Create a free account now and get 3 full lessons, completely free. Get my 3 free lessons

Worldwide

Notes from the field ·

Where Does a Tree's Mass Come From? The Willow Experiment That Surprised Scientists

By Ahmed, founder

Ask a group of people: “A tree grows from a tiny seed into a giant. Where does all that wood come from?” Many say soil, and it seems sensible. Trees are rooted in soil, and soil seems to be what they take. It’s one of the best examples of a wrong idea that real data can overturn.

The willow experiment

In the 1600s, a Flemish scientist, Jan Baptist van Helmont, did something careful for his time. He dried and weighed some soil, put it in a pot, planted a small willow tree weighing about 2 kg (5 lb), and covered the pot to keep out dust. He watered it with rainwater for five years.

At the end, the tree weighed about 77 kg (169 lb). He dried the soil again and found it had lost only a tiny amount, about 57 grams (2 ounces).

So the tree gained roughly 74 kg, and the soil lost almost nothing. The mass can’t have come from the soil.

Van Helmont concluded it came from the water. That was a reasonable guess given the evidence he had, and it was partly right. The part he missed came later.

The modern answer

Most of a tree’s mass comes from carbon dioxide in the air. Through photosynthesis, leaves take in carbon dioxide and, using energy from sunlight, combine it with water to make sugars and oxygen:

carbon dioxide + water → sugar + oxygen (using light energy)

The tree builds the sugar into wood, leaves and roots. The wood is mostly carbon, which came from the air. It sounds strange that air can become wood, but carbon dioxide is a gas made of carbon and oxygen, and the tree keeps the carbon and releases most of the oxygen.

Soil provides small amounts of minerals and water, which are essential but a tiny share of the mass.

How to teach it

  1. Predict. Before showing the data, have students guess where the mass comes from, and why.
  2. Examine the data. Give teams the willow numbers. Ask: “What does the evidence say about soil?”
  3. Add modern evidence. Show data cards, such as plants grown without soil in water (hydroponics), carbon dioxide measurements near a growing plant and the chemistry of wood.
  4. Argue from evidence. What do we now think and why?
  5. Write the equation. Introduce photosynthesis as a model that matches the evidence.

Students are often surprised that the answer is “mostly from air,” and surprise is a good learning moment.

Common mix-ups

  • “Plants eat soil.” Plants make their own food.
  • “Plants get food from the roots.” Roots take in water and minerals. Food is made in leaves.
  • “Wood is made of water.” Water is part of it, but carbon is the bulk of dry wood.

This is the lesson Where does a tree’s mass come from? from How Plants Make Food: Photosynthesis and Respiration, which continues with What happens to the food a plant makes? and How does energy from the sun move through a whole ecosystem? Sign up for three lessons free.