Director's Brief
The Big Picture🎬 Plants Are Not Passive — They Respond
A plant cannot run from a drought, hide from a predator, or chase the sun across the sky on legs. But it can grow toward light, bend roots away from toxins, close stomata in drought, and even release chemical signals to warn neighbouring plants. These responses are slower than animal responses — but they are just as purposeful and just as precisely controlled.
Plant responses fall into two main categories: tropic responses (directional growth movements caused by a directional stimulus — the direction of the response depends on where the stimulus comes from) and nastic responses (non-directional — the response is the same regardless of stimulus direction).
| Response Type | Directional? | Stimulus Example | Plant Example |
|---|---|---|---|
| Positive tropism | ✅ Yes — toward stimulus | Light from one side | Shoot grows toward light |
| Negative tropism | ✅ Yes — away from stimulus | Gravity pulling down | Shoot grows upward (away from gravity) |
| Nastic response | ❌ No — same regardless of direction | Touch anywhere on leaf | Mimosa leaves fold shut |
| Taxis | ✅ Yes — whole organism moves | Light source | Single-celled algae swim toward light |
🎥 The Cast — Types of Tropism
☀️ Phototropism
Response to light. Shoots are positively phototropic — grow toward light. Roots are negatively phototropic — grow away from light.
⬇️ Gravitropism
Response to gravity. Roots are positively gravitropic — grow downward. Shoots are negatively gravitropic — grow upward.
💧 Hydrotropism
Response to water. Roots are positively hydrotropic — grow toward moisture. Can override gravitropism in dry conditions.
👆 Thigmotropism
Response to touch/contact. Tendrils of climbing plants curl around supports when they make contact — allowing the plant to climb.
Tropisms
Episode by EpisodeEach tropism is a documentary episode. Tap each card to see the narration and full mechanism.
Episode 1 — Phototropism
Chasing the Light
The stem bends toward the window. The roots grow away. One hormone — two opposite results.
🎙️ Narrator
"A young seedling sits on a windowsill. Within hours it has curved dramatically toward the light source. But the roots, hidden beneath the soil, have done the opposite — turning away from the window entirely. Remarkably, the same hormone has produced two completely opposite responses in two different organs. This is phototropism — and auxin is the director behind the performance."
⚙️ The Mechanism — Shoot
- Light causes unequal distribution of auxin (IAA)
- Auxin migrates away from the light — accumulates on the shaded side
- Higher auxin on dark side → faster cell elongation on that side
- Shaded side grows longer → shoot curves TOWARD light
- = Positive phototropism
⚙️ The Mechanism — Root
- Same auxin redistribution occurs in roots
- BUT roots are MORE sensitive to auxin than shoots
- High auxin concentration on shaded side INHIBITS root elongation
- Lit side grows longer → root curves AWAY from light
- = Negative phototropism
⚠️ The Key Exam Principle
The same auxin concentration that promotes elongation in a shoot will inhibit elongation in a root — because roots are far more sensitive to auxin. This single principle explains why shoots and roots respond in opposite directions to the same hormone redistribution. Always link concentration to organ type in your answer.
Auxin moves to shaded side
Shoots: high auxin = more growth
Roots: high auxin = less growth
Same hormone — opposite results
Episode 2 — Gravitropism
Earth's Pull
A seed placed sideways in darkness. The root curves down. The shoot curves up. No eyes, no light — just gravity.
🎙️ Narrator
"We have placed a germinating seed on its side in a clear gel chamber — in complete darkness. Within 24 hours, the root tip has curved unmistakably downward. The young shoot has simultaneously arched upward. Neither structure has any light to guide it. The plant is reading gravity itself, using specialised cells that sense which way is down."
🔬 How Roots Detect Gravity
- Root cap cells contain statoliths — dense starch grains
- Statoliths sink to the lowest point in the cell in response to gravity
- Their position triggers redistribution of auxin toward the lower side
- High auxin on lower side inhibits root elongation there
- Upper side grows longer → root bends downward
- Root = positively gravitropic
🌱 How Shoots Respond
- Auxin also accumulates on the lower side of horizontal shoots
- In shoots, high auxin = more elongation (not inhibition)
- Lower side grows faster than upper side
- Shoot curves upward — away from gravity
- Shoot = negatively gravitropic
🎬 Documentary Insight
Gravitropism ensures seeds germinate correctly regardless of how they land in the soil. A seed buried upside down will still send its root downward and shoot upward. This is why you don't need to orient seeds correctly when planting — the plant always finds its way.
Statoliths detect gravity
Roots: positively gravitropic
Shoots: negatively gravitropic
Works in complete darkness
Episode 3 — Thigmotropism
The Climbers
A tendril touches a stake and within minutes begins to coil. Touch is all it takes.
🎙️ Narrator
"The tendril of a pea plant sweeps slowly through the air. Then — contact. It brushes against a bamboo stake. Within minutes, the tendril has begun to curl around the support, tightening its grip. Within an hour, it is firmly anchored. The plant has used touch alone to solve the engineering problem of climbing upward toward the light."
