Where does a tree come from?
A tree weighing many tonnes grows from a seed you could lose in a pocket. Where does all that material come from? The obvious guess is the soil. In the seventeenth century the Flemish physician Jan Baptist van Helmont put the guess to a test. He planted a willow of 5 pounds in a pot holding 200 pounds of dried soil, and for five years gave it nothing but water. The tree grew to 169 pounds. The soil had lost two ounces.
Van Helmont concluded that the tree was made of water. He was half right. What he missed was the other ingredient, which he could neither see nor weigh: a gas in the air. About half of a tree’s dry weight is carbon, and every atom of it was once in carbon dioxide, a gas that makes up only about 0.04 per cent of the air.
That answers the question you started with: An oak that weighs several tonnes grew from an acorn, and the soil around it has hardly been used up. Where did all that wood come from?
Food from light
What the tree does with that gas is one of the most important chemical processes on Earth. In its green parts it joins carbon dioxide from the air to water from the soil and makes sugar, using the energy of sunlight to do it. This is photosynthesis, a word that means “putting together with light”.
The sugar is glucose. It is the plant’s fuel, and also its building material: linked into long chains, glucose becomes the cellulose of wood and the starch of a potato. The process leaves something over. For every molecule of carbon dioxide taken in, one molecule of oxygen is released into the air as waste. Nearly all the oxygen in the air was put there in this way, by plants on land and by algae and bacteria in the sea that do the same thing. Animals breathe a waste product.
Why leaves are green
The work is done inside the cells of a leaf, in small green bodies called chloroplasts. A single leaf cell may hold dozens. What makes them green is chlorophyll, the pigment that traps the light. Chlorophyll absorbs red and blue light strongly and reflects most of the green, and the reflected green is what reaches your eye.
Trapping light is a change of energy from one form to another. Light is energy on the move, and it cannot be stored. The chloroplast uses it to build glucose, and the energy ends up held in the bonds of the sugar as chemical energy, the kind stored in any food or fuel. There it can wait for hours or for centuries. When you burn a log, the warmth you feel is sunlight that fell on a leaf years ago.
What holds a leaf back
Give a plant more light and it makes sugar faster, but only up to a point. Measure the rate of photosynthesis, the amount of sugar a leaf makes each hour, as the light is turned up. The line climbs steeply at first, then bends over and runs flat. Beyond that brightness, extra light is wasted.
The reason is that photosynthesis needs several things at once, and it can go only as fast as the scarcest of them allows. Whichever is scarcest is called the limiting factor. In dim light it is the light itself. In bright light it is usually carbon dioxide, of which the air holds so little. Temperature matters too. Growers put this to use. In a closed greenhouse on a sunny day they pump in extra carbon dioxide, and the crop grows faster.
The price of breathing in
Carbon dioxide has to get into the leaf, and a leaf’s skin is waxed and waterproof. The gas enters through thousands of tiny pores, mostly on the underside, called stomata. Each pore is a slit between two curved cells that swell to open it and go slack to close it.
An open pore works in both directions. While carbon dioxide drifts in, water vapour escapes from the damp inside of the leaf. This loss of water through the leaves is called transpiration, and it is enormous. Of every hundred litres a plant takes up, it keeps three at most. The rest goes out through the stomata. So a plant is always choosing. On a hot, dry day it shuts its stomata to save water, and for as long as they are shut it can take in no carbon dioxide and make no sugar.
Drinking through the roots
All that lost water has to be replaced from the soil. It enters a root by osmosis: the movement of water across a membrane, the thin skin around a living cell, from the side where the water is purer to the side where more is dissolved in it. The cells of a root are full of dissolved sugars and salts, and the water in damp soil is nearly pure, so water moves in.
How fast it moves depends on how much root is touching soil. For this reason the young tips of a root are covered in root hairs, single cells drawn out into threads far finer than a human hair. They slip between the grains of soil and give the root an enormous surface area for its size. Most of the water a plant drinks comes in through them, and so do the minerals it needs.
A hundred metres with no pump
Water enters at the roots and is needed in the leaves, which in the tallest trees are more than a hundred metres above. It travels in the xylem, bundles of fine tubes made from dead, hollow cells joined end to end, running unbroken from root to leaf. Wood is old xylem.
