Plant Growth and Development Class 11 Biology: Growth Rates, Differentiation and Plant Growth Regulators

What Plant Growth and Development is about

Weightage: This chapter is part of Unit IV, Plant Physiology, which carries 12 of the 70 theory marks in the school annual exam together with Photosynthesis in Higher Plants and Respiration in Plants.Source: CBSE Biology (044) syllabus 2026-27, Class XI course structure, Unit IV Plant Physiology

Key concepts

Growth: seed germination, indeterminate growth and measurement

NCERT section Introduction (seed germination); 13.1 Growth (13.1.1 Plant Growth Generally is Indeterminate; 13.1.2 Growth is Measurable)

Seed germination is the first step of plant growth. A seed germinates when the environment is favourable (water, oxygen, suitable temperature). If not, it does not germinate and stays in suspended growth; when conditions improve it resumes metabolic activity and grows. In bean germination, the hypocotyl emerges and forms a hook, the cotyledons come above ground, and the epicotyl grows to form the shoot.

Growth defined: an irreversible permanent increase in size of an organ, its parts or even a single cell. It is accompanied by metabolic processes (both anabolic and catabolic) that use energy. (Swelling of a dry piece of wood in water is not growth because it is reversible and not due to living processes.)

Plant growth is generally indeterminate (open form of growth):

• Plants keep the capacity for unlimited growth throughout life, because of meristems whose cells can divide and self-perpetuate; their products soon stop dividing and form the plant body.
• Root and shoot apical meristems: primary growth, elongation along the axis.
• Lateral meristems (vascular cambium and cork cambium), appearing later in dicots and gymnosperms: secondary growth, increase in girth.

Growth is measurable: at cell level, growth mainly means more protoplasm, which is hard to measure directly, so we measure something roughly proportional to it: fresh weight, dry weight, length, area, volume and cell number.

• One maize root apical meristem can produce more than 17,500 new cells per hour (growth as cell number).
• Cells of a watermelon may increase in size up to 3,50,000 times (growth as cell size).
• Pollen tube: growth measured as length; dorsiventral leaf: as surface area.

Worked example

Why is plant growth called 'open'? Name the meristems responsible for growth in length and in girth. (3 marks)

Try it first, then show the solutionSolution

1. Meristems keep dividing and self-perpetuating throughout life, so new cells are always being added: growth is open/indeterminate. (1)
2. Length: root and shoot apical meristems (primary growth). (1)
3. Girth: vascular cambium and cork cambium (lateral meristems; secondary growth in dicots and gymnosperms). (1)

Answer: Meristems keep adding cells; apical meristems for length, lateral meristems for girth.

Phases of growth

NCERT section 13.1.3 Phases of Growth

Looking along a root or shoot tip, three zones (phases) are seen one after another:

1. Meristematic phase: constantly dividing cells at the root apex and shoot apex.

• Cells are rich in protoplasm with large, conspicuous nuclei.
• Walls are primary, thin and cellulosic, with many plasmodesmata.

2. Elongation phase: cells just behind (proximal to) the meristematic zone.

• Increased vacuolation, cell enlargement and new cell wall deposition.

3. Maturation phase: further away from the apex.

• Cells reach their maximum size in wall thickening and protoplasmic changes; most familiar tissues belong here.

Parallel line technique: mark equally spaced lines on a growing root tip. After some time, the lines just behind the apex (zones A-D in the textbook figure) are spaced farthest apart, showing that the elongation zone grows most.

Worked example

In a root marked with ink lines 1 mm apart, which lines move apart most after 24 hours, and what are the features of cells in that region? (2 marks)

Try it first, then show the solutionSolution

1. Lines just behind the root tip (the elongation zone) move apart most. (1)
2. Cells there show increased vacuolation, enlargement and new wall deposition. (1)

Answer: The elongation zone just behind the tip; vacuolation, enlargement, wall deposition.

