Plants are an inevitable part of the ecosystem. Every life on the earth is directly or indirectly dependent on plants. Among the different parts of shoot system of a plant, the leaf is the essential one. Primarily, leaves have two functions: photosynthesis and transpiration. In some plants, it takes up the responsibility of reproduction also. Let’s learn more about leaves, parts of a leaf, its types and modifications.
Structure and Parts of a Leaf
By structure, a leaf is a thin, flattened structure develops laterally at the node. It is an important part of shoot system and it originates from shoot apical meristems. Generally, leaf base, petiole, and lamina, together form the main parts of a typical leaf.
- Leaf Base: This is the part where a leaf attaches to the stem. Leaf base has two small leaf-like structure called stipules. In plants like paddy, wheat, and other monocotyledons, this leaf base is wide and masks the stem.
- Petiole: Petiole is the long, thin, stalk that links the leaf blade to the stem.
- Lamina: Also known as leaf blade. It is the green, flat surface of the leaves. It consists of small branched vein and veinlets. The vein that runs along the middle of the lamina is called midrib. Midrib divides the surface of the lamina into two. These veins and veinlets give rigidity to the leaf blade and help in transportation of water and other substances.
Venation is defined as the arrangement of veins and the veinlets in the leaves. Different plants show different types of venation. Generally, there are two types of venation:
- Reticulate venation: In a reticulate venation, the veinlets are randomly arranged and form a complex network of veinlets. Ex: Dicotyledonous plants like a rose plant.
- Parallel venation: In a parallel venation, the veinlets run parallel to each other. Ex: In monocotyledons like paddy.
Types of Leaves
When a single lamina is connected to the main stem by a petiole, the leaf is said to be simple. Ex: Guava leaves.
In a compound leaf, the midrib of the leaf is branched into different leaflets and is connected by a single petiole. Ex: Palm leaves.
Compound leaves are further divided into two: pinnately compound leaf and palmately compound leaf.
- In a pinnately compound leaf, the midrib of the leaf is divided into numerous leaflets and all connected by a common axis. Ex: Neem.
- In a palmately compound leaf, the leaflets are attached at the tip of the petiole. Ex: Silk cotton.
The patterns of arrangement of leaves on the stem are called Phyllotaxy. Plants show three types of phyllotaxy- alternate, opposite and whorled types of phyllotaxy.
- When only a single leaf develops at each node alternatively, it is an alternate type of phyllotaxy.E.g. China rose.
- When a pair of leaves develops at each node opposite to each other, it is called opposite phyllotaxy.E.g. Guava plants.
- When more than two leaves develop at the nodes to form a whorl of leaves, it is called whorled phyllotaxy. E.g. Sunflower.
We know leaves are specialized to perform photosynthesis. In addition, they also have other major roles to play such as support, storage of food, defense, etc. For each of these functions, they have been modified into different forms.
For example, tendrils of peas, spines of cacti, onion bulb, leave of insectivorous plants, etc are different modified leaves. The modifications depend on the functions they perform.
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