Chapter 8 of 8

All 8 chapters
  1. 1The nervous system, readunderstood
  2. 2Reflexes, readunderstood
  3. 3The eye, readunderstood
  4. 4Hormones, readunderstood
  5. 5Blood sugar and diabetes, readunderstood
  6. 6Temperature and water balance, readunderstood
  7. 7Hormones and the menstrual cycle, readunderstood
  8. 8Plant responses, readunderstood

S4.5.8 Chapter 8

Plant responses

Seedlings grow in a closed box with one hole in its side. Which way will their shoots grow?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how auxin makes shoots grow towards light
  • Show seedlings bending towards light in a box with one hole

A plant on a windowsill leans towards the window, and its roots grow down. A plant cannot run from danger or walk to food. It responds by growing. The way a plant grows is set by what acts on it. Growth in a direction set by a stimulus is a tropism. The two you will study are light and gravity.

Take seedlings in a closed box. The box has one hole in one side, so light comes from that side only. Look at the seedlings at the start, and again after a few days.

Draw it first

Seedlings in a box with one hole

  1. Two pictures of seedlings in a box with one hole in the left wall, one after another. First, at the start, the labels are light, hole, seedlings and at the start, and the shoots stand straight up. Then, a few days later, the labels are light, hole, seedlings and a few days later, and every shoot bends towards the hole.lightholeseedlingsat the start

    1 At the start, the shoots stand straight up. Light comes in through the hole.

  2. Two pictures of seedlings in a box with one hole in the left wall, one after another. First, at the start, the labels are light, hole, seedlings and at the start, and the shoots stand straight up. Then, a few days later, the labels are light, hole, seedlings and a few days later, and every shoot bends towards the hole.lightholeseedlingsa few days later

    2 A few days later, every shoot has bent so that its tip points at the hole.

This is phototropism: growth in a direction set by light. A shoot grows towards the light. This helps the plant, because its leaves then get more light for photosynthesis.

How does the shoot know where the light is? In the 1880s Charles Darwin and his son Francis tested young grass shoots with light from one side. A shoot with its tip covered by a small cap did not bend. A shoot with its base covered still did. So the tip is what senses the light.

A table with the columns Test and Result. Tip uncovered: bends. Tip covered: no bend. Tip cut off: no bend. Base covered: bends.TestResulttip uncoveredbendstip coveredno bendtip cut offno bendbase coveredbends
Shoots with light from one side. The tip senses the light.

The tip makes a plant hormone, auxin. Auxin moves down the shoot to the cells behind the tip, and it makes them grow longer. Cells grow longer by taking in water and stretching. A plant has no blood, so auxin passes from cell to cell.

When light comes from one side, auxin moves to the shaded side. There is more auxin on the shaded side, so the cells there grow longer than the cells on the lit side. The shaded side becomes longer, and the shoot bends towards the light.

A shoot tip bent towards light that comes from one side. The labels are shaded side, longer cells, auxin, light, lit side and shorter cells. Many dots of auxin are on the shaded side, where the cells are longer, and few are on the lit side, where the cells are shorter.auxinshadedsidelongercellslightlit sideshortercells
A model of a shoot tip. The dots show auxin. The cells on the shaded side are longer. The longer side is on the outside of the bend.

Gravity acts on plants too. Gravitropism is growth in a direction set by gravity. A shoot grows up, away from the pull of gravity. A root grows down, in the direction of the pull. Even a seed that is planted upside down sends its root down and its shoot up.

Lay a seedling on its side. Auxin collects on the lower side of both the shoot and the root. In a shoot, more auxin makes the cells grow longer. So the lower side grows faster, and the shoot turns up. In a root, a lot of auxin slows growth. So the lower side grows more slowly, and the root turns down.

A seedling lying on its side at the start. The labels are shoot, seed, root, pull of gravity and lying on its side at the start. The shoot and the root both point sideways, level with the seed, and an arrow points down for gravity.lying on its side at the startshootseedpull ofgravityroot
A seedling on its side at the start. The shoot and the root both point sideways.
The same seedling a few days later. The labels are shoot, seed, root, pull of gravity and a few days later. The shoot has curved up and the root has curved down. An arrow points down for gravity.a few days latershootseedpull ofgravityroot
A few days later. The shoot has turned up and the root has turned down. This happens in the dark too, so it is not a response to light.
A table with the columns Part, Grows and Auxin below. Shoot: up, speeds growth. Root: down, slows growth.PartGrowsAuxin belowshootupspeeds growthrootdownslows growth
On a seedling lying on its side, auxin collects on the lower side. It speeds growth in a shoot and slows it in a root.

Plants respond to other things too. Roots grow towards water. The tendrils of a pea or a vine curl round whatever they touch. Each response helps the plant to find what it needs. A plant that grows towards light and its roots towards water has more to live on.

Now some numbers. A learner grew a seedling in a box with a hole. Each day they measured the bend of its shoot, as an angle from straight up. The graph shows example results. Read it, then work out how fast the shoot bent.