Growth rates: arithmetic, geometric, absolute and relative

NCERT section 13.1.4 Growth Rates

Growth rate = increase in growth per unit time.

Arithmetic growth:

• After mitosis, only one daughter cell keeps dividing; the other differentiates and matures.
• Example: a root elongating at a constant rate.
• Plot of length vs time: a straight line.
• Lt = L₀ + rt (Lt = length at time t; L₀ = length at time zero; r = growth rate, i.e. elongation per unit time).

Geometric growth:

• Both daughter cells keep the ability to divide.
• Growth is slow at first (lag phase), then rapid and exponential (log/exponential phase); with limited nutrients it slows to a stationary phase.
• Plot vs time: an S-shaped (sigmoid) curve, typical of living organisms in natural environments and of all cells, tissues and organs of a plant.
• W₁ = W₀ e^(rt) (W₁ final size; W₀ initial size; r growth rate; t time; e base of natural logarithms).
• Here r is the relative growth rate, a measure of the plant's ability to produce new material, called the efficiency index. The final size W₁ depends on the initial size W₀.

Comparing growth:

• Absolute growth rate: total growth per unit time.
• Relative growth rate: growth per unit time per unit initial size (expressed on a common basis).
• Example: two leaves both gain 5 cm² in the same time. Leaf A grows from 5 to 10 cm²; leaf B from 50 to 55 cm². Absolute growth is equal (5 cm²), but A's relative growth is 5/5 = 100% while B's is 5/50 = 10%. So A has the higher relative growth rate.

Worked example

A root is 4 cm long and grows arithmetically at 0.5 cm per day. Find its length after 10 days, and state what kind of curve its growth would give. (2 marks)

Try it first, then show the solutionSolution

1. Lt = L₀ + rt = 4 + 0.5 × 10 = 4 + 5 = 9 cm. Check: 0.5 cm/day for 10 days adds 5 cm to 4 cm = 9 cm. (1)
2. A straight line (linear curve) when length is plotted against time. (1)

Answer: 9 cm; linear graph.

Conditions for growth

NCERT section 13.1.5 Conditions for Growth

Water:

• Plant cells grow mainly by cell enlargement, which needs water.
• Turgidity of cells drives extension growth, so growth and development are closely tied to the plant's water status.
• Water is also the medium for the enzyme reactions of growth.

Oxygen: helps release metabolic energy (respiration) for growth.

Nutrients: macro- and micro-nutrients are needed to make protoplasm and act as an energy source.

Temperature: each plant has an optimum temperature range; going far outside it can harm survival.

Environmental signals: light and gravity also affect certain phases of growth (e.g. direction of shoot and root growth).

Worked example

Explain why a plant grows less during a drought even when nutrients are plentiful. Give two reasons. (2 marks)

Try it first, then show the solutionSolution

1. Cell enlargement needs water; without turgidity cells cannot extend. (1)
2. Water is the medium for enzymatic reactions; with little water, metabolism and synthesis of protoplasm slow down. (1)

Answer: No turgor for enlargement, and enzymes lack their watery medium.

Differentiation, dedifferentiation, redifferentiation and development

NCERT section 13.2 Differentiation, Dedifferentiation and Redifferentiation; 13.3 Development

Differentiation: cells from apical meristems and cambium mature to perform specific functions. They undergo few to major structural changes in walls and protoplasm.

• Example: to form a tracheary element, a cell loses its protoplasm and develops a strong, elastic, lignocellulosic secondary wall to carry water long distances even under extreme tension.

Dedifferentiation: living differentiated cells that had lost the capacity to divide regain it under certain conditions.

• Example: interfascicular cambium and cork cambium form from fully differentiated parenchyma cells.
• Callus formed in plant tissue culture is also produced by dedifferentiation of parenchyma.

Redifferentiation: cells produced by the dedifferentiated meristem again lose the capacity to divide and mature for specific functions.

