Plant Growth and Development — Biology Class 11 Notes (CBSE & HBSE)
Free NCERT Biology notes for Plant Growth and Development (Class 11) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Plant Growth and Development (CBSE & HBSE)
Plant Growth and Development explains how plants increase in size irreversibly and progress through orderly developmental phases from seed to senescence. The chapter covers growth phases and rates, the roles of differentiation, dedifferentiation and redifferentiation, the five major plant growth regulators, and the environmental control of development through photoperiodism, vernalisation and seed dormancy. It is a high-yield NEET and board chapter where regulator functions and PGR-specific effects are repeatedly tested.
Growth, Phases, Rates and Differentiation
What is Growth?
Growth is an irreversible permanent increase in size of an organ or its parts or even of an individual cell. It is generally accompanied by metabolic processes (anabolic and catabolic) that occur at the expense of energy.
Plants retain the capacity for unlimited growth throughout their life because of meristems (regions of active cell division) at certain locations of the plant body.
Plant Growth is Indeterminate
- Cells of apical and lateral cambial meristems divide and add new cells continuously.
- Apical meristems → primary growth → increase in length.
- Lateral meristems (vascular and cork cambium) → secondary growth → increase in girth.
Phases of Growth
The period of growth is divided into three phases:
| Phase | Key Feature |
|---|---|
| Meristematic | Cells at root/shoot apex divide repeatedly; rich protoplasm, large nuclei, thin primary walls |
| Elongation | Increased vacuolation, cell enlargement, new cell wall deposition |
| Maturation | Cells attain maximal size with wall thickening and protoplasmic modification |
Growth Rates
The increased growth per unit time is termed growth rate. Growth can be arithmetic or geometric.
- Arithmetic growth: Only one daughter cell continues to divide while the other differentiates. Constant linear increase. Equation: Lt = L0 + rt (Lt = length at time t, L0 = length at t=0, r = growth rate).
- Geometric growth: Both progeny cells retain the ability to divide initially. Slow lag phase, rapid exponential (log) phase, then a stationary phase — producing a sigmoid (S-shaped) curve.
Exponential growth is expressed as: W1 = W0 e^rt where W1 = final size, W0 = initial size, r = growth rate (efficiency index), t = time, e = base of natural logarithms.
The sigmoid curve is characteristic of all living organisms growing in a natural environment.
Conditions for Growth
- Water — turgidity needed for cell extension; provides medium for enzymatic activity.
- Oxygen — releases metabolic energy for growth.
- Nutrients (macro & micro) — supply carbon and energy and build protoplasm.
- Temperature & light — optimum range required for division and elongation.
Differentiation, Dedifferentiation, Redifferentiation
- Differentiation: Cells derived from apical and cambial meristems mature to perform specific functions (e.g., tracheary elements lose protoplasm, develop strong lignified walls).
- Dedifferentiation: Living differentiated cells regain the capacity to divide (e.g., formation of interfascicular cambium and cork cambium from parenchyma).
- Redifferentiation: Dedifferentiated tissues again lose the capacity to divide and mature to perform specific functions.
Development = sum of growth + differentiation; a cell's fate may follow different pathways called plasticity (e.g., heterophylly in cotton vs. buttercup).
Plant Growth Regulators (PGRs)
What are PGRs?
Plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition that control growth and developmental responses. They are also called plant hormones or phytohormones.
PGRs fall into two broad groups:
- Growth promoters: auxins, gibberellins, cytokinins.
- Growth inhibitors: abscisic acid (ABA) and the gaseous regulator ethylene (involved in many inhibitory/promoting roles).
