🌿 Chemical Coordination in Plants – Full Breakdown
ICSE Class 10
Biology
June 2026
Chemical coordination in plants is the shortest and most straightforward plant physiology chapter in ICSE Class 10 Biology. The entire chapter boils down to 5 plant hormones (auxin, gibberellin, cytokinin, ABA, ethylene) and how they control tropic and nastic movements. The key to scoring full marks is memorising the one unique function of each hormone and knowing the experiment that proves phototropism. Written by someone who scored 93.4% in ICSE 2026.
1) Why Do Plants Need Chemical Coordination?
Plants cannot move from place to place like animals, but they still need to respond to environmental stimuli (light, gravity, water, touch) for survival. Responses in plants are brought about by chemical messengers called hormones (phytohormones).
Phytohormones / Plant Hormones: Naturally produced organic compounds in plants that regulate growth, development, and responses to environmental stimuli. They are produced in small quantities and transported to the target site.
Unlike animal hormones (which are produced in glands), plant hormones are produced in meristematic tissues (root tip, shoot tip, young leaves, cambium). They act locally or are transported through diffusion, active transport, or via the xylem/phloem.
2) The 5 Major Plant Hormones
| Hormone | Site of Production | Major Functions | ICSE Significance |
|---|---|---|---|
| Auxin (IAA — Indole-3-acetic acid) | Shoot apical meristem, young leaves, developing seeds | Cell elongation, phototropism, geotropism, apical dominance, root initiation, prevention of abscission (leaf fall) | Most important hormone for ICSE — phototropism experiment, apical dominance, synthetic auxins as weed killers |
| Gibberellin (GA — Gibberellic acid) | Young leaves, shoot tips, roots, seeds | Stem elongation (internode growth), seed germination, fruit development (parthenocarpy), bolting in rosette plants | Elongation of stems (e.g., sugarcane), seed dormancy breaking, parthenocarpy (seedless fruits like grapes) |
| Cytokinin | Root tips, developing fruits and seeds | Cell division (cytokinesis), delay of senescence (ageing), promotion of lateral bud growth (opposite of auxin), nutrient mobilisation | Delays ageing of leaves (keeping them green longer), promotes lateral branching |
| Abscisic Acid (ABA) | Mature leaves (in chloroplasts), root tips | Stomatal closure (during water stress), seed dormancy promotion, inhibition of growth (stress hormone), abscission (leaf fall) | The "stress hormone" — closes stomata when water is scarce, induces seed dormancy |
| Ethylene (C₂H₄ — gaseous hormone) | All parts of the plant, especially ripening fruits and senescing tissues | Fruit ripening, abscission (leaf & fruit fall), senescence, inhibition of stem elongation, promotion of root hair formation | Only gaseous plant hormone. Used to artificially ripen fruits (bananas, tomatoes). "One bad apple spoils the bunch" — ethylene gas from the bad apple ripens (and spoils) the rest |
⚠️ ICSE loves comparisons — remember these pairs:
• Auxin vs Cytokinin: Auxin promotes apical dominance (main shoot grows, side shoots suppressed). Cytokinin promotes lateral bud growth (opposite effect).
• Gibberellin vs ABA: Gibberellin breaks seed dormancy and promotes germination. ABA induces seed dormancy and prevents germination. They are antagonistic.
• Ethylene: The only gaseous hormone. Accelerates ripening and senescence.
• Auxin vs Cytokinin: Auxin promotes apical dominance (main shoot grows, side shoots suppressed). Cytokinin promotes lateral bud growth (opposite effect).
• Gibberellin vs ABA: Gibberellin breaks seed dormancy and promotes germination. ABA induces seed dormancy and prevents germination. They are antagonistic.
• Ethylene: The only gaseous hormone. Accelerates ripening and senescence.
📌 FIGURE 1 — Phototropism Experiment (Darwin & Boysen-Jensen)
Fig: Darwin's experiment showing the tip of the coleoptile detects light, and auxin diffuses down the shaded side to cause bending. Boysen-Jensen showed auxin is a chemical that diffuses through gelatin. ICSE 2019, 2021, 2023.
