Plants are remarkable transporters. A fully grown tree moves hundreds of litres of water from its roots to its topmost leaves every single day — with no heart, no pump, and no electricity. Just quiet, invisible chemistry called capillary action.
The good news: you can make this invisible process visible in your own kitchen, with a white flower and a few drops of food colouring.
What actually happens when a plant drinks?
Capillary action is the primary force behind plant water transport. You have seen it many times: touch a corner of a paper towel to a glass of water and watch the water creep upward on its own, well above the water's surface. That is capillary action.
Inside a plant, long narrow tubes called xylem run from the root tips to the leaf edges — the plant's own water pipes. The narrower a tube, the higher water molecules climb. Two properties of water molecules make this work: cohesion (water molecules attracting each other, forming a continuous chain) and adhesion (water molecules sticking to the xylem walls, pulling the chain upward).
Capillary action alone cannot account for the tallest trees. A second force takes over: transpiration pull. Leaves lose water vapour through tiny pores called stomata. This evaporation creates low pressure at the top of the water column — like sucking on a straw — drawing a continuous supply of water up from the roots below.
Together, capillary action and transpiration form a two-stage, fully passive motor that runs on physics alone. Water is also one of the two raw ingredients in photosynthesis, the process plants use to make food from sunlight.
Why this matters for children's learning
Norway's national curriculum LK20 emphasises inquiry and direct observation in science from early primary school onwards. Capillary action is a concrete, observable example of the kind of biological process children are expected to explore. Educators generally find that children who connect an abstract concept to something they can see and touch retain that understanding more durably.
Capillary action is not limited to plants. It moves oxygen through the smallest blood vessels in the human body, holds rainwater in grains of sand, and draws ink into paper fibres when you write. Understanding it well opens a window onto many other phenomena.
Try it at home: white flowers in coloured water
Suitable for: 5–14 years — five minutes to set up, results in a few hours.
You need: a white flower (carnation or white gerbera), a transparent glass, food colouring (red or blue shows best), water, and scissors.
- Fill the glass with about 10 cm of water.
- Add 10–15 drops of food colouring and stir.
- Cut the stem at a diagonal angle — a larger cut surface absorbs more water. ⚠️ Adult helps with scissors.
- Place the flower immediately in the glass.
- Take a "before" photo.
- Wait 2–6 hours (or overnight for the clearest result). Watch the petals change colour.
The petals change because coloured water has travelled up through the xylem tubes and into the flower tissue itself. No pump. Just capillary action and transpiration.
What happens if you split the stem lengthwise and place one half in red water and the other in blue? Try it and see.
Three questions to wonder about
- If a pine tree is 50 metres tall and has millions of leaves — how many litres of water do you think it lifts from the ground every day?
- Plants release both oxygen and water vapour through the same leaf pores. Is that a coincidence, or a clever design?
- On a planet with no gravity, would plants still be able to move water from roots to leaves?
Every child is made of good atoms. At Good Atoms, we help children discover that science is not something that happens in a distant laboratory — it happens everywhere, all the time, including inside the flower on the windowsill.
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