Internal Structure of Leaf- Parts of Leaf and their Functions

Have you ever wondered how a small leaf performs such a complex function of preparing food? We need to learn about the anatomy of the Internal Structure of Leaf to understand the mechanism involved. Various internal structures of the leaf facilitate the process of photosynthesis, transpiration, gaseous exchange, and transfer of prepared food to other parts of the plant. Angiosperms can be divided into two types, i.e. monocots and dicots, and the internal structure of a leaf also varies depending upon the type of plant too.

Internal Structure of Monocot Leaf

Monocotyledonous leaves are isobilateral, thin and elongated with parallel venation, which is often used to differentiate monocot leaf from dicot leaf. The anatomy or internal structure of the monocot leaf has the following layers:

1. Upper Epidermis
2. Mesophyll
3. Vascular Bundles            
4. Lower Epidermis

Upper Epidermis

1. The upper epidermis is single-layered, and it is also made up of parenchymatous tissue.
2. Parenchyma in the upper epidermis is barrel-shaped or cubical shaped without any intercellular space between them.
3. The upper epidermis is covered with a thin cuticle.
4. Stomata are present on both the upper and lower epidermis; such a condition is known as an amphistomatic condition.
5. Few cells in the upper epidermis are enlarged and form motor cells or bulliform cells.
6. Bulliform cells are larger in size compared to other epidermal cells.
7. Bulliform cells cause the rolling of the leaves and thus, reduce the surface area of the leaf, thereby reducing the rate of transpiration in hot conditions.
8. In monocots, the two guard cells present surrounding the stomata are dumbbell-shaped.
9. The epidermal cells do not have chloroplasts. However, the guard cells have chloroplasts in them.
10. The epidermal cells of some monocot leaves, like grasses, have silica in them, known as silica cells.

Mesophyll

1. Mesophyll is present in between the upper epidermis and lower epidermis.
2. It is the ground tissue of the leaves.
3. Mesophyll cells are not differentiated into palisade and spongy parenchyma.
4. There is only one kind of cell that are small, oval or spherical or irregular in shape, which is similar to spongy parenchyma.
5. These spongy parenchyma cells contain chlorophyll and chloroplast in it. These cells are isodiametric and thin-walled. Thus, mesophyll cells perform the process of photosynthesis.
6. There are large intercellular spaces in between the cells, and these cells are 6 to 7 layered. These air cavities help in the exchange of gases.

Fig: T.S of monocot leaf

Vascular Bundles

Veins represent the vascular bundle of the leaves. Vascular bundles are present in the
mesophyll.

1. Each vascular bundle has xylem and phloem surrounded by a bundle sheath.
2. Bundle sheath is made up of endodermal cells, which are large and barrel-shaped.
3. These bundle sheaths store starch granules and hence are known as starch sheaths.
4. Xylem vessels are present towards the upper epidermis and consist of xylem fibres, xylem tracheids and xylem parenchyma.
5. Xylem is concerned with the conduction of water and dissolved minerals.
6. Phloem is present towards the lower epidermis and consists of sieve pores, sieve tubes, companion cells, phloem parenchyma and phloem fibres.
7. Phloem tissue is concerned with the conduction of dissolved food materials (glucose).
8. Vascular bundles are conjoint, collateral and closed with endarch xylem. Vascular bundles are known as conjoint and collateral because xylem and phloem are present on the same radius and closed because there is no cambium.

Lower Epidermis

Lower epidermis is present below the mesophyll cells.
1. The cells are barrel-shaped or cubical in shape without intercellular spaces.
2. Stomata are present. They help in the diffusion of gases.

Internal Structure of the Dicot Leaf

The anatomy or internal structure of dicot leaves can be understood on the basis of the following structures:
1. Upper Epidermis
2. Mesophyll
a. Palisade Tissue
b. Spongy Tissue
3. Vascular Tissue
4. Midrib
5. Lower Epidermis

Upper Epidermis

1. The upper epidermis is made up of a single layer of parenchymatous cells.
2. The cells in the outer wall are thick and are covered with a thick cuticle (a waxy coating).
3. Stomata are usually absent, but in some aquatic plants, stomata can be observed in the upper epidermis.
4. The upper epidermis is devoid of the chloroplast.
5. The upper epidermis also bears epidermal hairs or trichomes.
6. In the members of the Acanthaceae, Moraceae, and Cucurbitaceae family, the epidermal cells possess cystoliths (calcium carbonate crystals).
7. The cells that contain cystoliths are comparatively larger than the normal epidermal cell and are known as lithocytes.
8. In the members of the Piperaceae, Moraceae, and Begoniaceae family, the epidermal cells are multilayered.

