Unit 9 Tropic responses
Key unit competence: Investigate the responses of plants to external stimuli.
Key ideas
- Sensitivity is the ability to detect and respond to a stimulus (a change in the environment, such as light, gravity, water, touch or chemicals). Animals respond quickly using sense organs, nerves and muscles; plants respond slowly by growth.
- A tropism (tropic response) is a growth response of part of a plant whose direction depends on the direction of the stimulus. Positive tropism: growth towards the stimulus; negative tropism: growth away from it.
- Phototropism is a response to light from one direction. Shoots are positively phototropic (grow towards light); roots usually show no or negative response. It lets leaves get the most light for photosynthesis.
- Gravitropism (geotropism) is a response to gravity. Roots are positively gravitropic (grow down: anchorage, water and minerals); shoots are negatively gravitropic (grow up towards light). A seedling placed on its side bends its root down and its shoot up.
- Hydrotropism: roots grow towards water (positive). Thigmotropism: tendrils and stems of climbing beans and passion fruit coil around supports after touching them. Chemotropism: the pollen tube grows down the style towards the ovule, guided by chemicals.
- Tropisms are controlled by the plant hormone auxin, made in the shoot tip. With light from one side, auxin moves to the shaded side, where cells elongate more, so the shoot bends towards the light. Experiments: a shoot with its tip covered does not bend; a shoot with its tip cut off does not bend; covering only the lower part still allows bending, so the tip detects light.
- Investigating phototropism: grow seedlings in a box with a slit letting light in from one side (control: seedlings with light from all sides or on a klinostat). Investigating gravitropism: lay soaked bean seedlings in damp paper in different directions in the dark; after a few days all roots bend down.
- Differences: plant responses are slow, growth-based, controlled by hormones and usually permanent; animal responses are fast, use nerves and muscles and are often reversible.
- Importance: shoots reach light, roots anchor and find water and minerals, climbers reach light without thick stems, pollen tubes reach ovules. Farmers use this: staking climbing beans, spacing crops so they get light, watering so roots spread well.
Common misconceptions
- "Plants bend towards light because light pulls them." No: auxin collects on the shaded side, making those cells grow longer.
- "Roots grow down because the soil is below." No: roots respond to gravity; even in air or on damp paper they bend downwards.
- "Tropisms are fast like animal movements." No: tropic responses take hours or days because they depend on growth.
- "The whole stem detects light." No: the shoot tip detects light; the bending happens just below the tip.
Worked examples
Example 1. A pot plant on a windowsill bends towards the window. Explain why and state the advantage.
Solution
Light comes from one side. Auxin made in the tip moves to the shaded side, so cells there elongate more and the shoot bends towards the light (positive phototropism). The leaves receive more light for photosynthesis.
Example 2. Three seedlings are lit from one side: A uncovered, B tip covered with foil, C lower stem covered. Predict the results.
Solution
A bends towards light; B grows straight up (the tip cannot detect light); C bends towards light like A. This shows that the tip detects the light.
Practice questions
Define: (a) stimulus; (b) tropism; (c) positive and negative tropism.
Show answer
(a) A change in the environment that an organism detects. (b) A growth response of part of a plant to a stimulus coming from one direction. (c) Positive: growth towards the stimulus; negative: growth away from it.
Common mistake: Defining tropism as "movement of the whole plant".
Name the tropism shown by: shoot towards light; root towards gravity; tendril around a stick; pollen tube to ovule.
Show answer
Positive phototropism; positive gravitropism; thigmotropism; chemotropism.
Common mistake: Calling the tendril response phototropism.
Explain the role of auxin in phototropism.
Show answer
Auxin is made at the shoot tip; with light from one side it moves to the shaded side; it makes cells there elongate more, so the shoot bends towards the light.
Common mistake: Saying auxin collects on the lit side.
Describe an experiment to show that shoots are positively phototropic.
Show answer
Put potted seedlings in a box with a slit on one side (light from one direction) and others in uniform light as a control; water both; after a few days the seedlings in the box bend towards the slit, the controls grow straight.
Common mistake: Forgetting the control.
Describe an experiment to show that roots are positively gravitropic.
Show answer
Germinate bean seeds in damp paper towels; place seedlings in jars with roots pointing up, sideways and down, in the dark; keep damp; after a few days all roots bend downwards.
Common mistake: Doing the experiment in light, so light could be the cause.
Give three differences between plant and animal responses.
Show answer
Plants respond by growth, animals by movement; plants are slow, animals fast; plants use hormones (auxin), animals use nerves and muscles; plant responses are usually permanent.
Common mistake: Saying plants cannot respond at all.
Explain how tropisms help a bean plant to survive.
Show answer
Shoots grow towards light for photosynthesis; roots grow down and towards water for anchorage and absorption; tendrils climb supports to reach light; pollen tubes reach ovules for seed production.
Common mistake: Giving the definitions without the advantages.
How can farmers use knowledge of tropisms?
Show answer
Provide stakes for climbing beans; space and orient crops to receive sunlight; water so that roots grow well; use auxin-like chemicals as rooting powders for cuttings.
Common mistake: Giving answers with no link to tropisms.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Roots growing downwards show: a. a change that an organism can detect 2. Growth of a shoot towards light is called: b. positive gravitropism 3. A stimulus is: c. shaded side 4. In a shoot lit from one side, auxin collects on the: d. positive phototropism Show answer
1 → b (positive gravitropism) · 2 → d (positive phototropism) · 3 → a (a change that an organism can detect) · 4 → c (shaded side)
- Roots grow in the direction of gravity.
- Photo = light; positive = towards.
- Light and gravity are stimuli.
- More auxin on the shaded side makes those cells longer.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
shoot tip · leaves receive more light for photosynthesis · tropism · leaves only- A growth response of a plant to a stimulus from one direction is a .
- The main advantage of phototropism to a plant is that .
- Auxin is made mainly in the .
Show answer
1: tropism · 2: leaves receive more light for photosynthesis · 3: shoot tip
- Tropisms are directional growth responses.
- More light means more food.
- The tip produces auxin, which moves down.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Climbing beans coil around sticks because of: a. bends downwards 2. Compared with animal responses, plant responses are usually: b. slower and growth-based 3. Shoots growing upwards, against gravity, show: c. negative gravitropism 4. A seedling is laid on its side in the dark. After three days its root: d. thigmotropism Show answer
1 → d (thigmotropism) · 2 → b (slower and growth-based) · 3 → c (negative gravitropism) · 4 → a (bends downwards)
- Thigmo = touch.
- Plants have no nervous system.
- They grow away from the pull of gravity.
- Roots are positively gravitropic.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
sensitivity · excretion · positive hydrotropism · climb towards light using supports- Roots growing towards moist soil show .
- Tendrils of passion fruit help the plant to .
- The ability of organisms to detect and respond to stimuli is called .
Show answer
1: positive hydrotropism · 2: climb towards light using supports · 3: sensitivity
- Hydro = water.
- Climbing reaches light without a thick stem.
- It is one of the characteristics of living things.