In addition to the xylem and phloem, vascular tissues called cambium are present between them that create new vascular tissues as plants grow. There are open vascular bundles when a vascular cambium layer (in dicots) is present. Those devoid of it (in monocots) are closed vascular bundles. Thus, vascular bundles typically represent the organization of xylem and phloem and their association with other accessory transporting tissues. The vascular bundle arrangement differs in monocot and dicot plants.

Arrangement of Vascular Bundle

Functions of Vascular Bundle

Different Types of Vascular Bundles in Plants

In Roots

They have a radial vascular bundle. In monocot roots, both xylem and phloem are arranged in a circular pattern around the central pith, consisting of ground tissue (parenchyma). Monocots do not have vascular cambium. In contrast, the xylem in dicots is located in the middle, and phloem bundles are arranged around it, separated by the vascular cambium.

In Leaves

Like stems and roots, the leaf also contains vascular bundles composed of xylem and phloem. When the vascular bundle of the stem enters the leaf, usually the xylem faces outwards, whereas the phloem faces downward. The phloem is typically supported by stiff, sclerenchyma fibers that increase structural support for the veins.

Cambium

Form new vascular tissues.Increasing the girth of roots and stems in dicots.

  1. Radial: Xylem and phloem are in patches. They alternate and occupy the different radii of the axis separated by non-conducting tissues. They are found in monocot and dicot roots.
  2. Concentric: Here, one vascular tissue surrounds the other vascular tissue. It can be of 2 types: a) Amphivasal: Xylem surrounds the phloem as found in Dracena and Yucca b) Amphicribal: Phloem surrounds the xylem as found in Hydrilla and Pteris
  3. Conjoint: Xylem and phloem are on the same radii. It can also be of 2 types: a) Collateral: Xylem and phloem are arranged side by side on the same radius. It is collateral open when the xylem is on the inner side, and the phloem is on the outer side with cambium between them as found in the dicot stem. In contrast, it is collateral closed when the cambium is absent, as found in the monocot stem. b) Bicollateral: Phloem is present on both sides of the xylem with two patches of cambium. They are found in the family Solanaceae and Cucurbitaceae, among others.