• Examples in a woody dicot: secondary xylem, secondary phloem, cork (phellem) and secondary cortex.

Open differentiation: cells from the same meristem become different at maturity, and their fate depends partly on position. E.g. cells pushed away from the root apical meristem become root-cap cells; those pushed to the periphery become epidermis.

Development: all changes in the life cycle, from seed germination to senescence. Sequence for a cell: meristematic cell → division → plasmatic growth → expansion (elongation) → differentiation → maturation → mature cell → senescence → death.

Plasticity: plants follow different pathways in response to environment or life phase, forming different structures.

• Heterophylly due to life phase: in cotton, coriander and larkspur, juvenile leaves differ in shape from mature leaves.
• Heterophylly due to environment: in buttercup, leaves formed in water differ from those formed in air.

Control of development: intrinsic factors (intracellular: genetic; intercellular: chemicals such as PGRs) and extrinsic factors (light, temperature, water, oxygen, nutrition).

Worked example

During secondary growth of a dicot stem, parenchyma between bundles becomes cambium, which then forms secondary xylem. Identify the two processes involved, and name the process that gives leaves of two shapes on a buttercup. (3 marks)

Try it first, then show the solutionSolution

1. Parenchyma (non-dividing) regaining division to form interfascicular cambium: dedifferentiation. (1)
2. Cambium cells maturing into non-dividing secondary xylem: redifferentiation. (1)
3. Different leaf shapes in water and in air: heterophylly, an example of plasticity due to environment. (1)

Answer: Dedifferentiation, redifferentiation; heterophylly (plasticity).

Plant growth regulators: nature, groups and discovery

NCERT section 13.4 Plant Growth Regulators (13.4.1 Characteristics; 13.4.2 The Discovery of Plant Growth Regulators)

PGRs (also called plant growth substances, plant hormones or phytohormones) are small, simple molecules of diverse chemistry:

• Indole compounds: indole-3-acetic acid (IAA).
• Adenine derivatives: N⁶-furfurylamino purine (kinetin).
• Carotenoid derivatives: abscisic acid (ABA).
• Terpenes: gibberellic acid (GA₃).
• Gases: ethylene (C₂H₄).

Two functional groups:

• Plant growth promoters: cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, seed formation: auxins, gibberellins, cytokinins.
• Plant growth inhibitors: responses to wounds and stresses (biotic and abiotic), dormancy, abscission: abscisic acid.
• Ethylene could fit either group, but is largely an inhibitor.

Discoveries (all accidental):

• Auxin: Charles Darwin and his son Francis saw canary grass coleoptiles bend towards one-sided light (phototropism); the tip was the source of a transmissible influence. F.W. Went isolated auxin from oat coleoptile tips.
• Gibberellin: 'bakanae' (foolish seedling) disease of rice, caused by the fungus *Gibberella fujikuroi*. E. Kurosawa (1926) reproduced the symptoms with sterile fungal filtrates; the active substance was gibberellic acid.
• Cytokinin: F. Skoog and co-workers found tobacco internode callus grew only if, besides auxin, the medium had vascular tissue extract, yeast extract, coconut milk or DNA. Miller et al. (1955) crystallised the active substance, kinetin.
• Abscisic acid: in the mid-1960s, three inhibitors (inhibitor-B, abscission II and dormin) were found to be chemically identical and named ABA.
• Ethylene: H.H. Cousins (1910) found that a volatile substance from ripened oranges hastened ripening of stored unripe bananas; it was later identified as ethylene.

Worked example

Match each PGR with its discovery: (a) bakanae disease of rice (b) oat coleoptile tips (c) ripe oranges and bananas (d) coconut milk and herring sperm DNA. Name the chemical group of each. (3 marks)

Try it first, then show the solutionSolution

1. (a) Gibberellin, a terpene; (b) auxin, an indole compound. (1½)
2. (c) Ethylene, a gas; (d) cytokinin (kinetin), an adenine derivative. (1½)

Answer: GA (terpene), auxin (indole), ethylene (gas), cytokinin (adenine derivative).