3) Tropic Movements (Directional Growth Responses)
Tropic movements are growth movements in response to a unidirectional stimulus. The direction of growth depends on the direction of the stimulus. They are slow, irreversible growth movements.
| Type | Stimulus | Response | Hormone Involved | Example |
|---|---|---|---|---|
| Phototropism | Light | Shoot grows towards light (+ phototropism). Root grows away from light (− phototropism) | Auxin — accumulates on the shaded side of the shoot → cells elongate more → shoot bends towards light | Sunflower facing the sun, houseplants bending towards a window |
| Geotropism (Gravitropism) | Gravity | Root grows towards gravity (+ geotropism). Shoot grows away from gravity (− geotropism) | Auxin — in roots, high auxin on the lower side inhibits cell elongation (root bends down). In shoots, auxin on the lower side promotes cell elongation (shoot bends up) | Roots growing downwards, shoots growing upwards. A horizontally placed seed — root bends down, shoot bends up |
| Hydrotropism | Water | Roots grow towards water (+ hydrotropism) | Auxin (likely) — roots detect moisture gradient and grow towards higher moisture | Roots growing towards a water source in the soil |
| Chemotropism | Chemicals | Growth towards or away from chemicals | Various | Pollen tube growing towards the ovule (attracted by chemicals like calcium and sugars) — + chemotropism |
| Thigmotropism | Touch / physical contact | Growth in response to touch | Auxin | Tendrils of pea plant or grapevine coiling around a support when they touch it |
💡 Auxin — the dual-action secret:
Auxin has opposite effects on shoot and root cells:
• In shoots — high auxin → stimulates cell elongation
• In roots — high auxin → inhibits cell elongation
Why? Root cells are much more sensitive to auxin. The optimum auxin concentration for root growth is much lower than for shoot growth. The same concentration that promotes shoot elongation inhibits root elongation.
This explains geotropism: when a seed is placed horizontally, auxin accumulates on the lower side. In the shoot: lower side grows more → shoot bends up. In the root: lower side grows less → root bends down (the upper side grows relatively more).
Auxin has opposite effects on shoot and root cells:
• In shoots — high auxin → stimulates cell elongation
• In roots — high auxin → inhibits cell elongation
Why? Root cells are much more sensitive to auxin. The optimum auxin concentration for root growth is much lower than for shoot growth. The same concentration that promotes shoot elongation inhibits root elongation.
This explains geotropism: when a seed is placed horizontally, auxin accumulates on the lower side. In the shoot: lower side grows more → shoot bends up. In the root: lower side grows less → root bends down (the upper side grows relatively more).
📌 FIGURE 2 — Geotropism in a Horizontally Placed Seedling
Fig: Geotropic response of a horizontally placed seedling. The shoot grows upward against gravity, and the root grows downward towards gravity. ICSE 2018, 2020, 2022.
4) Nastic Movements (Non-Directional Responses)
Nastic movements are responses to stimuli where the direction of movement is independent of the stimulus direction. They are faster, reversible movements caused by changes in turgor pressure or differential growth.
| Type | Stimulus | Response | Example |
|---|---|---|---|
| Photonasty | Light | Opening/closing of flowers in response to light intensity (not direction) | Morning glory opens in morning (high light), closes in evening. Night-blooming jasmine (Cestrum) opens at night |
| Thigmonasty (Seismonasty) | Touch / vibration / shock | Rapid leaf folding or drooping in response to physical contact | Mimosa pudica (Touch-me-not): leaves fold and droop when touched. Caused by sudden loss of turgor in pulvini (specialised motor cells at leaf base) |
| Thermonasty | Temperature | Opening/closing of flowers in response to temperature | Tulip opens in warm temperatures, closes when cold |
| Nyctinasty (Sleep movements) | Day/night cycle (circadian rhythm) | Leaf movements in response to day/night | Plants that "sleep" — leaves fold up at night (e.g., bean, clover, tamarind) |
⚠️ Tropic vs Nastic — Key Difference for ICSE:
Tropic movements: Direction of response is determined by the direction of the stimulus. "When light comes from the right, the shoot bends right." → Directional.
