Grade by Grade
F. Plants: Anatomy, Growth, and Function · Plants: Anatomy, Growth, and Function

Tropisms: How Plants Respond to Light, Gravity and Touch

F3.4 F2.1 A1.1 A1.10 A2.2About 65 minutes
Learning goals
  • I can define tropism and describe phototropism, gravitropism and thigmotropism as positive or negative.
  • I can explain how auxin causes phototropism and gravitropism.
  • I can interpret the historic coleoptile experiments of Darwin, Boysen-Jensen and Went.
  • I can tell the difference between a tropism and a nastic movement.

Words to know

tropism
A growth response of a plant toward (positive) or away from (negative) a directional stimulus.
phototropism
Growth in response to light; shoots usually show positive phototropism.
gravitropism (geotropism)
Growth in response to gravity; roots are positively gravitropic and shoots are negatively gravitropic.
thigmotropism
Growth in response to touch, such as tendrils coiling around a support.
coleoptile
The protective sheath over a young grass shoot; used in classic experiments on phototropism.
statolith
A dense, starch-filled body in root-cap cells that settles to the bottom of the cell and lets the root sense gravity.
nastic movement
A plant movement that is not directed by the direction of the stimulus, such as leaves folding when touched or flowers closing at night.

Plants that move

Turn a potted plant on a windowsill so its leaves face the room. In a few days, the shoot tips will have turned back toward the window. Lay a sprouting seed on its side and its root will curve down while its shoot curves up. Plants can't walk, but they grow in useful directions. These directional growth responses are called tropisms.

Remember from Lesson 12

Auxin is made in the shoot tip, moves down the stem and makes cells elongate. In this lesson you'll see that if auxin is unevenly spread, one side of a stem grows longer than the other, and the stem bends.

Three main tropisms

A tropism is positive if growth is toward the stimulus and negative if it is away.

  • Phototropism (light): shoots grow toward light (positive). This places leaves where they can photosynthesize.
  • Gravitropism (gravity), also called geotropism: roots grow down (positive), shoots grow up (negative). A seed planted upside down still sends its root down and its shoot up.
  • Thigmotropism (touch): the tendrils of peas, cucumbers and grapes coil around anything they touch, helping the plant climb toward light.

Roots also grow toward moist soil (hydrotropism).

Not every plant movement is a tropism. When the leaves of a sensitive plant (Mimosa) fold up as soon as you touch them, or tulips close at night, the movement doesn't depend on the direction of the stimulus. These are nastic movements, usually caused by quick changes in water pressure in special cells, not by growth.

How phototropism works

  1. Auxin is made in the shoot tip and moves down the stem.
  2. When light comes from one side, auxin is moved to the shaded side of the shoot.
  3. Cells on the shaded side receive more auxin, so they elongate more than cells on the lit side.
  4. The shaded side becomes longer, so the shoot bends toward the light.

Notice that the plant doesn't "reach" for light by growing faster on the lit side. It grows faster on the dark side.

light1. auxin even2. auxin moves tothe shaded side3. shaded side growslonger → bends to lightlonger cellson shaded side
Phototropism. Light from one side moves auxin (dots) to the shaded side, where cells elongate more, so the shoot bends toward the light.

Discovering auxin: a century of clever experiments (A2.2)

  • Charles Darwin and his son Francis (1880) shone light from one side on grass seedlings (coleoptiles). A seedling bent toward the light if its tip was uncovered, but not if its tip was cut off or covered with an opaque cap. A clear cap still allowed bending, and covering the lower part didn't stop it. Conclusion: the tip detects light, but the bending happens lower down, so some signal must travel from the tip.
  • Peter Boysen-Jensen (1913) cut off coleoptile tips and put a block of gelatin between the tip and the stump: the seedling still bent toward light. A thin piece of mica (which chemicals can't pass through) inserted on the shaded side stopped bending. So the signal is a chemical that moves down the shaded side.
  • Frits Went (1926) placed coleoptile tips on blocks of agar so the chemical would soak in. When he put an agar block off-centre on a cut stump, in the dark, the stump bent away from the side with the block, because cells under the block grew longer. This growth chemical soon became known as auxin (from the Greek for "to grow"); its main natural form, IAA, was identified in the 1930s.

These experiments are a model of good inquiry: each one tested a specific hypothesis with a careful control.

lightintactbendstip removedno bendopaque capno bendclear capbendsbase coveredbendsDarwin and his son Francis (1880): the tip detects light; the bending happens lower down.
Darwin's coleoptile experiments (light from the left). Bending happens only when the tip can sense light, even though the bend occurs below the tip.

How gravitropism works

In the root cap, special cells contain statoliths: heavy, starch-filled bodies that sink to the bottom of each cell, like sand in a jar. Their position tells the root which way is down. If a root is turned sideways, the statoliths settle on the new lower side, and auxin is redistributed to the lower side of the root. Shoots have their own statolith-containing cells (in a layer around the vascular tissue), so auxin also collects on the lower side of a sideways shoot.

The key twist: root cells and shoot cells respond differently to auxin.

  • In shoots, more auxin on the lower side makes those cells elongate more, so the shoot curves up.
  • In roots, which are far more sensitive, the higher auxin level on the lower side slows elongation there. The upper side grows faster, so the root curves down.

Same hormone, same signal, opposite result.