A line graph. The x axis is day, from 0 to 6 in steps of 1. The y axis is bend in degrees, from 0 to 80 in steps of 10. The line starts at 0 and 0, rises slowly to 10 degrees on day 1 and 20 on day 2, then fast to 40 on day 3 and 60 on day 4, then slowly to 65 on day 5 and 70 on day 6.012345601020304050607080daybend in degrees
Example data. The bend of a shoot, in degrees from upright, each day. Real seedlings differ.

Worked example

What was the mean rate of bending from day 2 to day 4, in degrees per day?

  1. Read the graph. On day 2 the bend is 20 degrees. On day 4 it is 60 degrees.
  2. The bend grew by 60 − 20 = 40 degrees in 4 − 2 = 2 days.
  3. Divide: 40 ÷ 2 = 20 degrees per day.

Answer: 20 degrees per day

Worked example

What was the mean rate of bending over all six days, to 1 decimal place?

  1. Read the graph. On day 0 the bend is 0 degrees. On day 6 it is 70 degrees.
  2. The bend grew by 70 − 0 = 70 degrees in 6 days.
  3. Divide: 70 ÷ 6 = 11.7 degrees per day.

Answer: 11.7 degrees per day

Try it

Seedlings in a box with one hole

You need: a cardboard box, such as a shoebox, with a lid, two cups or tins with damp soil, or damp cotton wool or cloth, six beans or other seeds, water, a bright window or a bright place, a pencil and paper.

Be safe: Do not eat the seeds or the seedlings. Some raw seeds, such as beans, can make people ill. Ask an adult to cut the hole, or tear thin card. Do not use a knife yourself.

  1. Soak the seeds in water for a night. Put three in the damp soil or cotton wool in each cup, and keep both cups damp.
  2. Put one cup in the box, at the end opposite the hole. Ask an adult to cut a hole the size of a coin in one side wall. Or tear a hole in thin card. Put the lid on.
  3. Stand the box with the hole facing a bright window. Do not move it. Give the seedlings a little water each day, and open the box for a moment to check them.
  4. Put the second cup in the open, in the same room, with no box. Look at both after a few days.
  5. Draw the seedlings in the box and write down which way the shoots point.

What you will see: In the box the shoots bend towards the hole, where the light comes in. In the open the shoots grow straight up. The time it takes depends on the seeds and on how warm it is. It is often three to seven days. The picture shows an example, with the hole on the other side.

If you cannot try it, this is what happens.
Seedlings in a box with one hole in the right wall, a few days later. The labels are light, hole, seedlings and a few days later. Every shoot has bent towards the hole.lightholeseedlingsa few days later

A wrong idea many people have

A shoot bends towards light because the cells on the lit side grow more.

Why it feels right: Light helps a plant, so it seems that the side with the light should grow more.

The test that breaks it: Auxin moves to the shaded side. The cells there grow longer than the cells on the lit side. In the model picture, the longer cells are on the outside of the bend, on the side away from the light.

What is true: The shaded side grows more, not the lit side. Because the shaded side gets longer, the shoot curves round towards the light.

Key words

tropism
growth in a direction that is set by a stimulus.
phototropism
growth in a direction set by light. A shoot grows towards light.
gravitropism
growth in a direction set by gravity. A shoot grows up and a root grows down.
auxin
a plant hormone made in the shoot tip that makes cells grow longer.

Back to your guess

The answer is B: Towards the hole, where the light comes in.

The shoots bend towards the hole. A plant hormone called auxin gathers on the shaded side of the shoot, and the cells there grow longer. That bends the shoot towards the light.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What is phototropism?

    Tap your answer.

    1. Yes, that is right. A tropism is growth in a direction set by a stimulus. In phototropism the stimulus is light.

    2. Not this one. That is photosynthesis. Phototropism is about the direction of growth. Go to S4.1.6. Try another.

    3. Not this one. That is a quick response to touch. Phototropism is slow growth that is set by light. Try another.

    See the answer

    Growth in a direction that is set by the direction of light

    A tropism is growth in a direction set by a stimulus. In phototropism the stimulus is light.

    • If you chose B: That is photosynthesis. Phototropism is about the direction of growth. Go to S4.1.6.
    • If you chose C: That is a quick response to touch. Phototropism is slow growth that is set by light.

  2. Where is auxin made in a shoot?

    Tap your answer.

    1. Not this one. The roots do not make the auxin that bends a shoot. It is made in the tip of the shoot. Try another.

    2. Not this one. Auxin is made at the tip of the shoot, not only in the leaves. Try another.

    3. Yes, that is right. The tip makes auxin. It moves down to the cells behind the tip, and it makes them grow longer.

    See the answer

    In the shoot tip

    The tip makes auxin. It moves down to the cells behind the tip, and it makes them grow longer.

    • If you chose A: The roots do not make the auxin that bends a shoot. It is made in the tip of the shoot.
    • If you chose B: Auxin is made at the tip of the shoot, not only in the leaves.

  3. Light shines on a shoot from the right. What happens?

    Tap your answer.