⚙️ Mechanism
- Specialised cells on the contact side detect mechanical pressure
- Auxin redistributes away from the contact side
- Cells on the non-contact side elongate more
- Tendril curves toward the contact side — curling around the support
- Repeated coiling tightens the grip
🌿 Why It Matters
- Allows climbing plants to reach light without thick supporting stems
- Energy-efficient — uses other structures for support
- Examples: pea plants, passionflower, grape vines
- The response is directional (toward the contact point) — so it IS a tropism, not a nastic response
Response to touch/contact
Auxin moves away from contact side
Directional — IS a tropism
Tendrils curl toward support
Episode 4 — Hydrotropism
Following Water
Roots can sense moisture gradients and grow toward water — even against gravity.
🎙️ Narrator
"In this drought-stricken landscape, a tree root system has grown in an unexpected direction — horizontally toward an underground water pipe, overriding the gravitropic pull downward. The roots have detected a moisture gradient in the soil and followed it with extraordinary precision. Hydrotropism at work."
⚙️ Mechanism
- Root cap cells detect differences in water concentration in soil
- Auxin accumulates on the drier side — inhibiting growth there
- Wetter side grows faster → root curves toward moisture
- Roots = positively hydrotropic
- In dry conditions, hydrotropism can override gravitropism
🌍 Real World Significance
- Tree roots notoriously grow toward water pipes and foundations
- Important in agriculture — irrigation placement affects root direction
- Helps plants survive drought by locating deeper water sources
Response to water
Roots: positively hydrotropic
Can override gravitropism
Phytohormones
The Plant's Chemical Directors🧪 Plant Hormones — Slow Signals, Lasting Effects
Phytohormones (plant hormones) are chemical substances produced in small quantities in one part of a plant that are transported to other parts where they have specific effects. Unlike animal hormones, they don't travel in blood — they move through phloem, xylem, or by diffusing cell to cell. They control growth, development, responses to the environment, and reproduction.
Hormone 1
Auxin (IAA) — The Growth Architect
Produced at the shoot tip. Controls cell elongation. Drives all tropic responses.
📍 Production & Transport
- Produced in the apical meristem (shoot tip)
- Transported downward through the plant — polar auxin transport
- Can be redistributed laterally in response to light or gravity
- Moves by active transport — one direction only
⚙️ Effects
- Promotes cell elongation in shoot cells at low/moderate concentrations
- Inhibits cell elongation in roots at same concentrations (roots more sensitive)
- High concentrations inhibit lateral bud growth — apical dominance
- Promotes root formation on cuttings
- Synthetic auxins used as rooting powder and some herbicides
🎬 Director's Commentary
Apical dominance is when the main shoot tip suppresses side branches from growing — the auxin from the tip inhibits lateral buds. When you prune the tip of a plant, auxin levels drop and side branches start growing — making the plant bushier. Gardeners use this principle constantly.
⚠️ Exam Watch — Concentration Matters
Auxin effects depend entirely on concentration AND which organ it's acting on. Always specify: low concentration in shoot = promotes growth. Same concentration in root = inhibits growth. High concentration in shoot = inhibits growth (e.g. in lateral buds). Draw the concentration graph if asked — shoots and roots have different optimal concentration ranges.
Made at shoot tip
Promotes cell elongation in shoots
Inhibits root elongation
Causes apical dominance
Hormone 2
Gibberellins — The Stretch Factor
Promotes stem elongation, seed germination, and fruit development.
📍 What It Does
- Promotes internode elongation — makes stems grow taller
- Breaks seed dormancy — triggers germination
- Stimulates fruit development (sometimes without fertilisation)
- Works with auxin — together they produce more elongation than either alone
🌾 Applications
- Used commercially to produce seedless grapes (larger, without seeds)
- Applied to dwarf plants to make them grow to normal height
- Genetic dwarfism in plants = inability to produce gibberellins
- Barley germination uses gibberellins to activate enzymes that digest starch
Promotes stem elongation
Breaks seed dormancy
Seedless fruit production
Dwarfism = gibberellin deficiency
Hormone 3
Ethylene — The Ripening Signal
A gas produced by ripening fruit. Triggers ripening in neighbouring fruits. One rotten apple really does spoil the bunch.
🎙️ Narrator
"The old saying is true — one rotten apple really does spoil the barrel. The culprit is ethylene, a gas released by ripening fruit that triggers ripening in every fruit it touches. Fruit storage facilities flood their chambers with CO₂ to suppress ethylene and slow ripening. Supermarkets, meanwhile, do the opposite — releasing ethylene gas to ripen bananas on demand."