A plant has no heart to drive the flow. The pull comes from the top. As water evaporates from the leaves, it draws the water behind it up the tube, because water molecules cling to one another like the links of a chain. Evaporation from the leaves lifts the whole column.
A second set of tubes, the phloem, runs alongside and carries the other cargo. Phloem is made of living cells, and it moves the sugar made in the leaves to wherever it is needed: down to the roots, out to a growing shoot, or into a swelling fruit.
A plant on pause
A plant cannot walk to a better place, but its offspring can be sent. A seed is a complete young plant, an embryo with its first root and shoot already formed, packed together with a store of food inside a tough coat. The coat keeps water out, and without water the embryo’s chemistry almost stops. In that state it can wait for the right season, or for years.
How long is surprising. Wheat kept cold and dry in a seed bank still sprouts after more than twenty years. In 2005 a date seed dug from the ruins of an ancient desert fortress was planted, and it sprouted. Tests on fragments of its shell showed that it was about two thousand years old. It grew into a palm that has since flowered. A seed is the one stage of a plant’s life that is built for travelling and for waiting.
Waking up
What ends the wait is water, warmth and air. The seed soaks up water and swells, its coat splits, and a root pushes out and turns downward, followed by a shoot that turns up. The seed is now germinating.
Underground there is no light, so the young plant cannot yet make sugar. Until its first leaves open it lives on its own food store, burning it with oxygen from the air in the soil. This is why a seed buried too deep, or sown in waterlogged ground with no air in it, dies. The store is rich, since it has to build a whole root and shoot, and it is what we eat when we eat wheat, rice, beans or nuts. Once it has leaves above the ground and is feeding itself, the young plant is called a seedling.
Flowers and their visitors
A flower is the part of a plant that makes seeds. Its male parts produce pollen, a dust of tiny grains. A seed can form only when pollen reaches the female part of a flower of the same kind, and carrying it there is called pollination. Since a plant cannot move, something else must do the carrying.
Grasses, oaks and birches use the wind. Their flowers are small and dull, and they shed pollen in vast clouds, because nearly all of it is lost. Most flowering plants hire animals. They advertise with bright petals and scent, and pay with nectar, a sugary drink made at the base of the flower. A bee pushing in for the nectar is dusted with pollen and carries it to the next flower it visits. About three quarters of the world’s main food crops give more when animals pollinate them.
Why a cherry is sweet
Once a flower has been pollinated its petals fall, and the part that holds the young seeds swells around them. It becomes the fruit. To a botanist a pea pod, a grain of wheat, a tomato and an acorn are all fruits, whatever a cook may call them.
A fruit exists to move seeds. A seed that drops beneath its parent must compete with it for light and water, so plants have many ways of sending seeds away, and this scattering is called dispersal. A dandelion gives each seed a parachute. A burr hooks into fur. A cherry tree wraps its seed in sweet flesh. A bird swallows the cherry, flies off and drops the hard stone somewhere else, and so its seeds are carried away from the tree. The sweetness is the fare.
What plants do for everything else
Plants make their own food, and nothing that eats can say the same. A caterpillar eats a leaf, a bird eats the caterpillar, a hawk eats the bird, and every link in that chain runs on sugar first made in a leaf. For this reason plants, with the algae of the sea, are called producers. Everything else is a consumer.
A plant also uses its own sugar. In every living cell of the plant, by day and by night, glucose is broken down with oxygen to release its energy. This is respiration, the same process that powers an animal, and it gives off carbon dioxide. A plant therefore runs two processes at once. In daylight photosynthesis far outpaces respiration, and the plant takes in carbon dioxide and gives out oxygen. In the dark only respiration goes on, and the exchange runs the other way.
A botanist from Málaga
The careful study of plants is called botany, and for most of history it was practised by physicians, because plants were the medicine chest. One of the greatest was Ibn al-Bayṭār, born in Málaga in Muslim Spain in the 1190s. As a young man he set out along the coast of North Africa collecting plants. He settled in Cairo, where the sultan made him chief herbalist, and went on collecting in Syria.
His great book describes some 1,400 medicines and foods, most of them plants, in alphabetical order. For each he gives its names in several languages, what earlier authors had said about it, and what he had seen for himself. Where an old authority was wrong, he says so. Between 300 and 400 of the medicines were unknown to the ancient Greeks. Muslim physicians had found them, and he gathered them into one book. It was consulted for centuries.
