Plant Growth and Development ISC Class 11: Germination, Growth and Hormones

What Plant Growth and Development is about

Weightage: Plant Growth and Development belongs to the unit Plant Physiology, which CISCE weights at 17 of the 70 marks in the ISC Class XI Biology Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Biology syllabus, examination year 2028, Class XI unit weightage

Key concepts

Seed germination: epigeal, hypogeal and viviparous

Germination is the process by which the embryo in a seed resumes growth and forms a seedling. It needs water (to activate enzymes and soften the seed coat), oxygen (for aerobic respiration) and a suitable temperature; some seeds also need light. The radicle emerges first and forms the root; the plumule forms the shoot. Epigeal germination: the hypocotyl (part of the axis below the cotyledons) elongates rapidly and arches upwards, pulling the cotyledons above the soil, where they often turn green and photosynthesise for a while; examples: bean, castor, cotton, gourds, sunflower (and onion among monocots). Hypogeal germination: the epicotyl (part above the cotyledons) elongates, so the plumule comes out while the cotyledons remain below the soil; examples: pea, gram, maize, rice, mango. Viviparous germination (vivipary): the seed germinates while the fruit is still attached to the parent plant; the seedling grows a long, heavy radicle and hypocotyl, then drops into the mud and quickly anchors itself; examples: mangroves such as Rhizophora and Bruguiera. Vivipary helps mangrove seedlings establish in salty, waterlogged, tidal mud where ordinary seeds would be washed away.

Worked example

In a bean seedling the cotyledons appear above the soil, but in a pea seedling they stay underground. Which part elongates in each?

Try it first, then show the solutionSolution

1. Bean: the hypocotyl elongates and lifts the cotyledons above ground (epigeal)
2. Pea: the epicotyl elongates, carrying only the plumule upward (hypogeal)

Answer: Hypocotyl in bean (epigeal); epicotyl in pea (hypogeal)

Phases of growth and the sequence of development

Plant growth is indeterminate (open): it continues throughout life because meristems at root and shoot tips (and the cambium) keep producing cells. In a root or shoot tip three phases of growth can be seen. (1) Meristematic phase: at the apex; cells divide constantly; they have dense cytoplasm, large nuclei and thin primary cellulose walls with many plasmodesmata. (2) Phase of elongation: just behind the meristem; cells enlarge, mainly by increased vacuolation, and new wall material is deposited; this region contributes most of the increase in length. (3) Phase of maturation: further back; cells reach their maximum size, their walls thicken and their protoplasm changes as they differentiate into specific tissues. Sequence of developmental processes in a plant cell: meristematic cell → cell division → plasmatic growth → expansion (elongation) → differentiation → maturation → mature cell → senescence → death. Development is plastic: the same plant can form different structures in different conditions or stages, e.g. heterophylly, where juvenile and adult leaves differ (cotton, coriander, larkspur) or where leaves differ in air and water (buttercup).

Worked example

In a root tip, which region adds most to its length, and how do its cells grow?

Try it first, then show the solutionSolution

1. Cells produced in the meristem pass into the region of elongation
2. There they enlarge by taking up water into vacuoles, and new wall material is laid down

Answer: The region of elongation, by vacuolation and wall extension

Growth rate: arithmetic, geometric, absolute and relative

Growth rate is the increase in growth per unit time. (1) Arithmetic growth: after division only one daughter cell keeps dividing, while the other differentiates (as in a root elongating at a constant rate). Length rises linearly: $L_t=L_0+rt$, where $L_0$ is the initial length, $r$ the growth rate and $t$ time. A plot of length against time is a straight line. (2) Geometric growth: both daughters keep dividing, so cell number doubles each generation (1, 2, 4, 8, ...). In the early phase growth is slow (lag phase), then rapid and exponential (log phase), then slows as nutrients become limited (stationary phase), giving a sigmoid (S-shaped) curve, typical of cells, tissues, organs and whole organisms in natural conditions. Exponential growth is expressed as $W_1=W_0{e}^{rt}$, where $W_0$ and $W_1$ are initial and final size and $r$ is the relative growth rate (the efficiency index). Absolute growth rate: total growth per unit time. Relative growth rate: growth per unit time per unit initial size. Two organs may show the same absolute growth but very different relative growth.

Worked example

In two days leaf P grows from 4 cm² to 8 cm² and leaf Q from 40 cm² to 44 cm². Compare their absolute and relative growth.