Physiological effects of the five PGRs

NCERT section 13.4.3 Physiological Effects of Plant Growth Regulators

Auxins (Greek *auxein*, to grow; first isolated from human urine)

• Natural: IAA, IBA; synthetic: NAA, 2,4-D. Made at growing apices of stems and roots and move to sites of action.
• Rooting of stem cuttings (plant propagation).
• Flowering in pineapple.
• Prevent early fruit and leaf drop but promote abscission of older leaves and fruits.
• Apical dominance: the apical bud suppresses lateral buds; decapitation (removing the tip) makes lateral buds grow. Used in tea plantations and hedge-making to get bushy growth.
• Parthenocarpy (seedless fruit) in tomato.
• Herbicides: 2,4-D kills dicot weeds but not mature monocots; used for weed-free lawns.
• Controls xylem differentiation and helps cell division.

Gibberellins (more than 100, GA₁, GA₂, GA₃...; GA₃ studied most; all acidic)

• Increase length of the axis: longer grape stalks.
• Make apples elongate and improve their shape.
• Delay senescence, so fruits stay on the tree longer (extended market period).
• Speed up malting in brewing.
• Sugarcane: longer stems, yield up by as much as 20 tonnes per acre.
• Juvenile conifers: faster maturity and early seed production.
• Bolting (internode elongation just before flowering) in beet, cabbage and rosette plants.

Cytokinins (effects on cytokinesis)

• Kinetin (a modified adenine) was found in autoclaved herring sperm DNA; it does not occur naturally in plants. Natural cytokinin zeatin was isolated from corn kernels and coconut milk.
• Made where cells divide rapidly: root apices, shoot buds, young fruits.
• New leaves, chloroplasts in leaves, lateral shoot growth, adventitious shoots.
• Overcome apical dominance.
• Promote nutrient mobilisation, which delays leaf senescence.

Ethylene (simple gas)

• Made in large amounts by tissues undergoing senescence and by ripening fruits.
• Horizontal growth of seedlings, swelling of the axis, apical hook in dicot seedlings.
• Promotes senescence and abscission, especially of leaves and flowers.
• Highly effective in fruit ripening; increases respiration during ripening (respiratory climacteric).
• Breaks seed and bud dormancy; starts germination in peanut, sprouting of potato tubers.
• Rapid internode/petiole elongation in deep-water rice (keeps leaves above water).
• Promotes root growth and root hairs.
• Initiates flowering and synchronises fruit-set in pineapple; induces flowering in mango.
• Source compound: ethephon, absorbed in water and slowly releases ethylene. It hastens ripening of tomato and apple, accelerates abscission (thinning of cotton, cherry, walnut) and promotes female flowers in cucumber.

Abscisic acid (ABA)

• General growth inhibitor and inhibitor of metabolism.
• Inhibits seed germination.
• Stimulates stomatal closure and increases stress tolerance: the stress hormone.
• Important in seed development, maturation and dormancy, helping seeds withstand desiccation.
• Generally antagonistic to GAs.

Interactions: PGR roles can be complementary or antagonistic, individual or synergistic. Several PGRs together control events such as dormancy, abscission, senescence and apical dominance. PGRs are one kind of intrinsic control; extrinsic factors like light and temperature often act through them.

Worked example

Name the PGR you would use for each job: (a) making seedless tomatoes, (b) getting longer sugarcane stems, (c) promoting adventitious shoots on a callus, (d) helping maturing seeds withstand drying, (e) thinning excess young fruits on cherry trees, (f) killing dicot weeds in a lawn. (3 marks)

Try it first, then show the solutionSolution

1. (a) Auxin (parthenocarpy) (½) (b) gibberellin (lengthens the axis) (½)
2. (c) Cytokinin (½) (d) abscisic acid (seed maturation and desiccation tolerance) (½)
3. (e) Ethylene, applied as ethephon (accelerates abscission) (½) (f) 2,4-D, a synthetic auxin (½)

Answer: Auxin, gibberellin, cytokinin, ABA, ethylene (ethephon), 2,4-D.

