Internal Structure (Anatomy) of Dorsiventral Leaf

The internal structure of dorsiventral leaf (dicot leaf) exhibits upper epidermis, lower epidermis, mesophyll tissue (palisade parenchyma and spongy parenchyma) and vascular bundle.

Upper epidermis of dorsiventral leaf

The upper epidermis consists of a single layer of compactly arranged barrel-shaped parenchymatous cells. These epidermal cells are continuous, meaning there are no intercellular spaces between them. They are colorless and lack chloroplasts.

The outer surface of the epidermis is covered by a protective layer called the cuticle, which helps reduce water loss from the leaf. Multicellular hairs or trichomes may be present on the surface of the leaf which serves to protect it against herbivores, reduce water loss, and provide a physical barrier.

Lower epidermis of dorsiventral leaf

The epidermis of a dorsiventral leaf is composed of compactly arranged parenchymatous cells without intercellular spaces. It is externally covered by a cuticle. It contains a significant number of pores known as stomata.

The cuticle present on it, acts as a protective layer to reduce water loss from the leaf surface. The presence of open stomata facilitates gas exchange, enabling the leaf to take in carbon dioxide and release oxygen during photosynthesis, as well as allowing for the release of excess water vapor through transpiration.

Mesophyll tissue of dorsiventral leaf

The mesophyll is the tissue located between the upper and lower epidermis of a dorsiventral leaf. It contains specialized parenchyma cells called chlorenchyma, which contain chloroplasts for photosynthesis. The mesophyll is typically differentiated into two regions: The upper palisade parenchyma and the lower spongy parenchyma.

Palisade parenchyma

The palisade parenchyma is situated beneath the upper epidermis and consists of one to three layers of vertically elongated, parallel, and closely packed columnar or cylindrical cells. These cells have narrow intercellular spaces, allowing for the exchange of gases. They contain numerous chloroplasts arranged in linear rows, maximizing their ability to capture sunlight for photosynthesis.

Spongy parenchyma

The spongy parenchyma, located between the lower epidermis and the palisade parenchyma, is composed of loosely arranged oval, rounded, or irregular cells. These cells have large air cavities or intercellular spaces, except in the regions surrounding the vascular bundles. The spongy parenchyma cells also contain chloroplasts

Both the palisade parenchyma and the spongy parenchyma are primarily responsible for photosynthesis, with the palisade parenchyma being more specialized for light absorption and the spongy parenchyma facilitating gas exchange and providing aeration to the leaf.

Vascular bundle of dorsiventral leaf

The vascular bundles in the dicot leaf are classified as conjoint, collateral, and closed type.

This means that the xylem and phloem tissues within the bundle are arranged in a side-by-side manner, and there is no cambium present to allow for secondary growth.

The vascular bundles are surrounded by a layer of collenchyma tissue, which is present both above and below the bundles, extending towards the epidermis.

Each vascular bundle is enveloped by a sheath of compactly arranged parenchyma cells called the bundle sheath. This sheath further protects and supports the vascular bundle, aiding in the transport of fluids and providing structural integrity to the leaf.

In the vascular bundles of the dicot leaf, the xylem tissue is located towards the upper side of the leaf, while the phloem tissue is situated towards the lower side.

• Xylem component of the vascular bundles consists of various types of cells, including vessels, tracheids, xylem parenchyma, and a few xylem fibers. Vessels and tracheids are responsible for conducting water and mineral salts throughout the leaf. Xylem parenchyma cells, in addition to storing food reserves, facilitate lateral movement of water and mineral salts within the leaf. Xylem fibers, although present in small quantities, play a role in providing additional mechanical support to the leaf structure.

• Phloem component of vascular bundle consist of sieve tubes, companion cells and phloem parenchyma whereas phloem fibers are rare. Sieve tube is responsible for conduction of organic food, phloem parenchyma is responsible for storage of food and lateral conduction where as companion cells are responsible for controlling the function of sieve tubes.