Chapter 4 of 8

All 8 chapters
  1. 1Plant parts and their jobs, readunderstood
  2. 2From seed to seedling, readunderstood
  3. 3A fair test with plants, readunderstood
  4. 4Light, water and plant health, readunderstood
  5. 5Leaves make food, readunderstood
  6. 6Flowers and pollination, readunderstood
  7. 7How seeds travel, readunderstood
  8. 8The life cycle of a plant, readunderstood

S2.1.4 Chapter 4

Light, water and plant health

What do light and water do to a plant?

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

  • Compare plants grown with and without light
  • Describe what too little or too much water does to a plant

Plants need light. What happens to a plant that gets none?

Amina grows two bean seedlings of the same size. One stands by a window. The other stands in a dark box.

Draw it first

Draw a seedling in light and in the dark

  1. Two small seedlings in soil. Above the left one is a sun, labelled light. Above the right one is a black box, labelled dark. A week later the left seedling is short and strong, labelled green, strong. The right seedling is tall and thin with tiny leaves, drawn with dashed lines, labelled tall, pale, weak.

    1 Draw two small seedlings in soil.

  2. Two small seedlings in soil. Above the left one is a sun, labelled light. Above the right one is a black box, labelled dark. A week later the left seedling is short and strong, labelled green, strong. The right seedling is tall and thin with tiny leaves, drawn with dashed lines, labelled tall, pale, weak.lightdark

    2 Put a sun over one. Put a black box over the other. Write light and dark.

  3. Two small seedlings in soil. Above the left one is a sun, labelled light. Above the right one is a black box, labelled dark. A week later the left seedling is short and strong, labelled green, strong. The right seedling is tall and thin with tiny leaves, drawn with dashed lines, labelled tall, pale, weak.lightdarkgreen, strongtall, pale, weak

    3 A week later, draw both. The one in the dark is tall, thin and dashed.

A week later they look very different. The seedling in the dark is taller. It is thin and pale, and its leaves are small.

It stretched tall. It used the food in its seed. But it cannot make food without light. So it stays weak. If it stays in the dark, it dies.

The seedling by the window is green and strong. It is shorter. This is a healthy plant.

See it three ways

As a picture

A line graph of the seedling in the dark, with the labels day and height in cm. The points for days 1, 3, 5, 7 are at 3, 7, 11, 15 cm, joined by straight lines.04812161357dayheight in cm

In words

The dark seedling gets taller each day. On day 7 it is 15 cm. But it is pale and weak.

Try it

Light or dark?

You need: two cups of bean seedlings, the same size, a box with a lid, or a dark cupboard, a bright window, water and a spoon, a ruler marked in centimetres.

Be safe: Never look at the Sun, not even through a window. Wash your hands after you touch soil or plants. Do not eat the beans or seedlings.

  1. Measure both seedlings. They should be the same size.
  2. Stand one cup by the window. Put the other in the dark box.
  3. Give both the same water each day. Open the box for a moment only.
  4. After a week, take both out. Measure them.
  5. Look at their colour and their leaves.

What you will see: The table shows example numbers. Yours will differ. The seedling from the dark is taller and thin. It is pale, with small leaves. The seedling by the window is green, shorter and strong. Do not leave the dark one in the dark. It will die.

If you cannot try it, this is what happens.
A table with columns Plant, Height and Colour, after seven days. light: 9 cm, green. dark: 15 cm, pale. Labels: Plant, Height, Colour, light, 9 cm, green, dark, 15 cm, pale.PlantHeightColourlight9 cmgreendark15 cmpale

A wrong idea many people have

A plant in the dark grows less.

Why it feels right: Plants need light. So with no light, a plant should not grow.

The test that breaks it: Put two bean seedlings of the same size in two cups. Stand one in the light. Put the other in a dark box. Measure both after a week. The dark one is taller.

What is true: A seedling in the dark stretches tall and thin. But it is pale and weak. It cannot make food without light. If it stays in the dark, it dies.

Water matters too. A plant needs the right amount.

With too little water, a plant will wilt. To wilt is to droop. Its leaves dry out and go brown.

With too much water, the soil stays soaked. The roots cannot get air. They rot. Rotting means going soft and dying. The leaves go yellow. The plant can die.

Two plants in soil. On the left, the soil is dry with cracks, the stem bends over and the leaves hang down. Labels: too little, droops, leaves dry. On the right, the soil is soaked, shown with dots, the stem stands up and short roots lie in the soil. Labels: too much, roots rot, leaves yellow.too littledroopsleaves drytoo muchroots rotleaves yellow
Too little water and too much water both harm a plant.

Push a finger into the soil. If the top feels dry, give water. If it feels soaked, wait. A pot needs a hole at the bottom. Then extra water runs out.

A cup of soil with a plant in it. There is a hole in the bottom of the cup. Drops of water fall from the hole. Labels: hole, extra water runs out.holeextra water runs out
The hole lets extra water run out. The roots keep their air.

Key words

healthy
green and strong, and growing well.
wilt
to droop because there is too little water.
rot
to go soft and die. Roots rot in soaked soil.

Back to your guess

The answer is C: A seedling in the dark grows tall, thin and pale.

A seedling in the dark stretches tall and thin. It turns pale and weak.