Plant Growth and Development in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Growth Phases and Growth Rates
• Plant Growth Regulators: Types and Effects

Plant Growth and Development: key facts

Arithmetic growth

$L_t=L_0+rt$

r = elongation per unit time; linear graph.

Geometric (exponential) growth

$W_1=W_0{e}^{rt}$

r = relative growth rate (efficiency index); W₁ depends on W₀.

Absolute growth rate

$\text{AGR}=\frac{\text{increase in size}}{\text{time}}$

Total growth per unit time.

Relative growth rate

$\text{RGR}=\frac{\text{increase in size}}{\text{initial size}\times \text{time}}$

Growth on a common basis, per unit initial size.

Sigmoid curve phases

$\text{lag phase}\to \text{log (exponential) phase}\to \text{stationary phase}$

Typical of cells, tissues and organs in natural conditions.

Development of a plant cell

$\text{division}\to \text{plasmatic growth}\to \text{expansion}\to \text{differentiation}\to \text{maturation}\to \text{senescence}\to \text{death}$

Development = growth + differentiation.

Plant Growth and Development: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Plant Growth and Development

The question patterns to expect, and the method that works for each.

Tricks to remember Plant Growth and Development

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Meristem → Elongate → Mature

  'Make, Enlarge, Mature': divide at the tip, enlarge behind it, mature further back.

  Use it when: Phases of growth.

• Lag-Log-Stay

  Sigmoid curve: lag (slow start), log (rapid), stationary (plateau): like a new student settling in, racing ahead, then levelling off.

  Use it when: S-curve phases.

• A-G-C promote, A stops, E does a bit of both

  Auxin, Gibberellin, Cytokinin = promoters; ABA = stopper; Ethylene = mostly inhibitor.

  Use it when: PGR groups.

• GA = Grow tAll

  Gibberellins lengthen: grape stalks, sugarcane, bolting in cabbage.

  Use it when: Gibberellin effects.

• Cytokinin = Keep it young

  Cytokinins delay leaf senescence, make chloroplasts and new leaves, and break apical dominance.

  Use it when: Cytokinin effects.

• ABA = Always Brakes Activity

  Closes stomata, stops germination, causes dormancy: the stress hormone.

  Use it when: ABA effects.

Plant Growth and Development: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Plant Growth and Development

The slips examiners see most often, and what to write instead.

• ✗Mistake: Calling swelling of seeds or wood in water 'growth'

  Size increases visibly.

  ✓Do this: Growth must be irreversible and linked to metabolism; imbibition of dead wood is reversible physical swelling.

  Usually costs: 1 mark.

• ✗Mistake: Drawing the sigmoid curve as a straight line or bell shape

  Confusing it with arithmetic growth or enzyme graphs.

  ✓Do this: S-shape: slow lag, steep log phase, flat stationary phase; label all three.

  Usually costs: 1-2 marks.

• ✗Mistake: Saying absolute and relative growth rates are the same

  Both use 'growth per unit time'.

  ✓Do this: Relative growth rate divides by the initial size; it lets you compare organs of different sizes.

  Usually costs: 1 mark.

• ✗Mistake: Writing that kinetin is a natural plant hormone

  It is the best-known cytokinin.

  ✓Do this: Kinetin came from autoclaved herring sperm DNA and does not occur naturally in plants; zeatin is natural.

  Usually costs: ½-1 mark.

• ✗Mistake: Giving cytokinin as the cause of apical dominance

  Hormones in the same topic are mixed up.

  ✓Do this: Auxin causes apical dominance; cytokinin helps overcome it.

  Usually costs: 1 mark.