Nastic movements: Direction of response is independent of the stimulus direction. "Whether touched from the left or right, the leaf folds the same way." → Non-directional.
Example: A flower opens in response to light (photonasty) — it opens whether light comes from above, left, or right. But a shoot bends towards the light source (phototropism) — the direction matters.
Tropic movements: Direction of response is determined by the direction of the stimulus. "When light comes from the right, the shoot bends right." → Directional.
Nastic movements: Direction of response is independent of the stimulus direction. "Whether touched from the left or right, the leaf folds the same way." → Non-directional.
Example: A flower opens in response to light (photonasty) — it opens whether light comes from above, left, or right. But a shoot bends towards the light source (phototropism) — the direction matters.
5) Mechanism of Phototropism — The Classic Experiment
ICSE often asks the full experimental proof of phototropism (Darwin's experiment with oat coleoptiles + Boysen-Jensen's modification).
Darwin's Experiment (1880):
- Setup 1 — Normal coleoptile: Exposed to light from one side → bends towards the light (control)
- Setup 2 — Tip cut off: Tip of coleoptile removed → no bending (the tip is needed to detect light)
- Setup 3 — Tip covered with opaque cap: Tip covered with a light-proof cap → no bending (the tip must receive light to trigger bending)
- Setup 4 — Tip covered with transparent cap: Tip covered with a transparent cap → bending occurs normally (light reaches the tip)
- Conclusion: The tip of the coleoptile perceives light. A signal is transmitted from the tip to the elongating region below, causing curvature
Boysen-Jensen's Experiment (1913):
- Setup 5 — Mica sheet inserted on shaded side: A thin mica (impermeable) sheet is inserted on the shaded side of the coleoptile, just below the tip → no bending. This blocked the signal from travelling down the shaded side
- Setup 6 — Mica sheet inserted on illuminated side: Mica sheet on the illuminated side → bending still occurs. The signal on the shaded side remains unblocked
- Setup 7 — Gelatin block instead of mica: When the tip is cut off and a block of gelatin is inserted between the tip and the rest, bending occurs. Gelatin allows a chemical to diffuse through
- Conclusion: The signal from the tip is a chemical that can diffuse through gelatin. This chemical was later identified as auxin. Light causes auxin to move to the shaded side → shaded side cells elongate more → bend towards light
📌 FIGURE 3 — Thigmonasty in Mimosa pudica
Fig: Mimosa pudica (Touch-me-not plant). Left: normal state. Right: after being touched — leaflets fold and petiole droops due to loss of turgor in pulvinar cells. ICSE 2018, 2021.
Pulvinus: A specialised, swollen motor organ at the base of a leaf or leaflet in certain plants. Changes in turgor pressure of pulvinar cells (due to ion movement) cause leaf movements in Mimosa and nyctinastic plants.
