The big idea
A seed needs water and warmth to wake up. Then the little plant needs light too.
With sun, water and warmth, it grows into a tree. The tree makes flowers, the flowers make apples, and the apples hold new seeds.
A seed germinates with water, warmth and air. Its leaves then need light.
Until its first leaves open, the seedling lives on food stored in the seed. After that, the leaves make the plant's food from light, air and water. That is photosynthesis.
💧 Why water?
A seed soaks up water to wake up. Roots drink water from the soil and send it up to every leaf. Too little and the plant droops. Too much and the roots can't breathe.
Water makes the seed swell and switches on germination. Roots take up water, and it travels up the stem to the leaves. Too little and the plant wilts. Too much fills the air spaces in the soil, and roots need oxygen.
🌡️ Why warmth?
Plants grow when it's warm. When it's cold, seeds wait and plants rest until warmer days come back.
Warmth speeds up the chemistry inside a plant. Below about 10 °C, most garden plants stop growing, and very hot days slow them down too.
☀️ Why light?
A seed can sprout in the dark, using food packed inside it. But then the leaves need light to make more food. In the dark, a sprout grows tall, pale and weak.
Seeds don't need light to germinate; they run on stored food. Seedlings do: their leaves need light to make sugar. In the dark, a seedling stretches tall and pale (etiolation) until its stored food runs out.
Parts of a plant
- Roots hold the plant in the soil and drink water.
- The stem holds it up and carries water to the leaves. On a tree, the stem is the trunk.
- Leaves catch light and make food.
- Flowers turn into fruit, and the fruit holds the seeds.
- Roots anchor the plant and take up water and minerals through tiny root hairs.
- The stem (a trunk, once it turns woody) carries water up in tubes called xylem, and sugar around in phloem.
- Leaves are the plant's food factories: photosynthesis happens in their chloroplasts.
- Flowers are for reproduction. A pollinated flower becomes a fruit that protects the seeds.
Try this
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- ✓Grow it to a flowerWater it, turn on the sun, then hold GROW.Flowers! With light, water and warmth, it grew all the way.
- ✓What happens with no sun?Turn the sun off, then grow a new sprout.It grew tall, thin and pale, then stopped. Leaves need light to make food.
- ✓Too much water?Pour lots of water on a sprout and keep growing.The leaves turned yellow. Roots need air, and soggy soil has none.
- ✓Make it too coldTap Warmth for a cold day, then hold GROW.It stopped growing. Plants rest and wait for warm days.
- ✓Grow a seed from your plantGrow apples, then plant a seed from the one that falls.A new sprout! Seeds make new plants. That's a life cycle.
Grow one for real
- Fold a wet paper towel into a clear cup.
- Tuck a dry bean between the towel and the side of the cup.
- Keep the towel damp and the cup somewhere warm.
- In a few days, watch the root grow down and the shoot grow up.
How a plant makes its food
Leaves take in carbon dioxide from the air, and roots bring up water. Powered by light, the leaves turn them into sugar and give off oxygen. That is photosynthesis. Plants don't eat soil: the soil gives them water and a few minerals.
How the model works
Each day, growth goes ahead only as fast as the worst need allows: the growth rate is the light factor × the water factor × the warmth factor, each between 0 and 1. Good ranges: soil water 32 to 78%, warmth 15 to 30 °C (none below 10 °C), and light above about 45%. A seed germinates with water and warmth alone. In dim light a seedling stretches on its stored food until it runs out. The plant drinks more water when it is bigger, brighter and warmer.