Mesophyll

It is the second layer of the leaf and present in between the upper epidermis and the lower epidermis. It is the portion of the leaf where chloroplast is present. In the dicots, mesophyll is differentiated into two zones:
1. Palisade Tissue
a. These are present on the adaxial surface or dorsally to the surface of the upper epidermis.
b. The palisade cells are columnar or cylindrical and are arranged parallelly and vertically to each other.
c. The palisade cells are compactly packed without any intercellular spaces.
d. Palisade tissue contains a large number of chloroplasts and thus are mainly associated with photosynthesis.
e. In the leaves of Hakea, mechanical support is provided by osteosclereids.
f. In Nymphea leaves, mechanical support is provided by astrosclereids.

2. Spongy Tissue
a. Spongy tissue is present below the palisade tissue.
b. The cells are round or oval-shaped and are loosely arranged.
c. The spongy cells are composed of large intercellular spaces.
d. Spongy cells contain chloroplasts but are few in number in comparison to palisade cells.
e. These cells extend up to the lower epidermis.

Fig: Internal Structure of Dicot Leaf

Vascular Tissue

Vascular bundles in the leaf are known as veins. The arrangement of these veins is known as venation.
1. In leaves of dicots, the venation is net-like and is called reticulate venation.
2. Each vascular bundle is surrounded by a bundle of parenchymatous cells.
3. The xylem lies towards the upper side, and the phloem lies towards the lower side of the vascular bundle, and the xylem shows endarch arrangement.
4. Conjoint, collateral, and closed vascular bundle arrangement is seen.
5. Xylem is composed of Xylem vessels, xylem tracheids, xylem fibre, and xylem parenchyma
6. Phloem is composed of sieve tubes, phloem parenchyma, and companion cells.

Midrib

The midrib is present in the middle portion of the leaf. It lacks mesophyll cells.
1. The midrib portion is composed of vascular tissue, parenchyma, and collenchyma.
2. The xylem cells are present on the adaxial side of the midrib, whereas the phloem is present on the abaxial side of the midrib.
3. It gives off many lateral vascular strands that supply water and minerals to the leaf lamina.

Lower Epidermis

The lower epidermis is similar to that of the upper epidermis.
1. It is made up of a single layer of parenchymatous cells that are compactly arranged.
2. The lower epidermis is also covered with the cuticle, but it is thinner than the upper epidermis.
3. Stomata are present on the lower epidermis and thus are hypostomatic.
4. Stomata are surrounded by two guard cells and two to many subsidiary cells.
5. The stomata open into a small cavity called the substomatal cavity.
6. The two guard cells that surround the stomata are kidney-shaped or bean-shaped.
7. The number and distribution of stomata and arrangement and distribution of subsidiary cells vary in different plants.
8. In xerophytic plants, the stomata are sunken, known as sunken stomata and are responsible for reducing the rate of transpiration.
9. In succulents, the lower epidermis is multilayered.
10. In the leaves of Nerium, the stomata are present in pits surrounded by hairs.

Differences Between T.S of Dicot and Monocot Leaf

Fig: T.S of Leaf (a) Dicot and (b) Monocot

Summary

A leaf is a flattened appendage on the stem which is born in the node. The internal structure of the leaf tells us about the arrangement of the different cells or tissues. Typically the internal structure of leaves comprises the upper epidermis and lower epidermis encompassing the mesophyll cell. In the leaves of dicotyledonous plants (dorsiventral leaves), the mesophyll cells are differentiated into palisade tissue and spongy tissue, but in isobilateral leaf, it is not. Stomata in leaves of dicots is hypostomatic, while in monocots, it is amphistomatic.

Frequently Asked Questions (FAQs) on Internal Structure of Leaf

The most frequently asked questions about Internal Structure of Leaf are answered here:

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