6) Practical Applications of Plant Hormones
| Hormone | Application | How it Works |
|---|---|---|
| Auxin (synthetic — 2,4-D, NAA) | Weed killers (herbicides) | Synthetic auxin (2,4-D) selectively kills dicot weeds. At high concentrations, auxin becomes toxic by overstimulating growth → plant dies. Monocot crops (wheat, rice) are less affected |
| Auxin (IBA, NAA) | Rooting powder — stimulates root formation in cuttings | Stem cuttings dipped in auxin powder (IBA) develop roots faster and more abundantly. Used in plant propagation (e.g., rose, hibiscus) |
| Auxin | Prevention of fruit & leaf drop | Auxin delays abscission — prevents premature fruit/leaf fall. Applied to apple orchards to reduce fruit drop before harvest |
| Gibberellin | Parthenocarpy (seedless fruits) | Gibberellin spray on unpollinated flowers → fruit develops without fertilisation → seedless grapes, tomatoes, cucumbers |
| Gibberellin | Stem elongation in sugarcane | Gibberellin spray increases internode length → higher yield of sugarcane stems |
| Gibberellin | Breaking seed dormancy | Treat seeds with gibberellin to promote germination of dormant seeds (e.g., lettuce, barley seeds in malting for beer) |
| Cytokinin | Delaying leaf senescence | Cytokinin sprayed on harvested leafy vegetables (cabbage, lettuce) keeps them fresh longer by delaying chlorophyll breakdown |
| Ethylene | Artificial fruit ripening | Unripe fruits are exposed to ethylene gas (or ethephon, a liquid that releases ethylene) to ripen bananas, mangoes, tomatoes before market |
| Ethylene | Promotion of abscission | Ethylene accelerates leaf and fruit fall. Used in mechanical harvesting of nuts (e.g., walnut, almond) — spray ethylene → fruit loosens → shaken off easily |
| ABA | Inducing stomatal closure in dry conditions | Not directly applied commercially, but understanding ABA helps in developing drought-resistant crops |
📋 Syllabus Note: ICSE specifically expects you to know at least one practical application for each hormone. The most frequently asked: auxin as weed killer / rooting powder, gibberellin in parthenocarpy, ethylene in fruit ripening.
7) Common ICSE Board Questions
1-Mark Definitions:
- Define: Phytohormone, Phototropism, Geotropism, Hydrotropism, Chemotropism, Thigmotropism, Nastic movement, Photonasty, Thigmonasty, Nyctinasty
- Name the plant hormone responsible for phototropism.
- Which hormone is known as the "stress hormone" in plants?
- Name the gaseous plant hormone.
- Which hormone promotes fruit ripening?
- Name the hormone that breaks seed dormancy.
- Which hormone promotes cell division?
- Name the hormone that delays leaf senescence.
- What is the role of auxin in apical dominance?
- Give one example of a thigmonastic movement in plants.
- Which part of the coleoptile detects light in phototropism?
- What is a pulvinus?
3-Mark / Short Answer:
- Differentiate between tropic and nastic movements (any 4 points in a table). (ICSE 2019, 2021)
- Explain the role of auxin in phototropism. With a diagram, describe the distribution of auxin in the shoot when light comes from one side.
- Describe Darwin's experiment on phototropism. What did he conclude? (ICSE 2020, 2023)
- How did Boysen-Jensen prove that the signal in phototropism is a chemical? (ICSE 2021)
- Differentiate between auxin and cytokinin (any 4 points). (ICSE 2020)
- Explain why the shoot grows upward (against gravity) and the root grows downward (towards gravity) in a horizontally placed seed. (ICSE 2018, 2022)
- List four applications of plant hormones in agriculture/horticulture.
- What is the difference between phototropism and photonasty? Give one example of each.
- Explain the mechanism of thigmonasty in Mimosa pudica. (ICSE 2018, 2021)
- Write a note on the functions of gibberellins. Name two practical applications.
5-Mark / Long Answer:
- Describe the experiments of Darwin and Boysen-Jensen that demonstrated phototropism in oat coleoptiles. What conclusion was drawn from each experiment? Draw labelled diagrams to support your answer. (ICSE 2019, 2021, 2023)
- Explain what is meant by tropic movements. Describe phototropism and geotropism in detail. What is the role of auxin in each? (ICSE 2020, 2022)
- List the five major plant hormones. For each, mention: site of production, one major function, and one practical application in a table. (ICSE 2019, 2021)
- Differentiate between tropic and nastic movements. Explain phototropism and thigmonasty with the help of labelled diagrams. (ICSE 2020)
- Describe the response of a plant to (a) unidirectional light, (b) gravity when placed horizontally, (c) touch in Mimosa pudica. For each, state the type of movement, the stimulus, and the response.
- Write the functions of auxin, gibberellin, and cytokinin. How do auxin and cytokinin differ in their effects on plant growth? Give three practical applications of auxin in agriculture.
