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, species they belong to, and if any modifications are present. The structure of the leaf should be under different subheadings, namely:
Leaf Base: It is the place in the stem in which the leaf petiole attaches. Two stipules, which are a tiny structure 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 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.
Leaves can be divided into two main categories: Simple leaves and compound leaves.
Simple Leaves: These are the leaves that originate from the branch or stem and does 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:
Pinnately Compound Leaves: Here, the midrib of the leaf becomes the branch on which different leaflets arise. A common axis connects all of the brochures. These are further divided into:
a)Unipinnate: The leaflets arise on each side of the axis of the leaf. E.g., cassia
b)Bipinnate: The leaflets arise from a second axis, which originates from the central axis. E.g., Acacia
c)Tripinnate: The leaflets arise from the tertiary axis that arises from the secondary axis. E.g., Moringa
d)Decompound: If the leaflets have more than three pinnate, it is classified as decompound. E.g., Coriander
e) Paripinnate: The terminal leaflet is absent. E.g., Cassia.
f) Imparipinnate: If the terminal leaflet is odd, it is termed as imparipinnate. E.g., Pea plants.
2.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:
Unifoliate: Here, only one leaflet originates from the same point. E.g., citrus fruits.
Bifoliate: Here, two leaflets arise from the same point. Eg Balanites.
Trifoliate: There are three leaflets which originate from the same point. E.g., Oxalis.
Quadrifoliate: Four leaflets arise from the same point. Eg Marsilea.
Multifoliate: Many leaflets arise from the same point. Eg Bombax.
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: In this type, more than three leaves develop at the same node — Eg: Sunflower 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 the predators.
Tendrils of Leaf: In plants like Lathyrus aphaca, 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 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.
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 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.
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