Morphology of Flowering Plants - Notes | Class 11

Morphology of Flowering Plants - The Leaf

The Leaf

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:

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.
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.
Lamina (leaf blade): The green expanded part with veins & veinlets. The middle prominent vein is called midrib. Veins provide rigidity to lamina and act as channels of transport for water, minerals & food materials.

It is the arrangement of veins and veinlets in leaf lamina.

It is 2 types:

Reticulate venation: Here, the veinlets form a network. It is seen in dicotyledons.
Parallel venation: Here, the veins run parallel to each other within a lamina. It is seen in monocotyledons.

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.
- 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.
- 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:

Alternate: In this, a single leaf arises at each node in alternate manner. E.g. China rose, mustard & sun flower.
Opposite: In this, a pair of leaves arise at each node and lie opposite to each other. E.g. Calotropis and guava.
Whorled: In this, more than two leaves arise at a node and form a whorl. E.g. Alstonia.

Leaves are modified to perform functions other than photosynthesis. E.g.

Tendrils: For climbing. E.g. peas.
Spines: For defense. E.g. cacti.
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.

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