Plants exhibit continuous growth throughout their life due to the presence of meristematic tissues. Unlike animals, plant growth is indeterminate, meaning it continues as long as the plant is alive.
Plant growth and development include a series of events such as cell division, elongation, differentiation, and maturation, ultimately leading to the formation of a mature plant body.
What is Growth?
Growth is defined as a permanent and irreversible increase in size, mass, or number of cells.
Characteristics of Growth
- Irreversible
- Measurable
- Occurs due to cell division and elongation
- Influenced by internal and external factors
Phases of Plant Growth
Plant growth occurs in three distinct phases:
1. Meristematic Phase
- Active cell division
- Cells have dense cytoplasm and prominent nucleus
- No vacuoles
- Occurs in apical meristems
2. Elongation Phase
- Cells increase in size
- Vacuole formation begins
- Cell wall loosening occurs
- Maximum growth in length
3. Maturation Phase
- Cells attain final size and shape
- Differentiation occurs
- Cells become specialized
Growth Rate
Growth rate refers to the increase in growth per unit time.
Types of Growth
1. Arithmetic Growth
- Linear growth
- One daughter cell divides, other differentiates
Equation:
Lₜ = L₀ + rt
2. Geometric Growth
- Exponential growth
- Both daughter cells continue dividing
Equation:
Wₜ = W₀eʳᵗ
Growth Curve
The growth of a plant organ follows a sigmoid (S-shaped) curve:
Phases of Growth Curve
- Lag phase (slow growth)
- Log phase (rapid growth)
- Stationary phase (growth slows down)
Conditions for Growth
Plant growth depends on several factors:
1. Water
- Essential for cell expansion
- Maintains turgidity
2. Oxygen
- Required for respiration
- Provides energy
3. Nutrients
- Macronutrients: N, P, K
- Micronutrients: Fe, Zn, etc.
4. Temperature
- Optimal range required
- Extreme temperatures inhibit growth
5. Light
- Essential for photosynthesis
- Affects flowering and germination
Differentiation, Dedifferentiation and Redifferentiation
Differentiation
- Process where cells become specialized
- Example: Formation of xylem and phloem
Dedifferentiation
- Mature cells regain ability to divide
- Example: Formation of cambium
Redifferentiation
- Dedifferentiated cells become specialized again
- Example: Secondary tissues
Development
Development is the sum total of growth and differentiation.
It includes:
- Growth
- Differentiation
- Maturation
Plant Growth Regulators (PGRs)
Plant growth regulators are chemical substances that control plant growth and development.
Types of Plant Growth Regulators
1. Auxins
Functions:
- Promote cell elongation
- Apical dominance
- Root initiation
Examples:
- IAA (Indole-3-acetic acid)
- IBA
2. Gibberellins
Functions:
- Stem elongation
- Breaking dormancy
- Promote flowering
3. Cytokinins
Functions:
- Cell division
- Delay senescence
- Promote lateral bud growth
4. Abscisic Acid (ABA)
Functions:
- Growth inhibitor
- Induces dormancy
- Stress hormone
5. Ethylene
Functions:
- Fruit ripening
- Leaf abscission
- Senescence
Photoperiodism
Photoperiodism is the response of plants to the length of day and night.
Types of Plants
1. Short Day Plants
- Flower when day length is shorter
2. Long Day Plants
- Flower when day length is longer
3. Day Neutral Plants
- Not affected by day length
Vernalization
- Induction of flowering by low temperature
- Common in winter crops
- Example: Wheat
Seed Dormancy
Seed dormancy is a condition where seeds do not germinate even under favorable conditions.
Causes:
- Hard seed coat
- Immature embryo
- Chemical inhibitors
Senescence
- Aging process in plants
- Leads to death of cells/tissues
Types:
- Whole plant senescence
- Organ senescence
Abscission
- Shedding of leaves, flowers, or fruits
- Controlled by hormones (ethylene, ABA)
Important Differences
Growth vs Development
| Feature | Growth | Development |
|---|---|---|
| Meaning | Increase in size | Overall changes |
| Nature | Quantitative | Qualitative + Quantitative |
Important NEET & CBSE Points
- Growth is irreversible
- Sigmoid curve represents growth
- Auxin → cell elongation
- Cytokinin → cell division
- ABA → growth inhibitor
- Ethylene → fruit ripening
- Vernalization → low temperature requirement
Quick Revision Points
- Growth = increase in size
- Development = growth + differentiation
- PGRs regulate plant life
- Photoperiodism controls flowering
- Vernalization induces flowering
Frequently Asked Questions (FAQs)
Q1. What is plant growth?
Increase in size and number of cells.
Q2. What is the role of auxin?
Promotes cell elongation and apical dominance.
Q3. What is vernalization?
Induction of flowering by cold treatment.
Q4. Which hormone is responsible for fruit ripening?
Ethylene.
Conclusion
Plant growth and development involve complex interactions between internal genetic factors and external environmental conditions. Understanding this chapter is essential for mastering plant physiology concepts required for CBSE board exams and NEET preparation.