inner side and phloem towards the outer side. In between xylem and phloem, cambium is present. This type of vascular bundle is collateral open.
example: Dicot stem
a 2)Collateral closed: Xylem towards the inner side and phloem towards the outer side. In between this, there is no cambium. Therefore, the vascular bundle is called collateral closed.
example: Monocot stem
b)Bicollateral: Here two patches of cambium and one patch of xylem at the middle. This types of vascular bundles found in plants of members of family Cucurbitaceae, Solanaceae, Convolvulaceae.