6.2.5. Dorsiventral (Dicotyledonous) Leaf

The vertical section of a dorsiventral leaf through the lamina shows three main parts, namely, epidermis, mesophyll and vascular system.

In a dorsiventral leaf, which is typical in dicots, we can identify three primary components. The first is the epidermis, which serves as the outer protective layer on both the upper (adaxial) and lower (abaxial) surfaces. The second part is called mesophyll, the tissue located between the two epidermal layers. The third part is the vascular system, which includes the veins and supporting structures found within the leaf.

Think of the leaf like a sandwich. The outer bread represents the epidermis, the filling is the mesophyll, which contains the 'good stuff' like green chloroplasts that enable photosynthesis, and the 'toothpicks' or skewers acting as veins make sure everything holds together and connects to the rest of the plant.

The epidermis which covers both the upper surface (adaxial epidermis) and lower surface (abaxial epidermis) of the leaf has a conspicuous cuticle. The abaxial epidermis generally bears more stomata than the adaxial epidermis. The latter may even lack stomata.

The epidermis of a dorsiventral leaf is covered by a waxy layer called the cuticle that protects the leaf from water loss and outside elements. The lower surface (abaxial) typically has more stomata, which are small openings that allow for gas exchange — they enable the leaf to breathe by letting in carbon dioxide for photosynthesis and releasing oxygen. The upper surface (adaxial) might have fewer stomata or none at all, as it is more exposed to direct sunlight and potential water loss.

Imagine the cuticle as a raincoat for the leaf. The stomata are the zippers on a jacket; they can open when we want to let air in (like breathing during exercise) and close when we want to keep warmth or moisture inside (like putting on a jacket to keep warm in winter).

The tissue between the upper and the lower epidermis is called the mesophyll. Mesophyll, which possesses chloroplasts and carry out photosynthesis, is made up of parenchyma. It has two types of cells – the palisade parenchyma and the spongy parenchyma.

Mesophyll is crucial for the leaf's function in photosynthesis as it houses chloroplasts, which contain chlorophyll — the green pigment that captures light energy. Palisade parenchyma, located directly beneath the adaxial epidermis, consists of tightly packed elongated cells that maximize light absorption. Beneath this layer is the spongy parenchyma, which has loosely arranged cells creating air spaces that allow gas exchange and the circulation of carbon dioxide throughout the leaf.

Think of the mesophyll as the factory of the leaf. The palisade parenchyma are like assembly line workers who efficiently absorb sunlight, while the spongy parenchyma acts like a spacious storage area, allowing materials to move around and ensuring everything necessary for the 'production' of food can circulate freely.

The vascular system includes vascular bundles, which can be seen in the veins and the midrib. The size of the vascular bundles are dependent on the size of the veins. The veins vary in thickness in the reticulate venation of the dicot leaves. The vascular bundles are surrounded by a layer of thick walled bundle sheath cells.

The vascular system of a leaf consists of veins known as vascular bundles, which transport water, nutrients, and sugars throughout the leaf. In dicot leaves, the network of veins is reticulate, meaning they branch out in a net-like pattern, allowing for efficient distribution of resources. Each vascular bundle is encased in a sheath of thick-walled cells that provide protection and strength.

Think of the vascular bundles as highways in the city of the leaf. The thicker highways allow for larger transport vehicles (nutrients and water) to move quickly to various parts of the leaf, ensuring all areas get what they need to thrive. The bundle sheath is like the guardrails along the highway - they help keep everything within its lane and ensure safe transport.

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