Try it first, then show the solutionSolution

1. Absolute growth: both increase by 4 cm² (2 cm² per day each)
2. Relative growth: P = 4/4 = 100% over two days; Q = 4/40 = 10%

Answer: Same absolute growth; P's relative growth rate is ten times that of Q

Measuring growth: direct method and auxanometer; factors affecting growth

Growth can be measured as increase in fresh or dry weight, length, area, volume or cell number. Direct method: length is measured with a scale at regular intervals; for example, a root tip is marked with ink lines at equal intervals and remeasured after a day: the marks behind the tip spread out most in the region of elongation. Auxanometer: an instrument that magnifies small increases in length. In the arc auxanometer, a thread tied to the shoot tip passes over a small pulley and is kept taut by a weight; a long pointer fixed to the pulley moves along a graduated arc as the stem grows. Because the pointer is much longer than the pulley's radius, a small growth produces a large movement: actual growth = distance moved by the pointer × (radius of pulley ÷ length of pointer). Factors affecting growth: (1) water, for cell enlargement (turgor) and as the medium of enzyme reactions; (2) oxygen, for respiration to release energy; (3) nutrients (macro- and micro-elements) as raw materials and energy sources; (4) temperature, within an optimum range, since growth is enzyme-controlled; (5) light, for photosynthesis and for photomorphogenic responses; (6) gravity, which affects the direction of growth.

Worked example

In an arc auxanometer the pulley radius is 1.5 cm and the pointer is 30 cm long. The pointer moves 12 cm along the arc in 4 hours. Find the actual growth and the growth rate.

Try it first, then show the solutionSolution

1. Magnification = pointer length ÷ pulley radius = 30 ÷ 1.5 = 20
2. Actual growth = 12 ÷ 20 = 0.6 cm; rate = 0.6 cm ÷ 4 h = 0.15 cm per hour

Answer: 0.6 cm; 0.15 cm per hour

Differentiation, dedifferentiation and redifferentiation (brief idea)

Differentiation: cells derived from meristems mature to perform specific functions, undergoing changes in walls and protoplasm; e.g. to become a tracheary element a cell loses its protoplasm and develops a strong, lignified secondary wall suited to carrying water under tension. Dedifferentiation: living, differentiated cells that had stopped dividing regain the ability to divide; e.g. parenchyma cells forming the interfascicular cambium and cork cambium, or cells of an explant forming an undifferentiated callus in tissue culture. Redifferentiation: cells produced by such dedifferentiated tissue again lose the capacity to divide and mature into specialised cells; e.g. the secondary xylem and phloem formed from the cambium, and cork cells formed from cork cambium. These processes show that plant cells retain totipotency, the potential to form a whole plant.

Worked example

In plant tissue culture, a piece of leaf forms a callus, which later forms roots and shoots. Name the two processes involved.

Try it first, then show the solutionSolution

1. Leaf cells regain the ability to divide and form undifferentiated callus: dedifferentiation
2. Callus cells form organised roots and shoots: redifferentiation

Answer: Dedifferentiation followed by redifferentiation

Auxins and gibberellins: discovery, effects and uses

Auxins. Discovery: Charles Darwin and his son Francis noticed that coleoptiles of canary grass bend towards light only if the tip is present (phototropism), suggesting the tip sends a signal; F.W. Went later collected the substance from oat (*Avena*) coleoptile tips into agar blocks and showed it caused bending. Natural auxins: IAA (indole-3-acetic acid), IBA; synthetic: NAA, 2,4-D. Effects: (1) apical dominance: the shoot tip suppresses lateral buds; removing it (decapitation) lets side branches grow, used in tea plantations and hedge-making; (2) rooting of stem cuttings, used in propagation; (3) prevent premature drop of fruits and leaves, but promote abscission of older ones; (4) parthenocarpy (seedless fruits, e.g. tomato); (5) 2,4-D kills dicot weeds without harming monocot crops (weed-free lawns); (6) promote xylem differentiation and cell division. Gibberellins (GA). Discovery: E. Kurosawa found that rice seedlings infected with the fungus *Gibberella fujikuroi* grew abnormally tall ('bakanae', foolish seedling disease); the active substance was gibberellic acid. More than 100 gibberellins are known; GA₃ is the most studied. Effects: (1) stem elongation: GA sprayed on sugarcane lengthens internodes and increases sugar yield; (2) increase length of grape stalks and improve shape of apples; (3) bolting (sudden internode elongation before flowering) in rosette plants such as beet and cabbage; (4) break seed dormancy and speed up malting in brewing; (5) delay senescence, so fruits can stay on the tree longer; (6) hasten maturity and early seed production in conifers.

