What it shows
When a sprouting seed is turned on its side, the root bends to keep growing downwards and the shoot bends upwards. Plants sense gravity, and the response is called gravitropism (or geotropism).
The science
Cells at the tip of a root contain tiny starch grains that settle to the bottom of the cell, like sand in water. This tells the root which way is down. A plant hormone called auxin then collects on the lower side of the root tip and slows growth there, so the upper side grows faster and the root curves downwards. In the shoot the same hormone has the opposite effect, so the shoot curves upwards.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 4 sprouted chana or moong seeds with a root about 1 cm long
- A clear plastic box or a transparent food container with a lid
- Cotton wool or folded tissue
- Water
- A marker, tape and a notebook
Safety
For this project: Wash hands after handling damp cotton. No tools needed.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Sprout the seeds on damp cotton for 2 to 3 days until each has a root about 1 cm long.
Stand the clear box on one end, press damp cotton firmly against one clear wall, and wedge the seeds between the cotton and the wall with their roots all pointing down.
Close the lid and carefully turn the box a quarter turn onto its side, so the roots now point sideways, and mark the top with tape.
Keep the box in a warm place and look at it twice a day without moving it.
Draw the direction of each root and shoot every day in your notebook.
After 3 days, turn the box a quarter turn again and keep watching.
Keep the cotton damp but never soggy, adding water with a spoon.
Compare your drawings and describe how the roots and shoots changed direction.
What you should see
Within a day or two every root curves to point downwards again and every shoot curves upwards, whichever way the box is turned. The drawings show the bends happening at the growing tips.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What is gravitropism?
The growth of a plant part in response to gravity: roots grow towards it, shoots away from it.
02Why is it useful for roots to grow down?
Soil, water and minerals are below the seed, and a deep root anchors the plant.
03Does the root bend at its old part or near its tip?
Near the tip, just behind it, where the cells are still growing longer; the old part does not bend.
04Would this work in the dark?
Yes, the response to gravity does not need light.
Take it further
Turn the box upside down and time how long the roots of chana and moong take to bend back, and compare the two kinds of seed.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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