Leaves are lateral appendages borne on stem. They are the photosynthetic organs of a plant. They are green in colour due to the presence of chlorophyll pigment. Each leaf possesses a bud in its axil. This Axilliary bud later develops into branches.
Parts of a plant
- The point where leaf attaches to the stem is called leaf base.
- In monocots, the leaf base forms a sheath like structure encircling the stem.
- In dicots, leaf base bears stipules.
- If the leaf base is swollen (as in members of bean family), it is called pulvinous base or pulvinate.
- These are small outgrowths seen at the leaf base.
- Leaf with stipule is known as stipulate. When stipules are absent, the leaves are called exstipulate.
- The stalk of the leaf is called a petiole.
- Leaves which do not have petiole are called petiolate, and which do not have petioles are called sessile.
Lamina / Leaf blade
- It is the green expanded part of the leaf.
- Veins and veinlets are seen on the surface of the leaves.
- The main vein is called midrib.
- Veins provide rigidity to the leaf blade and act as channels of transport for water, minerals and food materials.
- The tip of the leaf is called leaf apex.
- Edge of the lamina is known as leaf margin.
- The upper side of the lamina is called ventral side or adaxial side.
- The lower side is known as dorsal side or abaxial side.
Types of Leaves
There are 2 types of leaves.
Simple Leaves: If a single lamina is attached to the petiole, it is called a simple leaf. (e.g. Hibiscus)
Compound leaves: If two or more lamina is attached to the petiole, it is called a compound leaf. Lamina of compound leaves is called leaflets. The main petiole is called rachis. There are two types of compound leaves: Pinnately and Palmately compound leaves.
- Pinnately compound leaves: The leaflets are arranged along the two margins of the rachis.
- Palmately compound leaves: The leaflets are attached at the tip of the rachis like fingers on a palm.
The arrangement of veins on the lamina is called venation. The main vein is called the midrib. There are two types of venation.
- Reticulate venation: The veins are branched repeatedly forming a net-like appearance. It is characteristically found in dicot plants.
- Parallel venation: The branchlets of the veins are running parallel to one another. It is found in monocot plants.