Sunday, July 26, 2020

Morphology of Flowering Plants - Notes | Class 11 | Part 3: The Leaf

5. MORPHOLOGY OF FLOWERING PLANTS


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.

Venation


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.

Types of Leaves


1. Simple leaf:

Here, leaf lamina is entire or when incised, the incisions do not touch the midrib.

2. 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.
  • 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.

Phyllotaxy

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.

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