[Biology Class Notes] on Morphology of Leaves Pdf for Exam

Plants are the primary source of food in the environment and are one of the basic units of the ecosystem. Plants make food for the consumers in the ecosystem by the process of photosynthesis. Do you know how photosynthesis takes place? Photosynthesis takes place in a structure of the plant called the leaf.

The leaf is an essential part of the plant, which contains nutrients and other ingredients necessary for preparing the edible portion of the plant, which may be a stem, leaf, or fruit. Leaves are the flat green portion of the plant, which is the main vascular supply of the plants. The stem, along with the leaf, is called the shoot. Apart from photosynthesis, the leaves also form the function of a process called transpiration. Now, we will learn the structure, parts, and function of the leaves in detail.

Structure of a Leaf

The leaf is a flat structure that is attached to the stem or branches of the plant or tree. Leaves come in different sizes and shapes depending on the environment they grow in, the species they belong to, and if any modifications are present. The structure of the leaf should be under different subheadings, namely:

Parts of the Leaf

• Leaf Base: It is the place in the stem in which the leaf petiole attaches. Two stipules, which are tiny structures similar to a leaf, are found here. Monocotyledons like paddy wheat have a large leaf base and can cover the stem. 

• Petiole: This is the structure that attaches the leaf blade of the plant to the base of the leaf. The length of the petiole depends on the species of the plant. 

• Leaf-Blade/Lamina: This is the main structure of the leaf, which is green in color. The leaf blade has a main vascular supply running in the center of the leaf called the midrib. The veins of the leaf run through the midrib, and veinlets may branch out of it. This structure of the leaf is the part where pigments like chlorophyll, xanthophyll are present. The leaf may contain structures like stomata, which are bean-like structures required for the transpiration process. 

Venation

Venation is the pattern or shape in which the vascular system of the venules of the leaves is arranged. It is classified into two types:

• Parallel Venation: In this type, the veins and veinlets are arranged parallel to one another. An example of this is the banana leaf. The veinlets if observed, are parallel to each other. All the monocotyledons have parallel venation, E.g., paddy, and wheat. 

• Reticulate Venation: In this type of venation, the veins form a mesh-like network, and there is no specific shape of the network. This network supplies all the nutrients to all parts of the leaf blade. All the Dicotyledons are an example of this kind of venation. E.g., Hibiscus, Rose, Mango, Jackfruit plant leaves, etc. 

Types of Leaves

Leaves can be divided into two main categories: 

1. Simple Leaves 

2. Compound Leaves. 

Simple Leaves: These are the leaves that originate from the branch or stem and do not divide any further into smaller leaflets.  Only one lamina is attached to the leaf base by the petiole. E.g., Mango leaves, black cherry leaves, Guava leaves. 

Compound Leaves: These are the leaves that divide further into different leaflets from a single leaf base and petiole. The lamina divides into subunits in two ways:

1. Unipinnate: The leaflets arise on each side of the axis of the leaf. E.g., cassia

2. Bipinnate: The leaflets arise from a second axis, which originates from the central axis. E.g., Acacia

3. Tripinnate: The leaflets arise from the tertiary axis that arises from the secondary axis. E.g., Moringa

4. Decompound: If the leaflets have more than three pinnate, it is classified as decompound. E.g., Coriander

5. Paripinnate: The terminal leaflet is absent. E.g., Cassia.

6. Imparipinnate: If the terminal leaflet is odd, it is termed imparipinnate. E.g., Pea plants.

• Palmately Compound: In this type, the leaflets arise from a single point of origin and hence form like a palm of the hand. E.g., cotton leaves. It is further classified into:

1. Unifoliate: Here, only one leaflet originates from the same point. E.g., citrus fruits.

2. Bifoliate: Here, two leaflets arise from the same point. Eg Balanites.

3. Trifoliate: There are three leaflets that originate from the same point. E.g., Oxalis.

4. Quadrifoliate: Four leaflets arise from the same point. Eg Marsilea.

5. Multifoliate: Many leaflets arise from the same point. Eg Bombax.

Phyllotaxy

The pattern in which the leaves are arranged on a stem is called phyllotaxy. Plants basically show three types of phyllotaxy.