Day 0: laid on its sideDay 2rootshootroot bendsdown (+)shoot bendsup (−)↓ gravity
A seedling laid on its side. Within about two days the root has curved down (positive gravitropism) and the shoot has curved up (negative gravitropism).
Worked example

Let's predict the results of Went's experiment

Went placed agar blocks that had held coleoptile tips on cut stumps in the dark. Predict the result for each: (a) block centred on the stump; (b) block on the left side of the stump; (c) plain agar block (never touched a tip) on the left side.

  1. (a) Auxin moves down evenly: both sides elongate equally, so the stump grows straight up (and taller than one with no block).
  2. (b) Auxin moves down the left side only: left-side cells elongate more, so the stump bends to the right (away from the block).
  3. (c) Plain agar has no auxin: no extra elongation, no bending. This control shows the bending in (b) is caused by the chemical from the tip, not by the agar.
  4. Because the experiment is done in the dark, light can't be the cause of any bending.

(a) Straight growth; (b) bends to the right, away from the block; (c) no bending (control).

Worked example

Let's explain a houseplant's lean

A spider plant on a windowsill leans toward the window. The owner turns it 180° every week. Explain the lean and why turning it helps.

  1. Light comes mainly from the window side, so auxin moves to the shaded (room) side of the growing shoots.
  2. Shaded-side cells elongate more, so the leaves and stems bend toward the window (positive phototropism).
  3. Turning the plant 180° puts the formerly lit side in shade, so the plant now bends the other way. Over time the bends cancel, keeping the plant even.

The lean is positive phototropism caused by auxin moving to the shaded side; regular turning balances the growth.

Just for fun

Young sunflowers really do follow the sun: their heads turn from east to west during the day and swing back east overnight. Once the flowers open, most stop moving and face east, where morning sun warms them up and attracts more bees. So a field of mature sunflowers is basically a crowd waiting for sunrise.

Hands-on activity

Seedling maze and the sideways jar

Option A: phototropism maze (1 to 2 weeks). You need: a shoebox with a lid, cardboard, tape, scissors (grown-up help with cutting), a small pot, soil and a bean or pea seed that has just sprouted.

  1. Cut a 3 cm hole in one end of the box. Tape two cardboard "shelves" inside, each covering about two-thirds of the box width, on alternating sides, to make a zigzag path.
  2. Put the potted sprout at the end opposite the hole. Close the lid and stand the box so the hole faces a window.
  3. Open briefly every 2 days to water and sketch the shoot.

Option B: gravitropism jar (2 to 3 days). You need: a clear jar, paper towel, 3 bean or radish seeds that have just started to show a radicle, tape.

  1. Line the jar with damp paper towel; place the seeds between the towel and glass with their radicles pointing down.
  2. After one day, turn the jar on its side. Mark the root tips with a dot on the glass.
  3. Check and sketch every 12 hours.

What to look for: in the maze, the shoot grows around the shelves toward the hole (positive phototropism), often pale and long. In the jar, root tips curve to grow down again within a day (positive gravitropism) and any shoots curve up. Write a hypothesis first and record observations with dated sketches.

Your turn

1

Define tropism, and explain the difference between positive and negative tropisms.

2

Describe the tropisms of a shoot and a root in response to gravity.

3

Explain how auxin causes a shoot to bend toward light.

4

What did Darwin conclude from covering coleoptile tips with opaque caps?

5

What are statoliths, and what do they do?

6

Is a Mimosa leaf folding when touched a tropism? Explain.

7

Give an example of thigmotropism and explain its benefit.

More practice: warm-up

1

What stimulus causes phototropism?

2

Which hormone controls phototropism?

3

What is a coleoptile?

4

Which scientist first collected the growth chemical from coleoptile tips in agar blocks?

5

Is a root's growth toward water positive or negative hydrotropism?

More practice: on your own

1

In the gelatin experiment, why did Boysen-Jensen use gelatin rather than a solid barrier?

2

Explain why the same auxin distribution makes a shoot bend up but a root bend down.

3

A seedling grown on a slowly rotating turntable placed on its side (a clinostat) grows straight out sideways. Explain.

4

Suggest a hypothesis and prediction for an experiment testing whether pea tendrils coil faster around rough or smooth supports.

5

Why is phototropism an advantage for a plant growing under a forest canopy?

Challenge

1

A student puts an agar block from a coleoptile tip on the right side of a cut stump and places the stump in light coming from the right. Predict and explain the result.

2

Roots of plants grown on the International Space Station grow in many directions. Explain, and suggest which other tropism might still guide them.

3

Design an experiment to test the hypothesis that the root cap is needed for gravitropism.

Check your understanding

Quiz

1.

A shoot bending toward a window shows:

2.

During phototropism, auxin collects on the:

3.

Roots show which type of gravitropism?

4.

Darwin's experiments showed that:

5.

What allows root cells to sense gravity?

6.

Which is a nastic movement rather than a tropism?

For parents and teachers

Big idea: F3.4 lists tropism as a factor affecting plant growth. Students should explain phototropism and gravitropism using auxin, and understand the historic experiments that discovered auxin, which are a great model of scientific inquiry (A1.1, A1.10) and of scientists' contributions (A2.2).

How to help: The sideways jar gives results in a day or two; the shoebox maze takes about two weeks but is memorable. Ask your teen to write a hypothesis before starting and to date every sketch. Help with cutting the box.

Common difficulties:

  • Thinking plants grow faster on the lit side. It is the shaded side that grows more.
  • Getting the root response backwards: auxin slows root cell elongation.
  • Calling every plant movement a tropism (nastic movements are not directional growth).