Anatomy of Leaf: Have you ever wondered how the tissue of plant leaves are arranged in plants? How does a leaf roll and unroll? A leaf is the main photosynthetic organ in plants as it possesses chloroplast. Leaf surfaces are covered with epidermis. The photosynthetic tissue is sandwiched between its upper and lower epidermis and is well supplied by the vascular system. The photosynthetic tissue in the leaf possesses chloroplasts. Read on to explore more about the anatomy of dicot and monocot leaves.

What is Leaf?

A leaf is a lateral, generally flattened structure born on a stem. It is divided into three parts: leaf base (Hypopodium), petiole (Mesopodium), lamina or leaf blade (Epipodium). It develops at the node and bears buds in its axil. It originates from the shoot apical meristem and is arranged in the acropetal order of the stem. The xylem always faces the upper epidermis while the phloem is towards the lower epidermis.

Based on anatomy, leaves are of two types:
a. Dorsiventral: Dorsiventral leaves are found in the dicotyledonous plants. Such leaves generally remain horizontal, and sunlight falls on their upper surface. The upper surface of a leaf is called the ventral surface or adaxial surface, and the lower surface is called the dorsal or abaxial surface. These are called the dorsiventral leaves as their dorsal and ventral surfaces differ in colour and anatomical features and are, therefore, distinguishable into two different surfaces. The upper surface is darker green as compared to the lower surface.
b.  Isobilateral: Isobilateral leaves are found in monocotyledonous plants. Such leaves generally remain vertical. These are called the isobilateral leaves as both surfaces (upper and lower) are equally green and similar to one another.

Dicotyledonous Leaf (Dorsiventral Leaf)

The vertical section of a dorsiventral leaf through the lamina shows three parts, namely: (a) Epidermis (b) Mesophyll (c) Vascular system

(a) Epidermis: It covers both the upper and the lower surface. The epidermis covering the upper or adaxial surface is called the adaxial epidermis, while the one covering the lower or abaxial surface is called the abaxial epidermis.
i. Adaxial or Upper Epidermis: This is the outermost layer of the leaf. It is single-layered, made up of parenchymatous cells. The outer walls of the epidermal cells are cuticularised. There are no chloroplasts in them. Stomata are lesser in number on the adaxial epidermis as compared to the abaxial epidermis. Stomata maybe even absent (hypostomatic) on the upper epidermis in dicot leaves.
ii. Abaxial or Lower Epidermis: The lower epidermis is like the upper epidermis in structure as it is also single-layered, made up of parenchymatous cells and covered with cuticles. It differs from the upper epidermis in bearing more stomata. Just below the stomata, a cavity is present, which is called the substomatal cavity. It helps in the exchange of gases. Chloroplasts are absent in the epidermal cells but are found in the guard cells surrounding the stoma.

(b) Mesophyll: The tissue present in between the upper and lower epidermis is called the mesophyll. It is the ground tissue of the leaf, which consists of thin-walled chloroplasts containing parenchymatous cells. The mesophyll is the photosynthetic tissue of the leaf. It is divided into two regions :
1. Palisade Parenchyma: It lies below the upper epidermis; hence it is adaxially placed. The palisade parenchyma extends up to one or two layers. It is made up of elongated cells arranged vertically, forming an angle of 90″ with the upper epidermis. These vertically arranged cells lie parallel to one another. These cells have numerous chloroplasts and take an active part in photosynthesis. The cells do not have intercellular spaces in between them and are closely packed.
2. Spongy Parenchyma: Lies below the palisade parenchyma and extends up to the lower epidermis; hence, it is abaxially placed. It is made up of round or oval cells, which are irregularly arranged without any particular arrangement. The cells possess large intercellular spaces in between them. Due to the presence of a large number of intercellular spaces and air cavities between these cells, this tissue is called spongy parenchyma (spongy-loosely arranged cells). These cells also have chloroplasts and participate in photosynthesis.

(c) Vascular System: Vascular system includes the vascular bundles. Bundles are scattered in the spongy parenchyma. These can be seen in the veins and the midrib of the leaves. (Middle prominent vein of the leaf is called the midrib). The dicotyledonous leaves possess reticulate venation in which the veins form a network. The veins vary in thickness in the reticulate venation. Vein size determines the size of the vascular bundles. Hence, vascular bundles are larger in the thicker veins and smaller in the thinner veins. The vascular bundles do not possess cambium. Hence, they are conjoint and closed. Around each vascular bundle, a layer of thick-walled bundle sheath cells is present.