    1. Not this one. Auxin gathers on the shaded side, which is the left. The lit side is the right. Try another.

    2. Not this one. Auxin makes the cells grow longer. They do not stop. It is the longer cells on the shaded side that bend the shoot. Try another.

    3. Not this one. Auxin does not stay even. Light from one side makes it move to the shaded side. Try another.

    4. Yes, that is right. Light from the right leaves the left side shaded. Auxin gathers there, and the cells grow longer. The longer left side makes the shoot bend to the right, towards the light.

    See the answer

    Auxin gathers on the left, and the cells there grow longer, so the shoot bends right.

    Light from the right leaves the left side shaded. Auxin gathers there, and the cells grow longer. The longer left side makes the shoot bend to the right, towards the light.

    • If you chose A: Auxin gathers on the shaded side, which is the left. The lit side is the right.
    • If you chose B: Auxin makes the cells grow longer. They do not stop. It is the longer cells on the shaded side that bend the shoot.
    • If you chose C: Auxin does not stay even. Light from one side makes it move to the shaded side.

  4. A bean seedling is laid on its side in the dark. A few days later, what has the root done?

    Tap your answer.

    1. Not this one. A root grows in the direction of gravity, which is down. The shoot turns up. Try another.

    2. Yes, that is right. Gravity makes auxin collect on the lower side of a root. A lot of auxin slows growth there, so the root curves down.

    3. Not this one. A root responds to gravity, even in the dark. It turns down. Try another.

    4. Not this one. A root does not stop. Auxin collects on its lower side and slows growth there, so the root curves down. Try another.

    See the answer

    It has turned down.

    Gravity makes auxin collect on the lower side of a root. A lot of auxin slows growth there, so the root curves down.

    • If you chose A: A root grows in the direction of gravity, which is down. The shoot turns up.
    • If you chose C: A root responds to gravity, even in the dark. It turns down.
    • If you chose D: A root does not stop. Auxin collects on its lower side and slows growth there, so the root curves down.

  5. How does it help a plant when its shoot grows towards light?

    Tap your answer.

    1. Not this one. Roots do not need light. It is the leaves that use light. Try another.

    2. Not this one. Cooling is not the reason. Light is what the leaves need for photosynthesis. Try another.

    3. Yes, that is right. The leaves use light to make glucose. A shoot that bends towards light puts its leaves where there is more of it.

    See the answer

    The leaves get more light for photosynthesis.

    The leaves use light to make glucose. A shoot that bends towards light puts its leaves where there is more of it.

    • If you chose A: Roots do not need light. It is the leaves that use light.
    • If you chose B: Cooling is not the reason. Light is what the leaves need for photosynthesis.

  6. In the box experiment, why does the box have only one hole?

    Tap your answer.

    1. Yes, that is right. A single hole makes the light come from one direction. Then you can see which way the shoots bend.

    2. Not this one. Some light comes in through the hole. The seedlings need it, and they grow towards it. Try another.

    3. Not this one. Air can get in. The hole is there to let the light in from one side. Try another.

    See the answer

    So that light reaches the seedlings from one side only

    A single hole makes the light come from one direction. Then you can see which way the shoots bend.

    • If you chose B: Some light comes in through the hole. The seedlings need it, and they grow towards it.
    • If you chose C: Air can get in. The hole is there to let the light in from one side.

  7. From an earlier chapter

    Which statement is true of both auxin and a hormone such as insulin?

    Tap your answer.

    1. Yes, that is right. A hormone is a chemical messenger. Auxin is a chemical messenger in plants, and insulin is one in animals. They differ in how they travel.

    2. Not this one. A plant has no blood. Auxin passes from cell to cell. Go to S4.5.4. Try another.

    3. Not this one. Insulin is made by a gland, the pancreas. A plant has no glands like this, and the shoot tip makes auxin. Go to S4.5.4. Try another.

    See the answer

    Both are chemical messengers.

    A hormone is a chemical messenger. Auxin is a chemical messenger in plants, and insulin is one in animals. They differ in how they travel.

    • If you chose B: A plant has no blood. Auxin passes from cell to cell. Go to S4.5.4.
    • If you chose C: Insulin is made by a gland, the pancreas. A plant has no glands like this, and the shoot tip makes auxin. Go to S4.5.4.

  8. A shoot bent from 20 degrees on day 2 to 60 degrees on day 4. What was the mean rate of bending, in degrees per day?

    See the answer

    20 degrees per day

    The bend grew by 60 − 20 = 40 degrees in 2 days. 40 ÷ 2 = 20 degrees per day.

  9. A shoot bends at a steady 12 degrees per day. How many degrees has it bent after 5 days?

    See the answer

    60 degrees

    12 × 5 = 60 degrees.

Keep it

Copy this into your exercise book.

Light from one side: auxin moves to the shaded side, the cells there grow longer, the shoot bends towards the light. Gravity: shoots grow up, roots grow down.

Draw: A shoot bending towards a light, with dots of auxin on the shaded side.

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