• Alternate: In this type, one leaf develops at every alternate node of the stem. E.g., China Rose.

• Opposite: In this type, both the leaves arise from the node opposite to each other Eg: Guava leaves.

• Whorls: I
  n this type, more than three leaves develop at the same node — Eg: Sunflower leaves.

Modifications of Leaves

When other structures of the plant cannot develop and perform a particular function, the leaves can be modified to perform their functions. These functions can include the storage of food, protection of the plant, and support to the plant. 

• Phyllode: Here, the petiole is modified as a leaf and is known as phyllode, which performs the function of the leaf. E.g., Australian Acacia.

• Leaf Spines: In plants like Opuntia, the leaves of the plants are modified into spines or thorns, which help in protecting the plant from predators.

• Tendrils of Leaf: In plants like Lathyrus aphaca, the leaf gets modified into a thread-like structure called tendrils to support the plants as the plant has weak stems.

• Leaflet Hooks: The terminal part of the leaf gets modified into a hook-like structure that helps it to climb. E.g., Bignonia unguis cati.

• Insectivorous Leaves: Nitrogen is required by a few plants to develop. Hence they derive it by modifying the leaves of the plant to catch and digest insects. E.g., Drosera, Venus flytrap.

Functions of Leaves

The primary function of the leaves is photosynthesis. Photosynthesis is a process in which the plant converts the sunlight, carbon dioxide, water, and other substances into glucose and other substances that can be consumed by organisms. The leaves contain a pigment called chlorophyll, which is essential in this process. It is also the pigment that gives the green color to the leaves.

• Transpiration: The process of removal of excess water from the plant is called transpiration. This takes place through the structure in the leaf called stomata. It is a kidney-shaped structure that sits in pairs. It opens to release excess water and closes when the water content is less.

• Guttation: It is also a process of removal of excess water from the plant. But it occurs when the stomata remain closed. It takes place through the edges of the leaves in which the xylem is present.

• Storage: Since leaves have to synthesize food, it has to store nutrients necessary for the process of photosynthesis.

• Protection: Few of the leaves get modified in order to protect the plants. E.g., Opuntia modifies the leaves into spines.

Evolutionary Adaptations Found in Leaves

In the course of their evolution, leaves have developed in many structural forms mainly to confer survival and/ or reproductive advantage to their plant species.

Some of these evolutionary adaptations are discussed  in the points below:

• Some leaves (especially in the plants that grow in aquatic environments and may be fully or partially submerged in water) have developed waxy coating surfaces to their leaves. On the other hand, leaves in other plants have micro and nanostructures on their surface to prevent wetting from rain and precipitation. This prevents adhesion of contaminants on leaf surface which may block light and opening of stomata for photosynthesis.

• Leaves that are divided into leaflets (compound leaves) face less resistance from gusty winds and promote the cooling effect.

• Some leaves (for plants found in dry climates) have evolved to produce hairs on their surfaces which helps them trap moisture. These hairs also help create a boundary layer and prevent moisture loss.

• In plants that grow in excessive sunlight (e.g. Fenestraria), the leaves may be opaque or partly buried and allow light to pass only through a translucent leaf window for photosynthetic activities in the inner leaf surface.

• Kranz’s anatomy and succulent property of leaves are also special leaf adaptations. 

Why do Some Plants Seasonally Lose their Leaves?

In many different climate zones of the world, such as the temperate, boreal (subarctic climates), and dry regions of Earth, leaves of trees fall off in particular seasons. Such leaves are said to be deciduous leaves. They fall off and die in their inclement season (more often in autumn). This is due to the process of abscission. The main cause for this phenomenon is the lack of enough sunlight in these times (shorter days, poorer sunlight, longer nights) of the year. The leaves cannot photosynthesize and the plants (or trees) are entirely dependent on their reserved food stores for survival. The leaves may also require this food. In order to cut off this “wastage” deciduous plants have evolved the abscission mechanism. As they are about to fall, the chlorophyll content in them is reduced (due to decreased chlorophyll production) and the leaves turn yellow, orange, and brown in color.