Leaves exhibit tissue system like stem and roots. The tissue system of leaves consists of epidermal tissues, ground tissues, and vascular tissues.

All the said tissues show a definite pattern of arrangement with certain differences in the dicotyledons and monocotyledons.

The arrangement of leaf lamina both in dicotyledons and monocotyledons exert certain influence on their internal structure and thus, they are being dealt under dorsiventral and isobilateral leaves respectively.

Dorsiventral Leaf?

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Dorsiventral leaves are found in dicot plants. Here a leaf-blade is held horizontally by the petiole. Its upper surface is directly exposed to the sun and the lower surface is less illuminated. Because of the unequal illumination, there occurs difference between the lower and upper sides of leaf. Thus the leaf is said to be dorsiventral.

Ficus bengalensis, Ficus religiosa, Citrus, Healianthus Mangifera indica etc. are examples of dorsiventral leaves.

Internal structure of leaf of Mango (Mangifera indica)

A thin transverse section passing through mid-rib region reveals the following internal structure.

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1) Upper Epidermis:

This consists of a single layer of cells covered by a thick cuticle. The cuticle prevents excess loss of water from the leaf surface.

2) Lower Epidermis:

This is also a single layer with a thin cuticle. Stomata occur in between the epidermal cells. Each stoma is surrounded by two guard cells.

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The guard cells contain chloroplasts. Remainder of the epidermal cells lack chloroplast. Internal to each stoma a large cavity called the respiratory cavity is seen. The stoma facilitates exchange of gases (Oxygen and Carbon dioxide) between the atmosphere and the internal tissues, besides loss of water by transpiration.

3) Mosophyll:

The ground tissue lying between the upper epidermis and the lower epidermis constitute mesophyll. It is differentiated into palisade parenchyma and spongy parenchyma.

a) Palisade Parenchyma:

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It consists of 2-3 layers of cells and occurs below the upper epidermis. The cells are elongated and closely packed with their long axis at right angles to the epidermis. The cells contain a large number of chloroplasts. Thus, the function of palisade parenchyma is synthesis of carbohydrate by the pocross of photosynthesis.

b) Spongy Parenchyma:

This zone is present below the palisade zone and extends up to the lower its cells are round, oval or irregular and are loosely arranged, enclosing large intercellular spaces and air cavities. The air spaces help in the exchange of with external atmosphere.

The cells contain a few chloroplasts, which is much than the palisade tissue. This is why the color of lower surface is lighter than upper surface of leaf.

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Isobilateral leaf?

Isobilateral leaves are also called unifacial leaves. These types of leaves are seen in most of the monocotyledons. An isobilateral leaf is more or less equally illuminated on both sides. Maize, bamboo, date palm leaves etc. are the typical examples of isobilateral leaves. A thin section at a right angle to one or more veins reveals the following internal structure.

Internal structure of isobilateral leaf:

A typical isobilateral leaf has the following internal structure.

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Epidermis

The structure of both upper and lower epidermis is more or less similar. Each of the epidermis consists of a single layer of non-chlorophyllous cells. The epidermis on either side bears more or less an equal number of stomata and is also uniformly thickened and cutinized. In grasses a few large, empty and colorless cells are also present at intervals on the upper epidermis. These are called bulli form cells.

Mesophyll

The entire tissue between the upper and lower epidermis is called the mesophyll. The mesophyll is not differentiated into palisade and spongy parenchyma.

Vascular bundles

Many vascular bundles are seen equally spaced in the mesophyll tissue. They are arranged in a parallel manner. The size of the vascular bundles may be equal in some plants and unequal in others.

Each vascular bundle is conjoint, cola end arch and closed. Vascular bundles are surrounded by a distinct parenchyma bundles sheath (border parenchyma). Two distinct patches of sclerenchyma are pr above and below each of the large vascular bundle and extend up to the upper lower epidermal layers. The xylem is located towards the upper epidermis and phloem towards the lower epidermis.