C. Leaf

Leaves are the most important appendages. They:

• Carry out photosynthesis

• Help remove excess water from plant body
• Develop from leaf primordium

• Are dorsiventrally flattened lateral appendage of stem

• Produced at nodal region

• Are thin, expanded, and green due to photosynthetic pigments

• Show exogenous origin

• Axil shows axillary bud

• Show limited growth (no apical bud or growing point)

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1. Typical Leaf Structure

A typical leaf shows three main parts:

i. Leaf Base (Hypopodium)

• Definition: Point by which leaf attaches to stem
• Features: Varies in different plants

• May be pulvinus (swollen)

• Sheathing or ligulate

• Stipules:
• Pair of lateral outgrowths in some plants
• Stipulate leaf: With stipules
• Exstipulate leaf: Without stipules
• Normally green protective structure

ii. Petiole (Mesopodium)

• Definition: Part connecting leaf lamina with leaf base
• Types:
• Petiolate leaf: With petiole
• Sessile leaf: Without petiole
• Functions:
• Helps lamina get exposed to light
• Helps in conduction

iii. Lamina (Epipodium)

• Structure: Large, expanded, flat, green part of leaf
• Functions: Photosynthesis, gaseous exchange, transpiration
• Anatomical types:

• Dorsiventral (common in Dicots)

• Isobilateral(common in Monocots)
• Centric or cylindrical: Both surfaces not distinctly different
• Variation: Lamina varies greatly in shape, margin, and apex

 

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2. Leaf Venation

Definition: Arrangement of veins and veinlets in leaf lamina

Functions of Veins:

• Conduction of water and minerals
• Conduction of food
• Development of structural framework of lamina

 

 

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3. Types of Leaf (Based on Incision of Lamina)

 

Simple Leaf

• Entire lamina (not divided)

Compound Leaf

• Lamina divided into number of leaflets

Pinnately Compound

• Leaflets present laterally on common axis called rachis
• Rachis represents the midrib of the leaf

Palmately Compound

• All leaflets attached at tip of petiole

 

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4. Modifications of Leaves

Leaves may undergo modifications to perform functions other than photosynthesis (transpiration, gaseous exchange, light perception for flowering).

Leaf Spines

• Modification types:
• Entire leaf modified into spines (Opuntia)
• Leaf margin becomes spiny (Agave)
• Stipule modified into spine (Acacia)
• Function: Check rate of transpiration and protect from grazing

• Examples: Zizyphus, etc.

Leaf Tendril

• Structure: Leaf, leaflet, or other part modified into thin, green, wiry, coiled structure

• Function: Provide additional support to weak stems for climbing
• Example: Weak-stemmed plants

Leaf Hooks

• Structure: Terminal leaflets modified into stiff, curved, pointed hooks
• Function: Cling to tree bark
• Example:

  Bignonia unguis-cati(Cat's nail)

Phyllode

• Definition: Petiole becomes flat, green, and leaf-like
• Structure: Modified petiole replaces normal leaf
• Example:Acacia auriculoformis
• Normal leaf is bipinnately compound and falls off soon
• Petiole modifies into phyllode
• Significance: Xerophytic adaptation

Leaf Modifications

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5. Phyllotaxy

Definition: Arrangement of leaves on stem and branches in specific manner

Function: Enables leaves to get sufficient light

Types: