Describe the internal structure of a dorsiventral leaf with the help of labelled diagrams.

A dorsiventral leaf, also known as a bifacial leaf, is a type of leaf with distinct upper (adaxial) and lower (abaxial) surfaces commonly found in dicotyledonous plants.

Epidermis

• The leaf has two epidermal layers: the upper (adaxial) and lower (abaxial) epidermis.

• The upper epidermis is usually a single layer of compact, transparent, parenchymatous cells covered with a thick cuticle to reduce water loss.

• Stomata are usually absent or fewer on the upper epidermis.

• The lower epidermis contains more stomata and is also made of parenchymatous cells with a thinner cuticle compared to the upper side.

• Guard cells regulate the opening and closing of stomata, which are crucial for gas exchange and transpiration.

Mesophyll

• The mesophyll is differentiated into two distinct regions: palisade parenchyma and spongy parenchyma.

• Palisade parenchyma is located just below the upper epidermis and consists of one or more layers of elongated, chloroplast-rich cells arranged closely. It is the main site of photosynthesis.

• Spongy parenchyma lies below the palisade layer toward the lower epidermis and has loosely arranged cells with large intercellular spaces. It facilitates gaseous exchange and temporary storage of water and nutrients.

Vascular Bundles

• Vascular bundles are arranged in a network of veins and veinlets, with a prominent midrib.

• Each vascular bundle is conjoint, collateral, and closed (having both xylem and phloem without cambium).

• Xylem is located on the upper side of the vascular bundle (facing the adaxial surface), and phloem is on the lower side (facing the abaxial surface).

• Bundle sheath cells surround the vascular bundles, sometimes forming a distinct layer.

Functions of Internal Structures

• The upper epidermis protects against physical damage and dehydration.

• The palisade layer maximizes light absorption for photosynthesis.

• The spongy parenchyma facilitates the diffusion of gases (CO₂, O₂) and the movement of water vapor.

• Stomata on the lower epidermis help regulate transpiration and gas exchange.

• Vascular tissues (xylem and phloem) transport water, nutrients, and photosynthates (sugars).

Adaptations

• Dorsiventral leaves are well-suited to horizontal orientation and receive maximum sunlight on the upper surface.

• Distribution of stomata primarily on the lower side reduces water loss.

Common Examples

• Found in dicots such as Mangifera indica (mango), Hibiscus rosa-sinensis, and Pisum sativum (pea).