In Class 11 Biology, "Plant Growth and Development" provides an authoritative, curriculum-verified master resource aligned with the 2026–27 NCERT syllabus.
Why do plant shoots bend gracefully towards a sunny window while roots burrow relentlessly downwards into dark soil, or why does a single ripe banana in a fruit bowl cause all surrounding green apples to ripen rapidly? Plant hormones coordinate plant growth.
Why This Chapter Matters
In Class 11 Biology, "Plant Growth and Development" provides an authoritative, curriculum-verified master resource aligned with the 2026–27 NCERT syllabus.
Before You Begin (Prerequisites)
Plant anatomy from Chapter 6.
Auxins from Class 10.
Cell division.
What You Will Learn (Core Objectives)
Explain Characteristics of Plant Growth: Open form of growth, Arithmetic ($L_t = L_0 + rt$) vs Geometric ($W_1 = W_0 e^{rt}$) growth, and Sigmoid Growth Curve.
Explain Differentiation, Dedifferentiation (interfascicular cambium), and Redifferentiation.
Analyze the 5 Major Phytohormones: Auxins (phototropism, apical dominance, 2,4-D herbicide), Gibberellins (bolting, internodal elongation), Cytokinins (cell division, anti-aging), Ethylene (gaseous, fruit ripening), and Abscisic Acid (ABA, 'stress hormone', stomatal closure).
Explain Photoperiodism: Short-day plants (SDP), Long-day plants (LDP), and Day-neutral plants (DNP); role of Florigen.
Arithmetic Growth: Following mitosis, only one daughter cell continues dividing while the other differentiates (e.g. Root elongation at constant rate): $$\mathbf{L_t = L_0 + rt} \quad (\text{Linear straight line!})$$
Geometric Growth: Both daughter cells retain division capability, producing exponential growth followed by nutrient limitation, forming a classic S-shaped (Sigmoid) Curve: $$\mathbf{W_1 = W_0 e^{rt}} \quad (\text{Lag phase } \to \text{ Log/Exponential phase } \to \text{ Stationary phase})$$
Plasticity: Ability to produce different structures in response to environment (e.g. Heterophylly in cotton, coriander, buttercup).
2. The Five Phytohormone Powerhouses
Auxin (IAA): Promotes cell elongation, initiates rooting in cuttings, causes Apical Dominance (terminal bud inhibits lateral buds; removing shoot tip 'pruning' promotes bushy tea gardens!). Synthetic 2,4-D is a selective herbicide killing broad-leaved dicot weeds!
Gibberellins ($GA_3$): Overcomes dwarfism; causes Bolting (internode elongation just prior to flowering in cabbage/beetroot); delays senescence in fruits.
Ethylene ($C_2H_4$): Gaseous hormone; triggers fruit ripening (climacteric respiration), promotes female flowers in cucumbers, and accelerates abscission.
Abscisic Acid (ABA):The Stress Hormone; induces stomatal closure during water deficit, promotes seed dormancy to survive freezing winters.
3. Photoperiodism & Vernalization
Response to day/night duration: Short-day plants require dark period exceeding critical photoperiod; mediated by light-sensitive pigment Phytochrome and hypothetical floral hormone Florigen synthesized in leaves! Vernalization: Quantitative or qualitative dependence on low winter temperatures ($1-5^\circ\text{C}$) to induce subsequent flowering.
Plant Growth and Development - Key Biological & Anatomical Model
Chapter Summary & 10 Key Takeaways
Takeaway 1
Apical Dominance: Auxin suppression of lateral axillary buds driven by apical meristem.
Takeaway 2
Bolting: Rapid internodal stem lengthening triggered by gibberellic acid in rosette plants.
Check Your Understanding (Diagnostic Practice Questions)
Diagnostic questions testing core conceptual clarity. Answers are hidden initially — solve each problem first, then click to reveal the step-by-step verified solution.
1
What is 'Apical Dominance'? Name the hormone responsible for it and explain how it is overcome in tea plantations.
Reveal Answer & Explanation
Answer: Apical dominance is the physiological phenomenon where the growing apical terminal shoot bud inhibits the growth of lateral (axillary) buds. It is caused by Auxins synthesized at shoot tips. In tea plantations and hedge-making, gardeners decapitate (prune) the shoot tips, removing the auxin source, which allows lateral buds to sprout vigorously into dense, bushy foliage. Apical bud inhibits lateral buds via auxin; overcome by pruning shoot tips.
2
Why is Abscisic Acid (ABA) called the 'Stress Hormone' in plants? Mention two of its protective actions.
Reveal Answer & Explanation
Answer: Because it is synthesized rapidly in response to environmental stresses like drought, extreme cold, and salinity. Protective actions: (1) It triggers immediate closure of stomata during water deficit, preventing catastrophic dehydration, (2) It induces seed dormancy, preventing seeds from germinating under harsh winter conditions. Synthesized under environmental stress; closes stomata and enforces seed dormancy.
3
Differentiate between Arithmetic Growth and Geometric Growth with mathematical expressions.
Reveal Answer & Explanation
Answer: In Arithmetic growth, following cell division, only one daughter cell continues to divide while the other differentiates and matures, producing a linear growth curve: $L_t = L_0 + rt$. In Geometric growth, both daughter cells retain the ability to divide, resulting in an initial slow lag phase followed by exponential explosive growth, producing a sigmoid curve: $W_1 = W_0 e^{rt}$. One dividing daughter cell (L_t = L0 + rt) vs both dividing (W1 = W0 e^rt).
4
What is 'Bolting'? Name the phytohormone that induces bolting in rosette plants.
Reveal Answer & Explanation
Answer: Bolting is the rapid internodal elongation of the stem just prior to flowering in plants with a rosette habit (like cabbage and beetroot). It is induced by Gibberellins. Rapid internodal elongation before flowering; induced by Gibberellins.
5
What is 'Photoperiodism'? Which organ of the plant perceives the light stimulus for flowering?
Reveal Answer & Explanation
Answer: Photoperiodism is the physiological response of plants to the relative duration of light and dark periods in a 24-hour day to induce flowering. The light stimulus is perceived by the Leaves, which synthesize the floral hormone Florigen. Response to day/night duration; perceived by Leaves.
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