Plants cannot walk to the light, but they can grow towards it. Shoots bend towards a window and grow up against gravity, while roots grow down into the soil. These tropic responses are growth responses to the direction of a stimulus. How the plant hormone auxin controls the bending of a shoot is Extended content.
Gravitropism is a response in which parts of a plant grow towards or away from gravity; phototropism is a response in which parts of a plant grow towards or away from the direction of the light source. Shoots grow towards light and away from gravity; roots grow towards gravity. Extended: these responses in a shoot are examples of chemical control of growth by auxin, which is made in the shoot tip, diffuses down the shoot, becomes unequally distributed and stimulates cell elongation.
Fig. 1: a shoot bending towards one-sided light, and a seedling laid on its side turning its shoot up and its root down
Section 01
Towards and away: tropisms.
Choose an experiment, press Grow for 3 days and watch the seedlings. Try light from one side, a seedling laid on its side, and the control with light from directly above.
Shoot
Root
Phototropism (response to the direction of light)
grows towards light: positive
grows away from light: negative
Gravitropism (response to gravity)
grows away from gravity: negative
grows towards gravity: positive
Why it matters: shoots growing towards light can photosynthesise more; roots growing down anchor the plant and reach water and mineral ions.
A growth response: tropisms happen because some cells grow more than others, so they are slow (hours to days), unlike a reflex.
Examiner trap
A tropism is a response to the direction of a stimulus. A shoot in light from directly above grows straight up: it still responds to light, but there is no bending because the light is even all round.
Stage 1 · Learn
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In the tropisms tab, how do the shoots grow with light from one side?
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When the seedling is laid on its side, how does the shoot respond?
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When the seedling is laid on its side, how does the root respond?
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Why does the sim include a control with light from directly above?
Section 02
Investigating tropisms.
Each investigation changes the direction of one stimulus and removes or controls the other.
Phototropism in shoots: grow seedlings in a box with a slit on one side so light comes from one direction; a control box has light from above (or seedlings rotated on a clinostat). After a few days the shoots in the slit box bend towards the light.
Gravitropism: pin germinating seeds (for example bean seeds) to damp cotton wool in a dish with their roots pointing in different directions, and keep them in the dark so light cannot affect them. Roots turn to grow downwards and shoots upwards.
Clinostat control: a seedling on a slowly rotating clinostat receives the stimulus equally on all sides, so it grows straight along its axis instead of bending.
Good practice: use several seedlings of the same type and age, keep temperature and water the same, and measure the angle of curvature or the direction of growth.
Examiner trap
In a gravitropism investigation the seeds must be kept in the dark (or lit evenly). Otherwise you cannot tell whether a shoot grew upwards because of gravity or because of light.
Section 03
Auxin and cell elongation.
Extended. Tick Show auxin and grow the seedlings again. The purple marks show where auxin collects in the shoot.
Made in the shoot tip: auxin is a plant hormone made by cells at the tip of the shoot.
Diffuses: it diffuses through the plant, back down the shoot from the tip.
Unequal distribution: in one-sided light, auxin collects on the shaded side; in a shoot laid on its side, it collects on the lower side.
Stimulates cell elongation: cells with more auxin elongate more, so that side grows longer and the shoot bends: towards the light, or upwards.
Chemical control: phototropism and gravitropism of the shoot are examples of the chemical control of plant growth.
Examiner trap
Auxin does not move to the lit side, and the lit side does not grow faster. More auxin on the shaded side makes those cells elongate more, so the shoot curves towards the light.
Stage 2 · Exam
Exam-style questions
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Which statement describes phototropism?
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Bean seeds are pinned to damp cotton wool in a dish kept in the dark. Why is the dish kept in the dark?
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Extended: where is auxin made in a shoot?
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Extended: a shoot is lit from the left. Which explains why it bends to the left?
Skill unlocked
Tropic responses, mastered.
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