A willow tree in a pot
In the 1640s a Flemish scientist, Jan Baptist van Helmont, planted a small willow in a pot of carefully weighed soil and gave it nothing but WATER for five years. The tree gained about 74 kilograms, while the soil lost only a few dozen grams. He concluded that the tree had been built from water. He was partly right, but he had missed the air. In 1771 Joseph Priestley found that a sprig of mint could keep a candle burning, and a mouse alive, in a sealed jar that had become stale, and in 1779 a Dutch doctor, Jan Ingenhousz, showed that PLANT leaves only did this in SUNLIGHT.
Put together, the answer is photosynthesis: plants take in carbon DIOXIDE from the air through their leaves and water through their ROOTS, and use the ENERGY of sunlight to turn them into GLUCOSE, a kind of SUGAR, giving off OXYGEN as a by-product. Most of a tree, wood and all, is built from carbon taken out of the air.
The oxygen, surprisingly, comes from the water rather than the carbon dioxide. Scientists proved it in 1941 by giving plants water made with a rare, traceable kind of oxygen atom, and finding that same kind of oxygen in the gas the plants gave off.
Why leaves are green, and why they change
The green PIGMENT inside each LEAF is chlorophyll, packed into tiny parts of the CELLS. It soaks up red and blue light for energy and reflects green, so most plants look GREEN to us. Leaves also hold yellow and orange pigments, hidden under all that green for most of the year.
In autumn, deciduous trees stop making new chlorophyll and start breaking down what they have, saving the useful parts before the leaf falls. As the green fades, the yellows and oranges show through. The reds and purples are different: some trees make those fresh in autumn, especially after sunny days and cold nights, so some autumns are far more colourful than others. Our forest page covers what happens under the trees when the leaves come back in spring.
Pipes, pores and stored food
A plant is full of plumbing. XYLEM tubes carry water and minerals up from the roots through the STEM, and PHLOEM tubes carry the sugar made in the leaves to every other part, including the roots. The VEINS you can see in a leaf are bundles of both. Tiny pores on the underside of leaves, called STOMATA, open to let carbon dioxide in and oxygen out, and close on hot days to save water.
Plants turn extra glucose into STARCH to store it, in potatoes, rice, wheat and the seeds of many other plants, and that stored sunlight is what we eat. The PROCESS is not very efficient, turning only a few per cent of the light that falls on a leaf into stored energy, yet it feeds almost every living thing on Earth and has put nearly all the oxygen into the air we breathe. Leaves ABSORB light and PRODUCE food on a scale nothing else can match; our plants page covers some of the stranger ways they do it.