🧠 From my exam — key things to focus on:
1. Darwin + Boysen-Jensen experiment — this is the most frequently asked long question in this chapter. Draw all setups side by side.
2. Tropic vs Nastic comparison — easy 3 marks. Know the key distinction: directional vs non-directional.
3. Auxin's dual role in shoot (stimulates elongation) vs root (inhibits elongation) — explains geotropism perfectly.
4. One practical application for each hormone — especially auxin (rooting powder/weed killer), gibberellin (parthenocarpy/stem elongation), ethylene (fruit ripening).
5. Mimosa pudica — the go-to example for thigmonasty. Know pulvinus and turgor loss.
Since this is a short chapter (usually 6-8 marks in the board exam), it's easy to score full marks with a well-organised revision sheet.
1. Darwin + Boysen-Jensen experiment — this is the most frequently asked long question in this chapter. Draw all setups side by side.
2. Tropic vs Nastic comparison — easy 3 marks. Know the key distinction: directional vs non-directional.
3. Auxin's dual role in shoot (stimulates elongation) vs root (inhibits elongation) — explains geotropism perfectly.
4. One practical application for each hormone — especially auxin (rooting powder/weed killer), gibberellin (parthenocarpy/stem elongation), ethylene (fruit ripening).
5. Mimosa pudica — the go-to example for thigmonasty. Know pulvinus and turgor loss.
Since this is a short chapter (usually 6-8 marks in the board exam), it's easy to score full marks with a well-organised revision sheet.
8) Quick Reference Tables (Print This)
Plant Hormones — One Look
| Hormone | Key Function | Unique Fact | Application |
|---|---|---|---|
| Auxin | Cell elongation, phototropism | Dual action — different effect on shoot vs root | Rooting powder, weed killer |
| Gibberellin | Stem elongation, seed germination | Breaks seed dormancy | Parthenocarpy, sugarcane yield |
| Cytokinin | Cell division, delays senescence | Promotes lateral buds (opposite of auxin) | Keeping vegetables fresh |
| ABA | Stomatal closure, seed dormancy | Stress hormone — closes stomata in drought | Understanding drought resistance |
| Ethylene | Fruit ripening, senescence | Only gaseous hormone | Artificial fruit ripening |
Tropic vs Nastic Movements
| Feature | Tropic | Nastic |
|---|---|---|
| Stimulus direction matters? | Yes — response is towards or away from the stimulus | No — response direction is independent of stimulus direction |
| Nature of movement | Growth movement (irreversible) | Often turgor-based (reversible) |
| Speed | Slow (hours/days) | Fast (seconds to minutes in Mimosa) |
| Hormone involvement | Mainly auxin | Turgor changes (Mimosa) or hormones (flowering) |
| Examples | Phototropism, geotropism, hydrotropism | Thigmonasty (Mimosa), photonasty (flower opening) |
⚠️ Final exam strategy: This is a 6-8 mark chapter in the board exam — don't over-study it, but don't ignore it either. The most common question pattern is:
1. "Explain phototropism with Darwin's experiment" (3-5 marks) — always prepared
2. "Differentiate between tropic and nastic movements with examples" (3 marks)
3. "Name the five plant hormones and write one function of each" (2-3 marks)
My advice: Create a one-page A4 sheet with the 5-hormone table and the Darwin/Boysen-Jensen experiment diagram. Review it once a day for 3 days before the exam. This chapter gave me full marks with minimal effort because it's so compact.
1. "Explain phototropism with Darwin's experiment" (3-5 marks) — always prepared
2. "Differentiate between tropic and nastic movements with examples" (3 marks)
3. "Name the five plant hormones and write one function of each" (2-3 marks)
My advice: Create a one-page A4 sheet with the 5-hormone table and the Darwin/Boysen-Jensen experiment diagram. Review it once a day for 3 days before the exam. This chapter gave me full marks with minimal effort because it's so compact.