Living Things Around Us
Primary 5 · Science and Elementary Technology
Living Things Around Us
Lesson 3 of 3
15 min
Let's begin
Keza put five bean seeds on wet cotton wool inside an old tin and left it on the window sill. On Monday the beans just sat there, hard as little stones. On Wednesday they had swollen up fat. On Thursday morning a small white root had pushed its way out of one of them.
Nobody helped that bean. It had everything it needed and it started itself. Today you will find out what living things do as they grow — and you will plan a real experiment to discover what a seed needs to start.
Worked example
the hen in your compound. Follow her back to the beginning.
And there we are, back at the egg. It is not a straight line — it is a circle, going round and round for as long as there are hens. That circle is what we call a life cycle: all the stages a living thing passes through, from its beginning, to being grown up, to producing young of its own.
A plant does the very same thing.
One seed, one full circle, and now there are dozens of seeds where there was one. That is why a farmer in Musanze can plant one small tin of beans and harvest a whole sack.
A newborn kid already looks like its mother: four legs, two ears, a little tail. It simply grows bigger and stronger until it is an adult that can have kids of its own. Three easy stages: newborn → young → adult. Humans, cows and dogs grow this way too.
Now look at one of the butterflies over Keza's flowers.
A caterpillar and a butterfly are the same animal at two stages of one life. That enormous change is called metamorphosis. Houseflies and mosquitoes do it too.
Egg (in a jelly, floating in water) → tadpole, which has a tail and gills and swims → froglet, growing back legs, then front legs, while the tail shrinks away → adult frog, breathing with lungs on the bank.
Egg → nymph (it looks like a tiny adult but has no wings yet) → adult. Only three stages, and no pupa.
Soak a bean in water overnight, then gently peel it. You will find three things:
So a seed is not dead. It is alive and resting, waiting for the right conditions. When it wakes up and starts to grow, we call that germination.
Three things, and only three:
Now the surprise. A seed does not need light to germinate. Think about where seeds normally germinate — buried in the soil, in total darkness. And it does not even need soil at first, because it carries its own packed lunch inside.
But be careful, because there is a “but”. Once the packed lunch runs out and the first green leaves open, the young plant must have light, or it will starve. Light is not needed to start the seed. Light is needed to keep the seedling alive.
Do not just believe what you read. Test it. Here is a complete investigation you can run at home this week.
Question: Do bean seeds need water in order to germinate?
Prediction: I think the seeds with water will germinate and the dry ones will not.
What you need: 2 clean tins, cotton wool (or a folded cloth), 10 bean seeds, water.
Method:
Here is the important part. Between Tin A and Tin B, you changed only one thing: the water. Everything else is identical — same seeds, same number of seeds, same cotton wool, same shelf, same warmth, same number of days. That is what makes it a fair test.
If you had also put Tin B in a cold, dark cupboard, then two things would be different (water AND warmth AND light), and when nothing grew in Tin B you would not know which of them was to blame. The whole experiment would be wasted.
Recording your results: each day, write down how many seeds have sprouted in each tin.
| Day | Tin A (watered) | Tin B (dry) |
|---|---|---|
| 2 | 0 sprouted, seeds swollen | 0, seeds still hard |
| 4 | 4 sprouted | 0 |
| 7 | 5 sprouted, shoots showing | 0 |
Conclusion: the watered seeds germinated and the dry ones did not. Bean seeds need water to germinate.
Why 5 seeds in each tin and not just 1? Because a single seed might be dead, or damaged, or eaten by an insect — and then you would draw the wrong conclusion from bad luck. Five seeds show you a pattern, and a pattern is far harder to argue with.
You can change one thing at a time and test warmth and air as well.
Tip
Two things to lock into your memory:
W-W-A — a seed needs Water, Warmth and Air. Not light. Seeds start in the dark, under the soil.
A fair test changes ONE thing and keeps everything else the same. If you change two things at once, you can never say which one caused the result.
Try each one, then tap Show answer to check.
Put these butterfly stages in the right order: pupa, egg, adult butterfly, caterpillar.
Answer: egg → caterpillar (larva) → pupa (chrysalis) → adult butterfly.
Ishimwe puts damp seeds in a warm cupboard with the door shut, so they are in complete darkness. Will they germinate?
Answer: Yes. They have water, warmth and air, and germination does not need light. But once the little green leaves open, he must bring them into the light or the seedlings will die.
Byiringiro wants to find out whether seeds need warmth. He puts Tin A on the sunny window sill and Tin B in a cold corner — but he waters Tin A and forgets Tin B. Is this a fair test? What must he fix?
Answer: It is not fair. He changed two things: the warmth and the water. If nothing grows in Tin B he cannot tell which one was responsible. He must water both tins exactly the same, so that the only difference left is the warmth.
Umutoni says a caterpillar and a butterfly are two different animals. Is she right?
Answer: No. They are the same animal at two stages of one life cycle. The caterpillar changes into the butterfly inside the pupa. That change is called metamorphosis.
Why do scientists test 10 seeds rather than 1?
Answer: One seed could be dead or damaged, so a single result could easily be a fluke. Testing many seeds shows a pattern and makes the conclusion far more trustworthy.
What you learned today
Quick check
Put the stages of a frog's life cycle in order.
Tap the cards in the right order.
