- It is a lateral, flattened structure borne on the stem.
- It develops at the node and bears a bud in its axil.
- The axillary bud later develops into a branch.
- Leaves originate from shoot apical meristems and are arranged in an acropetal order.
- They are important vegetative organs for photosynthesis.
A typical leaf has 3 main parts:
o Leaf base: With this, the leaf is attached to stem. It may bear two lateral small leaf-like structures called stipules. In monocots, the leaf base expands into a sheath covering the stem partially or wholly. In some leguminous plants, the leaf base may be swollen. It is called pulvinus.
o Petiole: It helps to hold the leaf blade to light. Long thin flexible petioles allow leaf blades to flutter in wind, thereby cooling leaf and bringing fresh air to leaf surface.
- It is the arrangement of veins and veinlets in leaf lamina.
- It is 2 types:
o Reticulate venation: Here, the veinlets form a network. It is seen in dicotyledons.
o Parallel venation: Here, the veins run parallel to each other within a lamina. It is seen in monocotyledons.
Types of Leaves
- Simple leaf: Here, leaf lamina is entire or when incised, the incisions do not touch the midrib.
- Compound leaf: Here, the incisions of the lamina reach up to the midrib breaking it into several leaflets.
A bud is seen in the axil of petiole in simple & compound leaves, but not in the axil of leaflets of the compound leaf.
The compound leaves are 2 types.
o Pinnately compound leaf: In this, many leaflets are present on a common axis, the rachis, which represents the midrib of the leaf. E.g. neem.
o Palmately compound leaf: In this, leaflets are attached at a common point (at the tip of petiole). E.g. silk cotton.
It is the pattern of arrangement of leaves on the stem or branch. It is 3 types:
o Alternate: In this, a single leaf arises at each node in alternate manner. E.g. China rose, mustard & sun flower.
o Opposite: In this, a pair of leaves arise at each node and lie opposite to each other. E.g. Calotropis and guava.
o Whorled: In this, more than two leaves arise at a node and form a whorl. E.g. Alstonia.
Modifications of Leaves
- Leaves are modified to perform functions other than photosynthesis. E.g.
o Tendrils: For climbing. E.g. peas.
o Spines: For defense. E.g. cacti.
o Fleshy leaves: To store food. E.g. onion and garlic.
- In plants such as Australian acacia, the leaves are small and short-lived. The petioles in these plants expand, become green and synthesise food.
- Leaves of some insectivorous plants (e.g. pitcher plant, Venus-fly trap) are also modified leaves.