The science idea
A seedling in a dark cupboard is pale, thin and long, and it is leaning towards whatever small gap of light there is. A plant cannot walk to the light, but it can grow towards it. This bending of a plant’s shoot towards the light is called phototropism (photo means light and tropism means turning). It is a response to a stimulus, one of the characteristics of living things, and it is easy to see in a plant maze, where the seedling must find its way up through two gaps.
The cause is a plant hormone called auxin. Auxin is made in the tip of the shoot and travels down the stem, where it makes cells lengthen. When light comes from one side, receptors in the shoot tip (phototropins, which respond to blue light) detect it, and auxin moves across the stem away from the light, to the shaded side. The cells on the shaded side receive more auxin, so they elongate faster than the cells on the lit side. The stem therefore bends towards the light. This explanation of the response is called the Cholodny-Went theory.
There are two more responses at work. Gravitropism is growth in response to gravity: shoots grow upwards (against gravity), and roots grow downwards. In your maze the shoot grows up, and the light tells it which way to turn at each shelf. And in the dark, a seedling shows etiolation: it grows tall and spindly, and its leaves stay small and yellowish-white because it cannot make green chlorophyll without light. This is a strategy for reaching light quickly before its food store runs out. When the seedling reaches the light it turns green.
It is not enough to see the seedling bend, because a scientist asks whether the light was really the cause. So the project includes controls: a seedling in a similar box with no shelves, and one in a box with no hole at all. If the seedling in the box with no hole does not bend, and the one in the maze does, then light is the cause of the bending.
Plants differ in how fast and how clearly they respond. Moong, mustard and bean shoots bend within a day or two of light from one side. Writing down the direction of growth and measuring the height every day gives you data to draw a graph and to describe the path of the seedling through the maze.
Materials and tools
Cost: Free
A shoe box, cardboard, a cup, soil and a few moong, mustard or bean seeds from the kitchen are all you need.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Shoe box with a lid | 1 (and 1 or 2 more for controls) | Ask at a shoe shop or use any cardboard box about 30 cm × 20 cm × 12 cm. |
| Stiff card for the two shelves | 2 pieces, each as wide as the box and about two thirds as deep as it is | Cut from a cereal box or the side of a carton. |
| Small cup (paper or plastic), 100 to 150 mL | 1 per box | A used cup from the kitchen, with a drainage hole made by an adult. |
| Garden soil or potting mix | 1 cup per box | From the garden. Remove stones. |
| Seeds (moong, mustard, methi or a bean) | 5 to 6 per cup | From the kitchen. Soak them overnight for faster sprouting. |
| Water in a spray bottle or a syringe | 1 | A small bottle with a pump or a medicine syringe (no needle). |
| Tape and glue | 1 roll | Strong parcel tape keeps light out of the seams. |
| Ruler and notebook | 1 each | For measuring the seedling and recording the days. |
| Black paper or black marker (optional) | 1 sheet | To darken the inside of the box. |
Tools
- Scissors
- A craft knife for the hole (adult)
- Ruler and pencil
- Spoon for soil
- Camera or phone to photograph the maze each day (optional)
Safety first
Read this before you start
- An adult should cut the light hole and the drainage hole in the cup. Cut on a board.
- Wash your hands with soap after handling soil, seeds and wet cups.
- Do not eat seeds or seedlings from this project. They have grown in soil and in a box.
- Keep the box in a bright place, but not in direct strong sunshine for hours, because the closed box can get hot and cook the seedling.
- Keep the soil damp, not wet. A very wet closed box can grow mould and attract mosquitoes.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Step-by-step build
12 steps. Do them in order, and test each part before you go on to the next.
Prepare the box
Stand the shoe box on one of its short ends so it is tall. The lid is now on the front: tape it on along one long edge so that it opens like a door. Check that no light comes through the door by closing it and looking for cracks.
Tip: Tape the other three sides with black or dark tape, so that light cannot get in anywhere else.
Cut the light hole
On the top face (the box end that is now at the top), an adult cuts a round or square hole about 3 cm wide, near one side. This is the only place where light enters the box.
Common mistake: Cutting more than one hole. Light from two directions will confuse the seedling and spoil the test.
Darken the inside
If the inside is white or shiny, colour it with a black marker or glue black paper on it so that light is not reflected around the box.
Tip: A matt black inside makes only the hole bright, so the direction of light is clear.
Cut the shelves
Measure the inside width and depth of the box. Cut two shelves from stiff card as wide as the box and about two thirds as deep, leaving a gap of about 4 cm.
Common mistake: Cutting the shelves too big. They must fit snugly, or they will not stay in place, and there should be a clear gap at one side to let the seedling through.
Fix the shelves
Glue or tape shelf 1 inside the box at about one third of the way up, fixed to the left wall, with the gap on the right. Glue shelf 2 at about two thirds of the way up, fixed to the right wall, with the gap on the left. Seal all the edges against the walls.
Tip: Put a small card tab under each shelf to hold it level while the glue dries.
Plant the seeds
Fill the cup with damp soil. Press 5 or 6 soaked seeds about 1 cm deep. Water lightly with the spray until the soil is damp but not dripping. Stand the cup on the floor of the box, on the side where it is under the gap in shelf 1.
Common mistake: Planting in very wet soil. The seeds rot, so keep it just damp.
Set up the controls
Make Box B with the same light hole but no shelves, and Box C with no light hole at all (a closed dark box), and a cup in the open on the window sill as Box D. Use the same soil, seeds and water in all.
Tip: Label the boxes with A, B, C and D. Every box is kept in the same place.
Place the boxes
Put the boxes side by side near a bright window, all facing the same way, with the light hole turned toward the window. Do not move them during the experiment.