Monocotyledonous Leaf (Isobilateral Leaf)

The anatomy of an isobilateral leaf is similar to that of the dorsiventral loaf in many ways. It shows the following characteristic features:

(a) Epidermis: There is an upper (adaxial) and lower (abaxial) layer of the epidermis. Both the layers are made up of a single layer of cells and possess stomata. The stomata are present on both surfaces in near equal numbers. That is why the leaf is called isobilateral. Both layers of the epidermis are cuticularised.
i. In grasses, some cells in the upper epidermis become large. These adaxial epidermal cells along the veins modify themselves into large, empty and colourless cells. These are called bulliform cells. Such cells occur in groups. They help in the rolling of leaves during drought or lack of water. This can be explained as: When the water supply is sufficient to the plant, the bulliform cells in the leaves absorb water due to water absorption; they become turgid when they are turgid. The leaf surface straightens and is exposed. The leaf straightening permits the loss of water.
ii. On the other hand, these cells lose the water and become flaccid due to the water loss at the time of insufficient water supply. When flaccid during the water stress, the leaves curl inwards not to expose the leaf surface. The curling of leaves minimises the water loss. Hence, the bulliform cells minimise the loss of water during the conditions of water stress.
iii. The sub-stomatal cavity is present below the stoma of the abaxial epidermis.
iv. The monocot leaves are amphistomatic, i.e., have an equal number of stomata on both sides of the leaf.

(b) Mesophyll: The mesophyll is present between the upper and the lower epidermis. It is not differentiated into the palisade and spongy parenchyma. The cells are almost spherical and are irregularly arranged. These cells contain chloroplasts and carry out photosynthesis.

(c) Vascular Bundles: There are many large and small vascular bundles. Each vascular bundle is enclosed by a layer of thin-walled cells that form a bundle sheath around them. The vascular bundles are conjoint and closed. Xylem lies towards the upper epidermis and phloem towards the lower epidermis. These complex tissues are distinct in the larger bundles. Unlike the dicot leaf, in which larger vascular bundles are present in the larger veins and smaller in the narrower veins, near similar vascular bundles, are seen in the monocot leaf due to the parallel venation in the monocot leaves. In parallel venation, veins are present parallel to one another and are of nearly equal size.

Comparison of Anatomy of Dicot and Monocot Leaf

Character	Dicot Leaf	Monocot Leaf
Leaf	Dorsiventral	Isobilateral
Stomata	More in number on the lower epidermis	Equal in number on the lower and upper epidermis
Mesophyll	Differentiated into palisade and spongy parenchyma	Not Differentiated
Bulliform Cells	Absent	Present(mainly in grasses)
Vascular Bundles	Differ in size due to reticulate venation	Similar in size due to parallel venation
Bundle Sheath	Parenchymatous	Sclerenchymatous

Summary

Leaves are present on the stem, which are mainly photosynthetic. Leaf outer covering is the epidermis, which is covered by a waxy layer cuticle that prevents water loss. They have minute pores present for gaseous exchange and transpiration called stomata, whose opening is regulated by the turgidity and flaccidity of the guard cells. In between the epidermal layer, mesophyll cells are present, which possess chloroplast for photosynthesis. The mesophyll is differentiated into palisade and spongy parenchyma in the dicot leaf but not in the monocot leaf. The vascular bundle is scattered and of different sizes; due to reticulate venation in the dicot leaf and monocot leaf, vascular bundles are nearly similar due to the presence of parallel venation. 

Frequently Asked Questions (FAQs) on Anatomy of Leaf

Q.1. Which leaves show reticulate venation?
Ans: The dicot leaf shows reticulate venation.

Q.2. What is the main function of the cuticle?
Ans: The main function of the cuticle is to prevent water loss from the leaf.

Q.3. Which leaves show an isobilateral arrangement?
Ans: The monocot leaves show the isobilateral arrangement.

Q.4. What is the meaning of the adaxial and abaxial surface of the leaf?
Ans: The adaxial surface means the upper surface of the leaf, and the adaxial surface means the lower surface of the leaf.

Q.5. What is spongy parenchyma?
Ans: It is a round or oval cell with intercellular spaces which possess chloroplast present in the leaf.

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