Plants Around Us
Primary 3 · Science and Elementary Technology
Plants Around Us
Lesson 2 of 3
14 min
Let's begin
Ishimwe grew two bean plants from the very same packet of beans. One sits on the windowsill. It is green and strong and taller than his water bottle.
The other one he forgot in the dark store room. It is long, thin, pale yellow, and it flops over like a tired snake.
Same beans. Same soil. So why are they so different? Today you will find out — not by being told, but by doing a real test with five cups.
You need five old cups or tins, some soil, five dry beans from the kitchen, water, a saucer with a wet cloth on it, and a box with a lid.
Write A, B, C, D and E on the cups. Then set them up like this.
| Cup | How you set it up | What it is missing |
|---|---|---|
| A | Soil, one bean, water every day, on the windowsill | Nothing — it gets everything |
| B | Soil, one bean, on the windowsill — but you never water it | Water |
| C | Soil, one bean, water every day — but kept inside the closed box | Light |
| D | Soil, one bean, then fill the cup right to the top with water so the soil is drowned; on the windowsill | Air |
| E | One bean on a wet cloth on a saucer, kept wet, on the windowsill | Soil |
Look again at cup C. It gets soil. It gets water. It gets air. It is warm. Only one thing is taken away: light.
That is on purpose. If you took away the light and the water at the same time, and the bean died, you would not know which one killed it. By changing one thing only, the result can only point at one cause.
A test built like this is called a fair test. Scientists all over the world work this way, and now you do too.
Ishimwe checked every day and wrote it down. Here is his record.
| Cup | Day 3 | Day 7 | Day 14 |
|---|---|---|---|
| A (everything) | A little root pushing down | Green shoot up, two leaves open | Strong green plant, as tall as his hand |
| B (no water) | Nothing | Nothing | Still a hard, dry bean |
| C (no light) | A little root pushing down | A pale shoot, growing fast | Long, thin, yellow-white, floppy, falls over |
| D (no air) | Nothing | The bean has gone soft | Rotten and smelly. It never sprouted |
| E (no soil) | A little root | Shoot and two leaves | Alive, but small, and its leaves are going pale |
Cup B teaches you about water. A dry bean is hard as a stone. Water soaks into it and makes it swell until the skin splits and the root can push out. With no water, nothing ever begins. Water is need number one.
Cup D teaches you about air. This surprises people. Cup D had plenty of water — too much! The soil was drowned, so there was no air left in the spaces between the bits of soil. A seed has to breathe. With no air it did not sprout; it just rotted. This is why farmers do not plant beans in a puddle.
Cup C teaches you about light. Look carefully: the bean in the dark box did sprout. How? Because a seed carries a small packet of food inside it, and that food is enough to get it started, even in the dark. But that packet runs out. After that, the plant must make its own food, and leaves can only make food when sunlight falls on them. In the dark box there was no sunlight, so the plant grew pale and weak, and could not stand up.
Cup E teaches you about soil. Cup E also started well, using the food packet inside the bean. But then it stalled. Soil does two jobs: it grips the roots so the plant can stand, and it holds the water and the mineral salts that the roots drink. Without soil, the little plant simply runs out of what it needs.
And one more: warmth. If you had put a sixth cup in a cold place, it would have been much slower. Seeds wake up in a warm place. Luckily, most of Rwanda is warm enough all year.
Umutoni brings you her bean plant. It is two weeks old. It is long and thin, it is yellow-white instead of green, and it keeps flopping over.
She tells you: "It has good soil. I water it every single day. It sits on a shelf in our store room."
Step 1 — Write down the four needs: water, light, air, soil.
Step 2 — Tick off the ones she has.
Step 3 — One need is left, so that is your answer: the plant is missing light.
Step 4 — Check that the clues agree. Pale instead of green, long, thin and floppy. That is exactly what cup C looked like. The clues agree.
Step 5 — Fix it: move the plant to a sunny windowsill and keep watering it. In a few days the new leaves will come out green.
That is how a scientist thinks: list the needs, tick off what is there, and the missing one is your answer.
Tip
Four needs, four words: Water, Light, Air, Soil
Say it like this and it sticks:
"Water to drink. Light to cook. Air to breathe. Soil to stand."
When a plant looks sick, go through those four words in order. One of them is always the problem.
Work each one out first, then tap Show answer to check.
Habimana's cup sits in the sun. It has good soil, plenty of air, and it is nice and warm. Two weeks later the bean is still a hard dry bean. What is missing?
Answer: water. Without water the bean cannot swell and split open, so it never even starts.
Gatete filled his cup right to the brim with water so the soil was under water for a whole week. His bean went soft and smelly and never sprouted. What was it missing?
Answer: air. Drowned soil has no air in it, and a seed must breathe. Too much water pushes the air out.
Keza wants to test whether beans really need light. She puts cup A on the windowsill and cup B in a dark box. What else must she do to make it a fair test?
Answer: keep everything else exactly the same. The same kind of bean, the same soil, the same amount of water, both in a warm place. Only the light may be different — otherwise she will not know what caused the result.
Two cups are watered every day. One is on the windowsill, one is in a cupboard. In three weeks, which plant will be greener and stronger, and why?
Answer: the one on the windowsill. Its leaves get sunlight, so they can keep making food. The cupboard plant uses up the food stored in its seed and then goes pale and weak.
Why does a farmer near the Nyabarongo river avoid planting beans in a spot where rainwater stands in a pool?
Answer: because standing water pushes all the air out of the soil. The seeds cannot breathe, so instead of sprouting they rot.
What you learned today
Quick check
A bean plant grew in a dark store room. It is pale, thin and floppy. What is it missing?