Check yourself

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

  1. A bean seedling stays in a dark cupboard for a week. What does it look like?

    Tap your answer.

    1. Not this one. Light keeps a plant green and strong. In the dark it turns pale. Try another.

    2. Not this one. It does grow. It stretches tall, but it is weak. Try another.

    3. Not this one. It stretches tall and thin. It does turn pale. Try another.

    4. Yes, that is right. A seedling in the dark stretches tall and thin. It turns pale. It cannot make food without light.

    See the answer

    Tall, thin and pale

    A seedling in the dark stretches tall and thin. It turns pale. It cannot make food without light.

    • If you chose A: Light keeps a plant green and strong. In the dark it turns pale.
    • If you chose B: It does grow. It stretches tall, but it is weak.
    • If you chose C: It stretches tall and thin. It does turn pale.

  2. A plant droops. Its leaves are dry and brown. What is wrong?

    Tap your answer.

    1. Yes, that is right. A plant with too little water wilts. Its leaves dry out and go brown.

    2. Not this one. Too much water gives soaked soil and yellow leaves. Try another.

    3. Not this one. Drooping with dry, brown leaves comes from too little water. Try another.

    See the answer

    It has too little water.

    A plant with too little water wilts. Its leaves dry out and go brown.

    • If you chose B: Too much water gives soaked soil and yellow leaves.
    • If you chose C: Drooping with dry, brown leaves comes from too little water.

  3. Why do roots rot in soaked soil?

    Tap your answer.

    1. Not this one. Soaked soil has plenty of water. The roots lack air. Try another.

    2. Yes, that is right. Water fills the soil and pushes the air out. The roots rot and the plant can die.

    3. Not this one. Roots grow in the dark soil. The trouble is air, not light. Try another.

    See the answer

    They cannot get air.

    Water fills the soil and pushes the air out. The roots rot and the plant can die.

    • If you chose A: Soaked soil has plenty of water. The roots lack air.
    • If you chose C: Roots grow in the dark soil. The trouble is air, not light.

  4. Look at the table. How much taller is the dark seedling than the light one?

    A table with columns Plant, Height and Colour, after seven days. light: 9 cm, green. dark: 15 cm, pale. Labels: Plant, Height, Colour, light, 9 cm, green, dark, 15 cm, pale.PlantHeightColourlight9 cmgreendark15 cmpale
    See the answer

    6 cm

    The dark seedling is 15 cm. The light one is 9 cm. 15 − 9 = 6.

  5. From an earlier chapter

    Amina tests light on two seedlings. What must stay the same?

    Tap your answer.

    1. Yes, that is right. In a fair test you change one thing. Water, cups and place stay the same.

    2. Not this one. Light is the one thing she changes. Go to S2.1.3. Try another.

    3. Not this one. Height is what she measures. It will be different. Go to S2.1.3. Try another.

    See the answer

    The amount of water

    In a fair test you change one thing. Water, cups and place stay the same.

    • If you chose B: Light is the one thing she changes. Go to S2.1.3.
    • If you chose C: Height is what she measures. It will be different. Go to S2.1.3.

  6. Draw a plant with too little water and one with too much. Write what happens to each.

    See the answer

    Too little water: the plant droops and its leaves dry out. Too much water: the soil is soaked. The roots rot. The leaves go yellow.

    Both are harmful. A plant needs the right amount of water.

  7. Hana looks at three plants in cups. Leaves drawn with a dashed line are yellow. Which plant needs water?

    Three plants in cups, marked A, B and C from left to right. A stands up with solid leaves. B has water lying on dotted soil and dashed leaves. C has cracked soil, a bent stem and hanging leaves. Labels: A, B, C.ABC
    Tap your answer.

    1. Not this one. A stands up and has green leaves. It has the right amount of water. Try another.

    2. Not this one. B has soaked soil and yellow leaves. More water would harm its roots. Try another.

    3. Yes, that is right. C has dry, cracked soil. The stem has bent over and the leaves hang down. It has too little water.

    See the answer

    C

    C has dry, cracked soil. The stem has bent over and the leaves hang down. It has too little water.

    • If you chose A: A stands up and has green leaves. It has the right amount of water.
    • If you chose B: B has soaked soil and yellow leaves. More water would harm its roots.

  8. Seedling X is tall, thin and pale. Seedling Y is shorter and green. Which one is healthy?

    Tap your answer.

    1. Not this one. Tall and thin is not healthy. A pale seedling cannot make food without light. Try another.

    2. Yes, that is right. A healthy seedling is green and strong. A tall, thin, pale one has been without light.

    3. Not this one. Seedling X grew by using the food in its seed. Pale means it is weak. Try another.

    See the answer

    Seedling Y. Green leaves mean it makes food.

    A healthy seedling is green and strong. A tall, thin, pale one has been without light.

    • If you chose A: Tall and thin is not healthy. A pale seedling cannot make food without light.
    • If you chose C: Seedling X grew by using the food in its seed. Pale means it is weak.

Keep it

Copy this into your exercise book.

A seedling in the dark grows tall, thin and pale, then dies. A plant in light is green, shorter and strong. Too little water makes a plant droop. Too much water rots the roots.

Draw: Two seedlings, one in light and one in the dark. Label green and pale.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (922 KB). It opens on any phone.