Worked example

Why do gardeners pinch off the tips of plants to make hedges bushy?

Try it first, then show the solutionSolution

1. Auxin from the shoot apex suppresses growth of lateral buds (apical dominance)
2. Removing the tip removes the auxin source, so lateral buds grow into branches

Answer: Decapitation removes apical dominance caused by auxin

Cytokinins, ethylene and abscisic acid: discovery, effects and uses

Cytokinins. Discovery: F. Skoog and co-workers found that tobacco stem callus divided only when given extracts such as yeast extract, coconut milk or DNA; C. Miller and Skoog later isolated the active substance from autoclaved herring sperm DNA and named it kinetin (a modified adenine). A natural cytokinin, zeatin, was later found in corn kernels and coconut milk. Effects: (1) promote cell division (cytokinesis); (2) help form new leaves, chloroplasts and lateral shoots, and adventitious shoots in callus; (3) overcome apical dominance; (4) delay leaf senescence by promoting nutrient mobilisation, used to keep cut flowers and vegetables fresh. Ethylene. Discovery: H.H. Cousins observed that ripe oranges released a volatile substance that hastened ripening of stored bananas; this was later identified as ethylene, a simple gas. Effects: (1) fruit ripening with a rise in respiration (respiratory climacteric); (2) senescence and abscission of leaves, flowers and fruits; (3) in dicot seedlings, horizontal growth, swelling of the axis and apical hook formation; (4) breaks seed and bud dormancy and starts sprouting of potato tubers; (5) rapid internode and petiole elongation in deep-water rice; (6) promotes flowering in pineapple (synchronised fruit set) and mango, and more female flowers in cucumbers. Ethephon releases ethylene and is the common commercial source. Abscisic acid (ABA). Discovery: three groups independently found growth inhibitors, called inhibitor-B, abscission II and dormin; they turned out to be the same compound, ABA. Effects: (1) closes stomata under water stress, so it is called the stress hormone; (2) induces seed dormancy and helps seeds withstand desiccation; (3) inhibits growth and metabolism, acting as an antagonist to GA; (4) increases tolerance to various stresses and can promote abscission and senescence. Most effects result from interactions between hormones.

Worked example

A trader stores unripe mangoes with a few ripe ones in a closed box. Why do the unripe ones ripen faster?

Try it first, then show the solutionSolution

1. Ripe fruits release ethylene gas
2. In a closed box ethylene builds up and triggers ripening (respiratory climacteric) in the unripe fruits

Answer: Ethylene from ripe fruits hastens ripening

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, Growth Rates and Measurement
• Plant Growth Regulators and Photoperiodism

Plant Growth and Development: key facts

Arithmetic growth

$L_t=L_0+rt$

Straight-line graph of length vs time.

Geometric (exponential) growth

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

r is the relative growth rate; doubling time = ln 2 ÷ r.

Relative growth rate

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

Absolute growth rate = increase in size ÷ time.

Arc auxanometer

$\text{actual growth}=\text{pointer movement}\times \frac{\text{pulley radius}}{\text{pointer length}}$

Magnification = pointer length ÷ pulley radius.

Germination types

$\text{epigeal: hypocotyl elongates};\text{hypogeal: epicotyl elongates};\text{vivipary: on parent plant}$

Bean, castor; pea, gram; Rhizophora, Bruguiera.

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.

• Hypo-up, Epi-up

  Epigeal: hypocotyl grows (cotyledons up). Hypogeal: epicotyl grows (cotyledons stay down). The names are opposite to the part that grows.

  Use it when: Germination types.

• Rhizophora rides on mum

  Vivipary: seeds germinate on the parent (Rhizophora, Bruguiera).

  Use it when: Viviparous germination.

• Darwin-Went, Kurosawa, Skoog-Miller, Cousins, Dormin

  Auxin from oat tips; GA from foolish rice; kinetin from herring DNA; ethylene from oranges; ABA from three inhibitors.

  Use it when: Discovery of PGRs.

• Auxin ARPP

  Apical dominance, Rooting, Prevents drop, Parthenocarpy (plus weedicide).

  Use it when: Four effects of auxin.

