Science Puzzle
Where Does a Tree’s Mass Come From?
In the early 1600s the Flemish scientist Jan Baptist van Helmont planted a willow sapling weighing about 2.3 kilograms in a pot holding 90.7 kilograms of dried soil. For five years he gave it nothing but rainwater.
When he weighed everything again, the willow had grown to about 76.7 kilograms, yet the soil had lost only about 57 grams. Where did more than 74 kilograms of new wood, bark and roots come from?
The Answer
Mostly from the air. The solid stuff of a tree is built largely out of carbon dioxide gas.
Van Helmont was right about one thing: the soil supplied almost nothing. The 57 grams it lost were mainly minerals such as potassium and calcium, which a tree needs in small amounts. He concluded that the 74 kilograms of willow had come from water alone, but he missed an ingredient he couldn't see or weigh.
Leaves take in carbon dioxide from the air through tiny pores. Photosynthesis uses the energy of sunlight to split water and join its hydrogen to the carbon from the carbon dioxide, making sugar. The tree then builds that sugar into cellulose and lignin, the materials of wood. Dry a piece of wood completely and about half of what's left is carbon, and every atom of that carbon was once floating in the air as a gas.
Sunlight supplies the energy, not the matter, and the water provides hydrogen. A living tree is also full of water, so van Helmont was partly right. Burn the dry wood, though, and the reaction runs backwards: the carbon goes straight back into the air as carbon dioxide, leaving only a small heap of mineral ash from the soil.
The principle: Photosynthesis builds living matter from air and water. A plant's dry mass is mostly carbon taken in as carbon dioxide from the air, with only a small share of minerals coming from the soil.