• ✗Mistake: Writing about photoperiodism or vernalisation in detail

  They appear in older editions.

  ✓Do this: They are not in the 2026-27 syllabus; focus on growth, differentiation and the five PGRs.

  Usually costs: Time lost in the exam.

NCERT exercise guide: Plant Growth and Development

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Plant Growth and Development

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Which PGR is a derivative of carotenoids?
   (a) IAA (b) Kinetin (c) ABA (d) GA₃

   Hint

   The inhibitor.

   Answer

   (c) ABA

2. Q2 · 1 mark · MCQExam level

   Removal of the shoot tip results in growth of lateral buds. This shows:
   (a) apical dominance (b) parthenocarpy (c) bolting (d) senescence

   Hint

   Decapitation.

   Answer

   (a) apical dominance

3. Q3 · 2 marks · Very short answerExam level

   Name the scientists linked with (a) the coleoptile bending experiment and (b) the volatile substance from ripe oranges. (2 marks)

   Hint

   Father and son; 1910.

   Answer

   (a) Charles Darwin and Francis Darwin (canary grass); F.W. Went isolated auxin (1). (b) H.H. Cousins (1910) (1).

4. Q4 · 2 marks · Very short answerHarder exam level

   Why is a sigmoid curve considered typical of living organisms growing in nature? (2 marks)

   Hint

   Resources are finite.

   Answer

   Growth starts slowly, accelerates when many cells divide, but in nature nutrients and space are limited (1), so growth slows and levels off, giving an S-shaped curve for cells, tissues, organs and whole plants (1).

5. Q5 · 2 marks · Very short answerHarder exam level

   Give the characteristics of cells in the meristematic phase of growth. (2 marks)

   Hint

   Protoplasm, nucleus, walls.

   Answer

   Rich in protoplasm with large conspicuous nuclei (1); thin, primary, cellulosic walls with abundant plasmodesmata (1).

6. Q6 · 2 marks · Very short answerHarder exam level

   What is plasticity? Give one example each due to environment and due to life phase. (2 marks)

   Hint

   Heterophylly.

   Answer

   The ability of plants to follow different developmental pathways in response to environment or life phase (1). Environment: buttercup leaves in water vs air; life phase: juvenile vs mature leaves in cotton, coriander or larkspur (1).

7. Q7 · 3 marks · Short answerHarder exam level

   Give three effects of cytokinins and state where natural cytokinins are made. (3 marks)

   Hint

   Rapid cell division sites.

   Answer

   Promote new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoots; overcome apical dominance; delay leaf senescence by nutrient mobilisation (any three, 2). Made where cell division is rapid: root apices, developing shoot buds, young fruits (1).

8. Q8 · 3 marks · Short answerHarder exam level

   A single cell in culture divides every 6 hours, with both daughters always dividing again; a root 5 cm long elongates by 0.3 cm every 6 hours. (a) Which shows arithmetic and which geometric growth? (b) Find the number of cells and the length of the root after 24 hours. (c) What shape of graph does each give against time? (3 marks)

   Hint

   24 h = 4 periods of 6 h.

   Answer

   (a) The culture shows geometric growth (both daughters divide); the root shows arithmetic growth (constant elongation per unit time) (1). (b) 24 ÷ 6 = 4 rounds: 2⁴ = 16 cells; root: Lt = L₀ + rt = 5 + 0.3 × 4 = 6.2 cm (1). (c) Culture: exponential rise, becoming sigmoid once nutrients limit it; root: a straight line (1).

9. Q9 · 5 marks · Long answerHarder exam level

   (a) In a woody stem, a living cortical parenchyma cell becomes part of the cork cambium, which then produces cork and secondary cortex. Name the process at each step and explain. (b) A piece of carrot root on a nutrient medium forms a callus, which later gives rise to shoots and roots. Name the processes involved. (c) Why can a mature tracheary element never dedifferentiate? (5 marks)

   Hint

   Regaining division vs losing it again; living vs dead cells.