• Cytokinin = cell division and youth

  Cytokinins divide cells and delay ageing; ABA puts plants to sleep.

  Use it when: Contrasting 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 D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 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: Swapping hypocotyl and epicotyl

  The prefixes are confused with the germination names.

  Wrong working

  In epigeal germination the epicotyl elongates.

  Right working

  Epigeal: hypocotyl elongates (cotyledons above ground); hypogeal: epicotyl elongates.

  ✓Do this: Ask where the cotyledons end up, then which part pushed them.

  Usually costs: 1 mark.

• ✗Mistake: Giving fewer than four effects

  Students list two or three.

  Wrong working

  Gibberellins cause stem elongation and bolting.

  Right working

  The syllabus asks for four effects of each regulator.

  ✓Do this: Learn four numbered effects per hormone.

  Usually costs: 1-2 marks.

• ✗Mistake: Wrong discoverer

  Names are mixed up.

  Wrong working

  Kurosawa discovered kinetin.

  Right working

  Kurosawa: gibberellin (bakanae); Skoog and Miller: kinetin; Cousins: ethylene; Went: auxin.

  ✓Do this: Pair each scientist with the material used.

  Usually costs: 1 mark.

• ✗Mistake: Confusing absolute and relative growth

  Only the increase is compared.

  Wrong working

  Two leaves growing 5 cm² each have equal growth rates in every sense.

  Right working

  Absolute rates are equal; relative rates depend on initial size.

  ✓Do this: Always divide by initial size for relative rate.

  Usually costs: 1-2 marks.

• ✗Mistake: Forgetting to divide by magnification

  The pointer reading is taken as growth.

  Wrong working

  Pointer moved 12 cm, so the plant grew 12 cm.

  Right working

  Growth = 12 × (pulley radius ÷ pointer length).

  ✓Do this: Write the auxanometer formula before substituting.

  Usually costs: 1-2 marks.

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

   Hypogeal germination is seen in:
   (a) castor
   (b) bean
   (c) pea
   (d) cotton

   Hint

   Cotyledons stay underground.

   Answer

   (c)

2. Q2 · 1 mark · MCQExam level

   The gaseous plant hormone is:
   (a) ABA
   (b) ethylene
   (c) IAA
   (d) kinetin

   Hint

   Fruit ripening.

   Answer

   (b)

3. Q3 · 1 mark · Very short answerExam level

   Name a synthetic auxin used as a weedicide.

   Hint

   Kills dicots.

   Answer

   2,4-D

4. Q4 · 1 mark · Very short answerExam level

   What is bolting?

   Hint

   Rosette plants.

   Answer

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

5. Q5 · 2 marks · Short answerExam level

   Name the three phases of a sigmoid growth curve.

   Hint

   S-shape.

   Answer

   Lag phase (slow initial growth), log or exponential phase (rapid growth), stationary phase (growth slows and stops as resources become limiting).

6. Q6 · 2 marks · Short answerHarder exam level

   Give two differences between auxins and cytokinins in their effects on lateral buds and leaves.

   Hint

   Apical dominance, senescence.

   Answer

   Auxins maintain apical dominance (suppress lateral buds); cytokinins overcome it (promote lateral buds). Cytokinins delay leaf senescence; auxins mainly prevent premature leaf drop but promote abscission of older leaves.

7. Q7 · 3 marks · Short answerHarder exam level

   Name the factors affecting plant growth and explain the role of any two.

   Hint

   Six factors.

   Answer

   Water, oxygen, nutrients, temperature, light, gravity. Water: provides turgor for cell enlargement and is the medium for enzyme activity. Oxygen: needed for aerobic respiration to release energy for growth.

8. Q8 · 3 marks · Short answerHarder exam level

   Give the discovery and four effects of cytokinins.

   Hint

   Herring sperm DNA.

   Answer

   Skoog and Miller isolated kinetin from autoclaved herring sperm DNA after finding that tobacco callus needed such extracts to divide; zeatin found in corn kernels and coconut milk. Effects: cell division; new leaves, chloroplasts and adventitious shoots; overcome apical dominance; delay senescence by nutrient mobilisation.

9. Q9 · 3 marks · Short answerHarder exam level

   Give the discovery and four effects of abscisic acid.

   Hint

   Three names, one compound.

   Answer

   Three inhibitors, inhibitor-B, abscission II and dormin, were discovered independently and shown to be identical: ABA. Effects: closes stomata under water stress; induces seed dormancy and desiccation tolerance; inhibits growth (GA antagonist); increases stress tolerance and promotes abscission/senescence.

