The Fairy-Land of Science · Lecture VII, part 2 · Part 7: Lecture VII: The life of a primrose
20. How a plant makes its own food
Goal: Your child can say that a plant drinks water through its roots, takes in a gas from the air through its leaves, and uses sunlight to make food, giving off oxygen.
You'll need
- A handful of fresh green leaves (spinach or leaves from a garden plant)
- A clear glass or jar of water
- A sunny windowsill
- A magnifying glass
- Optional: a few raisins in a glass of water
- Science notebook and pencils
What's in the book
Pages 156 to 161. Plants can make living matter out of water, gases and minerals, which we cannot do; water creeps up from cell to cell; plants grown in the dark stay white; green leaves use sunlight to split carbon out of the air and give off oxygen (laurel leaves under water make bubbles); sugar is made of carbon; leaves are the plant's stomach; and tiny mouths under the leaf let out extra water.
The lesson, step by step
Hook question
SayYou eat food to grow. A plant has no mouth. What does it eat?
Let your child guess. Buckley says the plant is cleverer than we are: it makes food out of water and air.
Read together
Read pages 156 and 157.
If you have ever tried to grow a plant in a cellar, you will know that in the dark its leaves remain white and sickly.
SayThe green in leaves only comes with light. The green part is the plant's kitchen.
Try it: bubbles from leaves
Read pages 158 and 159.
you will soon see little bright bubbles rising up and clinging to the glass.
Put the fresh leaves in the jar of water, push them under, and set it in bright sunshine. After an hour or two, look closely with the magnifying glass for tiny bubbles on the leaves and glass. That is oxygen the leaves are giving off as they make food. (Buckley used laurel leaves; garden laurel is poisonous, so use spinach or another safe leaf, and wash hands after.)
Watch only: the black in white sugar
Watch only: don't try this at home. Buckley poured strong sulphuric acid onto a heap of white sugar:
See! in a few moments a black mass of carbon begins to rise, all of which has come out of the white sugar you saw just now.
Strong acid burns skin and eyes badly. Look up a video of a teacher doing it. The point: sugar, starch and wood are mostly carbon, and plants get that carbon from the air.
The plant's stomach
Read page 161.
In this way these leaves act, you see, as the stomach of the plant, and digest its food.
Under each leaf are thousands of tiny mouths that let out extra water. Optional: drop raisins into water and check tomorrow; they swell up as water creeps in through the skin, the way Buckley's water creeps from cell to cell.
Tell it back and check
Your child tells back how a plant makes its food. Then ask:
1. What does a plant drink through its roots?
2. What does it need to turn green and make food?
3. What gas did the bubbles in the jar give off?
What we know now
Plants take in carbon dioxide (Buckley's "carbonic acid") and water and, with sunlight, make sugar. Scientists later found that the oxygen they give off comes from splitting the water. Plants also breathe some oxygen, day and night, as we do. Some plants, like peas, beans and clover, get help from tiny living things in their roots to take nitrogen from the air.
Afterwards
Notice it this week: put one small potted plant (or a bean sprout) in a dark cupboard and one in the light for a few days. Which stays green? Bring it back to the light afterward.
Tip
The bubbles show best in strong, direct sun and with very fresh leaves. A cloudy day may give only a few.
Answers to the book's problems
1. Water, with tiny amounts of minerals dissolved in it. 2. Sunlight. 3. Oxygen.
Worked out for this site from the scan. If a number in your copy differs, trust the book.