Vascular Bundle

In Stems

Vascular bundles are arranged differently in monocot and dicot plants. In monocots, they are randomly scattered, with most of them near the outer edge just below the epidermis. The bundles are surrounded by large parenchyma cells in the cortex region. There is no pith region in monocots. In contrast, dicot stems have bundles in a ring surrounding parenchyma cells in a pith region. Smaller parenchyma cells make up the region between the bundles and the epidermis, the cortex. The vascular bundles are scattered in maize stems but smaller towards the periphery.  

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.

Xylem

• Transport water with minerals from roots to other plant parts. They are commonly called the ‘drinking’ tubes in plants.
• Transport hormones such as abscisic acid and cytokinin

Phloem

• Transport food and other organic nutrients from leaves to all other plant parts. They are commonly called the ‘eating’ tubes in plants.
• Provide the path for the translocation of peptides, proteins, and mRNAs involved in plant growth and development and defend against pathogens.

Both xylem and phloem form specialized vascular cell types such as tracheary elements in the xylem and sieve elements in the phloem. In addition, they together provide structural support to all parts of the plant.

Cambium

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

There are three main types of vascular bundles:

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.