⚙️ Effects
- Promotes fruit ripening — colour change, softening, sweetening
- Triggers leaf, flower, and fruit abscission (dropping)
- Promotes ageing (senescence) in plants
- Unusual hormone — it's a gas, not a liquid
🌍 Commercial Uses
- Bananas picked green → shipped → ethylene gas applied at destination to ripen
- Cold storage uses CO₂ to suppress ethylene — delays ripening
- Ethylene inhibitors extend shelf life of cut flowers
Promotes fruit ripening
Causes leaf drop
Gaseous hormone
Spreads ripening to neighbours
Hormone 4
Abscisic Acid (ABA) — The Stress Responder
The plant's emergency hormone. Closes stomata in drought. Triggers dormancy in winter.
⚙️ Effects
- Closes stomata rapidly in drought — prevents water loss
- Promotes seed dormancy — prevents germination in unfavourable conditions
- Inhibits growth — "the stress brake"
- Antagonist to gibberellins — opposes germination signals
- Promotes leaf senescence (ageing and dropping)
🔬 Stomatal Closure Mechanism
- ABA binds to receptors on guard cells
- Triggers K⁺ ions to leave guard cells
- Guard cells lose water by osmosis → become flaccid → stomata close
- This happens within minutes of water stress being detected
⚠️ Exam Watch — Guard Cells
Guard cells control stomatal opening and closing. When turgid (full of water) → stomata open. When flaccid (water lost) → stomata close. ABA causes K⁺ to leave guard cells → water follows by osmosis → cells become flaccid → stomata close. This is a classic structure-function-hormone question.
ABA = drought response
Closes stomata
Promotes dormancy
Opposes gibberellins
Nastic Responses
Non-Directional Movements✨ Nastic Responses — Same Regardless of Direction
A nastic response is a plant movement that occurs in response to a stimulus but is NOT directional — the movement is the same no matter which direction the stimulus comes from. These are often faster and more dramatic than tropic responses, and can involve changes in cell water pressure rather than actual growth.
🌸 Photonasty — Flowers Opening and Closing
🎙️ Narrator
"As dawn breaks over the meadow, tulips that were tightly closed during the cool night begin to unfurl. By midday they are fully open. As temperatures drop in late afternoon, they close again. This is photonasty — a response to light intensity that is the same whether light comes from the left, right, or directly above."
⚙️ Mechanism
Temperature and light changes trigger unequal growth of inner and outer petal cells. Inner cells elongate more with warmth/light → petals curve outward (open). Opposite at night → petals close. It is not reversible growth — it is differential growth rates.
🌺 Examples
- Tulips open in warmth, close in cold
- Daisies open in sunlight, close at night
- Responds to temperature AND light intensity
- NOT directional — opening is the same regardless of light direction
👆 Thigmonasty — The Touch Response
🎙️ Narrator
"We touch the leaf of a Mimosa pudica — the sensitive plant. Within one second, the leaflets snap shut. Within three seconds, the entire leaf has folded down. There is no growth happening here — instead, specialised cells at the base of each leaflet have rapidly lost water, collapsing and causing the dramatic folding."
⚙️ Mechanism — Mimosa pudica
- Touch triggers an electrical signal (similar to animal nerves)
- Signal reaches pulvini — specialised motor cells at leaf base
- K⁺ and water rapidly leave pulvinus cells (osmosis)
- Cells become flaccid instantly → leaf folds down
- Recovery takes 15–30 minutes as water re-enters
🎯 Why It's Nastic (Not Thigmotropic)
- Response is the same no matter where you touch the leaf
- NOT growth — it's a turgor pressure change (reversible)
- Thigmotropism (tendrils) = permanent directional growth toward contact
- Thigmonasty = non-directional, reversible turgor-based movement
| Feature | Tropism | Nastic Response |
|---|---|---|
| Directional? | Yes — depends on stimulus direction | No — same regardless of direction |
| Mechanism | Unequal cell elongation (growth) | Turgor pressure changes OR differential growth |
| Reversible? | No — growth is permanent | Often yes — turgor changes are reversible |
| Speed | Hours to days | Seconds to hours |
| Hormone involved? | Yes — mainly auxin | Often turgor-based — no hormones needed |
| Example | Shoot bending toward light | Mimosa leaf folding when touched |
🎯 Documentary Final Exam
Eight questions. Did you pay attention during filming?
Question 1 of 8
A sunflower shoot grows toward a light source on its left. This is an example of which type of response?
Question 2 of 8
Auxin accumulates on the shaded side of a shoot. How does this cause the shoot to bend toward the light?
Question 3 of 8
A root is exposed to the same auxin concentration as a shoot tip. The root grows LESS on the high-auxin side. Why?
Question 4 of 8
A gardener prunes the tip of a plant and notices the plant becomes much bushier. Which hormone principle explains this?
Question 5 of 8
A seed is placed on its side in complete darkness. After 48 hours, the root has curved downward. What does this demonstrate?
Question 6 of 8
During a drought, a plant rapidly closes its stomata. Which hormone triggers this response?
Question 7 of 8
Mimosa pudica leaves fold when touched anywhere on the leaf. Why is this classified as a nastic response rather than a tropism?
Question 8 of 8
Unripe bananas are shipped green and then treated with a gas at their destination to ripen on demand. Which hormone is used?