Common mistake: Turning the boxes around. The seedling then has to change direction, which confuses your results.
Water carefully
Every two days, open the door for a few seconds and spray a little water on the soil. Close it at once. Do not pour water.
Tip: A little water and a quick look is enough. The seedling does not need long in the light.
Observe daily
Each day, at the same time, open the door and note the height of the tallest seedling, the direction it is leaning and its colour. Draw a quick sketch and take a photograph if you can.
Common mistake: Leaving the door open. Even a short, bright burst of light from the front changes the direction of growth.
Find the path
After 10 to 14 days, open the box and sketch the path of the seedling through the maze. Measure the length of its stem along the curve with a piece of string, then straighten the string on a ruler.
Tip: Compare with the controls: the box with no hole should show straight, pale, thin seedlings; the open pot should show short, strong, green ones.
Draw your conclusion
Plot the height of the seedlings in each box against time. Describe the bending, the colour differences and what they show about light.
Tip: Say which result shows that light was the cause of the bending.
How it should look when it is finished
A tall shoe-box standing on its end, with a taped door on the front and a small round hole at the top. When you open the door you see two card shelves and, curving up through the gaps like a pale green snake, a seedling that has found its way to the hole and has leaves turning green near the light.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
Inside the closed box, the only light comes through the hole. As the seedling pushes up from the soil, the tip of the shoot senses this light. Special pigments in the tip detect the blue part of the light and cause the hormone auxin to move to the shaded side of the stem.
Cells on the shaded side stretch faster than the cells on the bright side, so the stem curves towards the light. Each time the shoot meets a shelf, it grows sideways towards the gap because that is where the light is stronger, and then up again. Gravity also keeps telling the shoot to grow upwards, so it climbs from gap to gap.
In the closed control box there is no light at all, so the seedlings grow straight up and show no bending. That comparison shows that light, not the box, is the cause.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The seedlings do not sprout | Old seeds, soil too dry or too wet, or too cold. | Use fresh seeds, soak them overnight, and keep the soil damp in a warm place. |
| The seedlings grow but do not bend towards the hole | Light leaks from other places, or the seedling is not tall enough yet. | Seal all cracks with dark tape and wait a few more days. Check the light hole is facing the window. |
| The seedlings turn yellow and fall over | Too little light for too long, or too wet and cold. | Reduce the number of seedlings and move the box to a brighter window. |
| Mould grows on the soil | The soil is too wet in a closed box. | Water less, take the cup out to dry for an hour and use fresh soil. |
| The seedling is stuck below a shelf | The gap is too small or the seedling does not reach it. | Widen the gap to 4 to 5 cm, and place the cup nearer to the gap. |
| The seedlings in all boxes look the same | The boxes are not different enough or light leaks into the control box. | Re-check the control box for leaks, and compare heights and colours carefully. |
Test and record
Measure the tallest seedling in each box every second day at the same time. Note the direction and the colour (pale yellow, light green or green).
| Day | Maze box A: height (cm) | Box B (no shelves): height (cm) | Box C (no light): height (cm) | Open pot D: height (cm) | Direction and colour notes |
|---|---|---|---|---|---|
| 2 | |||||
| 4 | |||||
| 6 | |||||
| 8 | |||||
| 10 | |||||
| 12 | |||||
| 14 |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- What is phototropism?
The growth of a plant towards or away from light. Shoots grow towards light (positive phototropism).
- Which hormone causes the bending and how?
Auxin. It moves to the shaded side of the stem and makes the cells there lengthen faster, so the stem bends towards the light.
- Why is a control experiment important?
A control shows what happens when only one thing is different. It shows that the bending is caused by the light and not by something else.
- Why were the seedlings in the dark pale and tall?
In the dark a seedling cannot make chlorophyll and it grows quickly in length to search for light. This is called etiolation.
- What is the difference between phototropism and gravitropism?
Phototropism is growth in response to light and gravitropism is growth in response to gravity. Shoots grow up (negative gravitropism) and roots grow down (positive gravitropism).
- Why is it important not to turn the boxes during the experiment?
The seedling responds to the direction of the light. Moving the box changes this direction and spoils the comparison.
- Why does the shoot need light after the seed food is used up?
The seed store of food runs out. Leaves make food by photosynthesis, and this needs light, so the plant must reach it quickly.
- Give another example of a tropism.
Roots growing downward (gravitropism), roots growing towards water (hydrotropism), tendrils twisting around a support (thigmotropism).
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To show that plant shoots grow towards light by making a plant maze, and to compare with control boxes.
- Apparatus
- Shoe boxes, cardboard shelves, cups, soil, seeds, a spray bottle, tape, a ruler.
- Procedure
- Describe how you made the maze box and the control boxes, planted the seeds, placed the boxes near the window, watered them and measured the seedlings every second day. Include the diagrams.
- Observation
- The table of heights, directions and colours, with photographs or sketches of the seedlings’ path through the maze.
- Result
- The seedling in the maze grew through both gaps towards the light hole. The seedling in the closed box did not bend. Give the heights.
- Conclusion
- Shoots grow towards light (positive phototropism) and the effect is caused by uneven distribution of auxin. The results from the control boxes show that light is the cause.
Take it further
- Test the colour of light: cover the hole with red, blue or green cellophane. Which colour attracts the shoots most?
- Place the light hole on the side and see whether the shoot bends sideways.
- Grow seedlings on a clinostat: turn a seedling slowly on a rotating card to remove the effect of gravity.
- Measure the angle of bending for different distances of the light.
- Compare the speed of bending in moong, mustard and bean seedlings.
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