   Answer

   (a) The parenchyma cell had lost the capacity to divide; regaining it to form cork cambium is dedifferentiation (1½). Cork and secondary cortex cells formed by this meristem lose the capacity to divide and mature for specific functions: redifferentiation (1½). (b) Callus forms by dedifferentiation of the parenchyma cells; shoots and roots arise from the callus by redifferentiation (1). (c) During differentiation a tracheary element loses its protoplasm and builds a lignocellulosic secondary wall, so it is dead; only living cells can regain the capacity to divide (1).

10. Q10 · 3 marks · Short answerHOTS / competency

    Suppose stomata of a wheat plant close during a hot afternoon even though light is good. Name the hormone likely responsible and evaluate whether this response helps or harms the plant. (3 marks)

    Hint

    Trade-off.

    Answer

    ABA, which stimulates stomatal closure under water stress (1). It helps by cutting water loss and preventing wilting, increasing stress tolerance (1). But it also reduces CO₂ entry, so photosynthesis and growth slow; the plant trades growth for survival (1).

11. Q11 · 3 marks · Short answerHOTS / competency

    A gardener wants a weed-free lawn of grass and sprays 2,4-D. Another person sprays it on a vegetable garden of beans and tomatoes. Predict the results and explain. (3 marks)

    Hint

    Monocot vs dicot.

    Answer

    2,4-D is a synthetic auxin used as a herbicide that kills dicot weeds (1). In the lawn, broad-leaved (dicot) weeds die while mature grass (monocot) is unaffected, giving a weed-free lawn (1). In the vegetable garden, beans and tomatoes are dicots, so the crops themselves would be damaged or killed (1).

12. Q12 · 1 mark · Assertion-reasonHOTS / competency

    Assertion (A): Spraying juvenile conifers with gibberellins leads to early seed production.
    Reason (R): Gibberellins hasten the maturity period of juvenile conifers.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Faster maturity → earlier seeds.

    Answer

    (a)

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.How many new cells can one maize root apical meristem produce per hour?Show answer

   More than 17,500.

2. 2.By how much can watermelon cells increase in size?Show answer

   Up to 3,50,000 times.

3. 3.In arithmetic growth, how many daughter cells keep dividing?Show answer

   Only one.

4. 4.What does r in W₁ = W₀e^(rt) represent?Show answer

   Relative growth rate (efficiency index).

5. 5.Give an example of dedifferentiation.Show answer

   Formation of interfascicular cambium or cork cambium from parenchyma.

6. 6.Name two plants showing heterophylly with age.Show answer

   Cotton, coriander or larkspur (any two).

7. 7.Which fungus causes bakanae disease?Show answer

   Gibberella fujikuroi.

8. 8.Who isolated auxin, and from which plant?Show answer

   F.W. Went, from oat coleoptile tips.

9. 9.Name a natural cytokinin and its sources.Show answer

   Zeatin, from corn kernels and coconut milk.

10. 10.What were the three names later found to be ABA?Show answer

    Inhibitor-B, abscission II and dormin.

11. 11.Which compound is used commercially as a source of ethylene?Show answer

    Ethephon.

12. 12.What is bolting?Show answer

    Internode elongation just before flowering in rosette plants such as cabbage and beet, promoted by gibberellins.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you define growth and give two reasons why plant growth is open?
• ☐Can you describe the meristematic, elongation and maturation phases?
• ☐Can you write and explain Lt = L₀ + rt and W₁ = W₀e^(rt)?
• ☐Can you draw and label the sigmoid growth curve?
• ☐Can you distinguish absolute and relative growth rates with a numerical example?
• ☐Can you define dedifferentiation and redifferentiation with examples?
• ☐Can you tell the discovery story of each PGR?
• ☐Can you give three uses each of auxin, gibberellin, cytokinin, ethylene and ABA?

Plant Growth and Development: frequently asked questions