10. Q10 · 3 marks · Short answerHarder exam level

    Explain the sequence of developmental processes in a plant cell.

    Hint

    From meristem to death.

    Answer

    Meristematic cell → cell division → plasmatic growth → expansion (elongation) → differentiation → maturation → mature cell → senescence → death. The pathway can be modified by environment and stage of life (plasticity), e.g. heterophylly.

11. Q11 · 5 marks · Long answerHarder exam level

    (a) Describe the discovery of gibberellins. [1]
    (b) Give four physiological effects and two commercial applications of gibberellins. [4]

    Hint

    Foolish seedlings.

    Answer

    (a) Kurosawa observed that rice seedlings infected with Gibberella fujikuroi grew abnormally tall (bakanae); the active substance was gibberellic acid. (b) Effects: stem/internode elongation; bolting in rosette plants; breaking seed dormancy; delaying senescence; elongation of grape stalks and improved apple shape. Applications: increased sugarcane yield by internode elongation; faster malting in brewing (or larger grapes, early seed production in conifers).

12. Q12 · 5 marks · Long answerHarder exam level

    (a) Explain how growth is measured with an arc auxanometer, with a labelled diagram. [3]
    (b) A pointer of length 40 cm on a pulley of radius 2 cm moves 16 cm in 8 hours. Calculate the growth rate. [2]

    Hint

    Divide by magnification.

    Answer

    (a) A thread tied to the stem tip passes over a pulley and carries a weight; a long pointer fixed to the pulley moves along a graduated arc as the stem grows, magnifying growth by pointer length ÷ pulley radius. (b) Magnification = 40 ÷ 2 = 20; growth = 16 ÷ 20 = 0.8 cm; rate = 0.8 ÷ 8 = 0.1 cm per hour.

13. Q13 · 2 marks · Short answerHarder exam level

    A plant tissue culture medium with a high auxin-to-cytokinin ratio makes callus form roots, while a high cytokinin-to-auxin ratio makes it form shoots. What does this show about plant hormones?

    Hint

    Interaction.

    Answer

    Hormones act in combination: the ratio between them, not each alone, decides the pattern of redifferentiation (auxin favours roots, cytokinin favours shoots).

14. Q14 · 3 marks · Short answerHOTS / competency

    Seeds of some desert plants germinate only after heavy rain washes the seed coat. Suggest which hormone the rain removes and why this is an advantage.

    Hint

    Dormancy.

    Answer

    Heavy rain likely leaches out abscisic acid, the inhibitor that keeps seeds dormant. This ensures seeds germinate only when enough water is available for the seedling to establish, not after a light shower that soon dries up.

15. Q15 · 3 marks · Short answerHOTS / competency

    A farmer sprays 2,4-D on a wheat field full of broad-leaved weeds. Explain why the wheat survives, and predict what would happen if the same spray were used on a field of mustard.

    Hint

    Monocot vs dicot.

    Answer

    2,4-D is a synthetic auxin that, at high concentration, disrupts growth of dicots (broad-leaved weeds) but has little effect on monocots such as wheat. Mustard is a dicot, so the spray would kill or badly damage the crop itself.

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.Part elongating in epigeal germination?Show answer

   Hypocotyl.

2. 2.Example of vivipary?Show answer

   Rhizophora.

3. 3.Who isolated auxin from Avena tips?Show answer

   F.W. Went.

4. 4.Fungus causing bakanae disease?Show answer

   Gibberella fujikuroi.

5. 5.Source of kinetin?Show answer

   Autoclaved herring sperm DNA.

6. 6.Commercial source of ethylene?Show answer

   Ethephon.

7. 7.Stress hormone?Show answer

   Abscisic acid.

8. 8.Instrument to magnify growth?Show answer

   Auxanometer.

Quick revision checklist

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

• ☐Can you give two examples each of epigeal, hypogeal and viviparous germination?
• ☐Can you describe the three phases of growth?
• ☐Can you write the equations for arithmetic and geometric growth?
• ☐Can you calculate actual growth from an auxanometer reading?
• ☐Can you define dedifferentiation with an example?
• ☐Can you describe the discovery of all five growth regulators?
• ☐Can you give four effects of each regulator?
• ☐Can you name two applications each of auxin, GA and ethylene?

Plant Growth and Development: frequently asked questions