Plant Growth and Development

NCERT Line-by-Line Breakdown for NEET 2026

Unit 4: Plant Physiology

Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being. It is an irreversible permanent increase in size of an organ or its parts. In this chapter, we decode the arithmetic of growth, the concept of Differentiation, and the crucial Plant Growth Regulators (PGRs).

1. Growth: Phases & Rates

A. Phases of Growth

Growth is open-ended and localized. Root and Shoot apical meristems are responsible for primary growth.

Meristematic Phase: Cells have rich protoplasm, large nuclei, and thin cellulosic walls with plasmodesmata. High respiration rate.
Elongation Phase: Increased vacuolation, cell enlargement, and new cell wall deposition.
Maturation Phase: Cells attain maximal size. Protoplasm modification and wall thickening occur.

B. Growth Rates

Arithmetic Growth
One daughter cell divides, the other differentiates.
Formula: L_t = L₀ + rt
Graph: Linear.

Geometric Growth
Both progeny cells retain ability to divide.
Formula: W₁ = W₀ e^rt
Graph: Sigmoid (S-curve).

⚠️ NEET Concept: In the geometric formula, ‘r’ is the relative growth rate and acts as the measure of the ability of the plant to produce new plant material (Efficiency Index).

2. Differentiation & Development

Term	Definition	Example
Differentiation	Cells lose capacity to divide and mature to perform specific functions.	Tracheary elements (lose protoplasm).
Dedifferentiation	Differentiated cells regain capacity to divide (Meristematic).	Formation of Cork Cambium.
Redifferentiation	Dedifferentiated cells lose division capacity again.	Secondary Cortex, Secondary Xylem.

Plasticity & Heterophylly

Plants follow different pathways in response to environment or phases of life. This ability is Plasticity.

Developmental Heterophylly: Leaves of juvenile vs mature plants (e.g., Cotton, Coriander, Larkspur).
Environmental Heterophylly: Leaves in air vs water (e.g., Buttercup).

3. Plant Growth Regulators (Hormones)

Small, simple molecules. Divided into Promoters (Auxins, GA, Cytokinins) and Inhibitors (ABA, Ethylene).

🅰️ Auxins (Indole Compounds)

Discovery: Charles & Francis Darwin (Canary grass). Isolated by F.W. Went (Oat seedlings).

Types: IAA, IBA (Natural); NAA, 2,4-D (Synthetic).

Apical Dominance: Inhibits growth of lateral buds (Decapitation promotes branching in tea plantations).
Rooting: Initiates rooting in stem cuttings.
Weedicide: 2,4-D kills dicot weeds (leaves monocots safe).

🍇 Gibberellins (Terpenes)

Discovery: E. Kurosawa (Bakane/Foolish Seedling disease in Rice – Gibberella fujikuroi).

Key Function: Bolting (Internode elongation prior to flowering) in beet/cabbage.

Increases length of Grape stalks.
Increases yield of Sugarcane (up to 20 tonnes/acre).
Speed up malting process in brewing industry.

🥥 Cytokinins (Adenine derivatives)

Discovery: Skoog & Miller (Kinetin from Herring sperm DNA). Zeatin (from Corn/Coconut milk).

Key Function: Cell Division.

Overcomes Apical Dominance (Antagonistic to Auxin).
Delays leaf senescence (Richmond-Lang effect) by nutrient mobilization.
Promotes formation of new leaves and chloroplasts.

💨 Ethylene (Gaseous Hormone)

Discovery: Cousins (Volatile substance from ripened oranges).

Key Function: Fruit Ripening (Respiratory Climactic – rise in respiration rate).

Breaks seed and bud dormancy (Peanut/Potato).
Promotes senescence and abscission.
Ethephon: Commercial source (hastens ripening in tomatoes/apples).

🛑 Abscisic Acid (ABA) – Stress Hormone

General plant growth inhibitor.

Stimulates closure of stomata under water stress.
Induces Seed Dormancy (Antagonist to Gibberellins).
Promotes abscission of leaves/fruits.

📝 Rapid Fire MCQs

Q1. Which hormone is used to induce bolting in rosette plants?

A) Auxin
B) Cytokinin
C) Gibberellin
D) Ethylene

Click to check Answer

Answer: C) Gibberellin.

Q2. The hormone antagonistic to Gibberellins regarding seed dormancy is:

A) ABA
B) IAA
C) Zeatin
D) Ethylene