Lesson overview
Plant responses: How does a shoot find the light?
Light four pots of oat seedlings from one side, with foil caps, clear caps or foil collars on their shoots, and measure how far each one bends with a protractor; then turn a dish of seedlings on its edge in the dark and light another from one side. For the Supplement, follow auxin from the shoot tip to the cells that elongate.
Suggested sequence
- Lesson 1 · Shoots that turn to the light. Phototropism: shoots grow towards light from one side; the tip detects the light; measuring the bend angle with a protractor. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Roots, shoots and gravity. Gravitropism: roots grow towards gravity and shoots away from it; investigating gravity and light with seedlings. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Auxin: the message from the tip. (Supplement) Auxin: made in the shoot tip, it moves away from the light or to the lower side and makes the cells there elongate. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 6 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 6 questions, 15 min, lesson closed.
Learning goals
Part 1: Shoots that turn to the light
- Describe phototropism as a response in which parts of a plant grow towards or away from the direction of the light source (14.5.2)
- Investigate and describe gravitropism and phototropism in shoots and roots (14.5.3)
Part 2: Roots, shoots and gravity
- Describe gravitropism as a response in which parts of a plant grow towards or away from gravity (14.5.1)
- Investigate and describe gravitropism and phototropism in shoots and roots (14.5.3)
Part 3: Auxin: the message from the tip
- Supplement: explain phototropism and gravitropism of a shoot as examples of the chemical control of plant growth (S14.5.4)
- Supplement: explain the role of auxin in controlling shoot growth: made in the shoot tip, diffuses from the shoot tip, unequally distributed in response to light and gravity, stimulates cell elongation (S14.5.5)
Misconceptions to listen for
- “A process in which a plant uses light to make glucose.” — That is photosynthesis. Phototropism is a growth response to the direction the light comes from.
- “A plant growing taller in bright light than in dim light.” — Phototropism is about the direction of the light, not how bright it is: the shoot curves towards one side.
- “A response in which parts of a plant grow towards or away from gravity.” — That is gravitropism, a response to gravity. Phototropism is a response to the direction of the light source.
- “The mimosa leaves folding when touched.” — That is a fast response to touch, a movement. Phototropism is growth towards or away from the direction of light.
- “The root growing down in the dark box.” — In the dark there is no light to respond to: the root is responding to gravity.
- “The tomatoes growing faster in the sun.” — Growing faster in bright light is not a response to where the light comes from. Phototropism means curving towards or away from the light source.
- “Shoots with black paper caps, kept in a dark cupboard.” — In the dark no shoot bends at all, so this group cannot tell you anything about the caps.
- “More shoots with black paper caps.” — More shoots make the result more reliable, but they do not test whether the cap itself stops the bending.
- “Shoots with their tips cut off.” — Cutting changes two things at once: the tip is gone and the shoot is wounded. It does not test the cap.
- “156°.” — That is the other scale, which runs in the opposite direction. Use the scale that starts at 0° where the shoot started, straight up.
Scope
Plant responses (14.5.1–14.5.5): gravitropism and phototropism as growth responses towards or away from gravity and light, and investigating them in shoots and roots; and, for the Supplement, gravitropism and phototropism as examples of the chemical control of plant growth, with auxin made in the shoot tip, diffusing through the plant and stimulating cell elongation, and unequal auxin explaining the bending of shoots.