The Five Major PGRs
| PGR | Discovery / Source | Key Functions |
|---|---|---|
| Auxins (IAA) | Isolated by F.W. Went from coleoptile tips | Apical dominance, root initiation, prevents fruit/leaf drop, parthenocarpy, used as herbicide (2,4-D) |
| Gibberellins (GA3) | From fungus Gibberella fujikuroi | Stem elongation (bolting), breaks dormancy, delays senescence, increases fruit size (apple), malting in brewing |
| Cytokinins (kinetin, zeatin) | First from herring sperm DNA; natural zeatin from corn-kernels/coconut milk | Promote cell division (cytokinesis), overcome apical dominance, delay leaf senescence (Richmond–Lang effect), nutrient mobilisation |
| Ethylene | Gaseous PGR | Fruit ripening, senescence and abscission, breaks seed/bud dormancy, promotes female flowers, triple response, root hair formation |
| Abscisic acid (ABA) | 'Stress hormone' | Inhibits growth, promotes dormancy, stomatal closure under water stress, antagonist of GA |
Important Applications
- Auxins: weed-free lawns via 2,4-D (kills dicot weeds), promote rooting in cuttings, control xylem differentiation.
- Gibberellins: speed up the malting process; spray on sugarcane to increase yield; promote bolting in beet, cabbage.
- Cytokinins: produce new leaves, chloroplasts; promote lateral shoot growth and adventitious shoot formation.
- Ethylene (Ethephon): synchronises fruit-set and accelerates ripening; the most widely used PGR in agriculture.
- ABA: stimulates closure of stomata and increases plant tolerance to various kinds of stress; also called stress hormone.
Memory aid: Auxin = Apical dominance; GA = Growth (elongation); Cytokinin = Cell division; Ethylene = Edible (ripening); ABA = Aestivation/dormancy & Anti-transpirant.
Photoperiodism, Vernalisation and Seed Dormancy
Photoperiodism
The response of plants to periods of day/night (light) is termed photoperiodism. Flowering in some plants depends not only on light/dark duration but also on their relative durations.
| Type | Requirement | Example |
|---|---|---|
| Long-day plants (LDP) | Light period longer than a critical period | Spinach, radish, wheat |
| Short-day plants (SDP) | Light period shorter than a critical period | Rice, soybean, chrysanthemum, tobacco |
| Day-neutral plants (DNP) | No correlation with light duration | Tomato, cucumber, maize |
The leaves perceive the photoperiodic stimulus. A hormonal substance (florigen, conceptual) is thought to migrate from leaves to shoot apices to induce flowering. The pigment phytochrome is involved in perceiving light.
Key point: It is the duration of the dark period that is actually critical. Interrupting the dark period with a flash of light can prevent flowering in SDP.
Vernalisation
Vernalisation is the qualitative or quantitative dependence of flowering on exposure to low temperature. It prevents precocious reproductive development late in the growing season and enables the plant to have sufficient time to reach maturity.
- Common in winter varieties of wheat, barley, rye.
- If winter varieties are planted in spring, they normally fail to flower unless given a low-temperature pretreatment.
- Example: Biennials (e.g., sugar beet, cabbage, carrot) which normally flower in the second season require cold exposure.
Seed Dormancy
Seed dormancy is a state in which viable seeds do not germinate even under favourable conditions (water, oxygen, temperature). It is an adaptation to avoid germination in unfavourable seasons.
Causes of dormancy:
- Impermeable/hard seed coats to water and oxygen (e.g., many legumes).
- Presence of chemical inhibitors such as abscisic acid (ABA), phenolic acids, para-ascorbic acid.
- Immature/rudimentary embryos.
Methods of breaking dormancy:
- Scarification — mechanical/chemical rupture of hard seed coats (e.g., abrasion, treatment with conc. H2SO4).
- Stratification (chilling) — exposure to low temperature in moist conditions.
- Gibberellin (GA) treatment — overcomes inhibitor-imposed dormancy; GA/ABA balance governs dormancy vs. germination.
- Light exposure for light-sensitive seeds.
NEET trap: GA and ABA are antagonists — GA breaks dormancy and promotes germination; ABA imposes dormancy. Their ratio decides whether a seed germinates or stays dormant.
Frequently asked questions
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Do these notes follow CBSE and HBSE?
Yes. The Plant Growth and Development notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.
What does the Plant Growth and Development chapter cover?
Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Plant Growth and Development.