How Does a Tree Know Which Direction Is Upward?

Plant a seed upside down and it will not grow upside down. Within days, the root curls round and heads down into the soil, and the shoot bends back on itself to climb towards the surface. It does this in complete darkness, before it has ever seen light, with no eyes, no inner ear and no brain. Somehow, a single seed knows which way is down.

We rarely think about this because it is so reliable. Forests stand upright. Crops grow in neat vertical rows. A tree on a hillside grows straight up rather than perpendicular to the slope. Yet every one of those trunks is the result of a sensing system operating continuously inside the plant, detecting the direction of gravity and correcting growth accordingly. Plant biologists call the response gravitropism, from the Greek for turning in response to weight.

People have been puzzling over it for centuries. In 1806 the English horticulturist Thomas Andrew Knight fixed germinating seeds to a wheel that spun rapidly, creating a force stronger than gravity pulling outward from the centre. The roots grew outward and the shoots grew towards the middle. Knight had shown that plants respond to gravity itself, or to any force that behaves like it, rather than to light or moisture. Later in the nineteenth century, Charles Darwin and his son Francis, experimenting with seedlings at their home in Kent, concluded that the tip of the root was where the sensing happened, calling it the part that acts like a brain. The comparison was provocative, and it was not entirely wrong.

The explanation lies in specialised cells. In the very tip of a root, inside a protective cap, sit cells called columella cells. In shoots, similar cells are found in a layer surrounding the plant's water-carrying tissues. Inside these cells are small, dense grains packed with starch, known as amyloplasts or, because of their role, statoliths: literally, stones that tell position. They are heavier than the fluid around them, and so they settle towards whichever side of the cell is lowest.

When a plant is standing upright, the grains rest on the bottom of each cell. Tip the plant on its side and the grains slide and resettle against what was previously a side wall. The plant has registered a change in the direction of gravity. Scientists are still working out exactly how the settling grains are converted into a signal, but it appears to involve pressure on internal membranes and structures within the cell, triggering a cascade of chemical changes.

What happens next involves one of the most important substances in plant life: the hormone auxin. Auxin is moved through plant tissues by transporter proteins that can be repositioned within cells. When the statoliths settle against a new side, these transporters shift, and auxin begins to accumulate along the lower side of the root or shoot. The idea that unequal auxin drives the bending was proposed in the 1920s by Nikolai Cholodny and Frits Went, working independently, and the basic principle has held up remarkably well.

The same hormone produces opposite results in different parts of the plant. In a shoot, more auxin makes cells elongate faster. The lower side grows more than the upper side, and the shoot curves upward. In a root, higher concentrations of auxin slow cell growth. The lower side grows less than the upper side, and the root curves downward. A single signal, read differently in two places, sends the plant in two directions at once.

A tree stands upright because, inside it, countless tiny grains keep falling.

The response is surprisingly fast. Time-lapse films of seedlings laid on their side show the tips beginning to lift within minutes to hours, correcting their direction as if feeling their way back to vertical. The plant does not overshoot wildly. It bends, checks and adjusts, continuously re-reading the position of its statoliths as it curves.

Trees add another layer to the story, because a trunk cannot simply bend like a seedling. Once wood has formed, it is largely fixed. Yet trees knocked off vertical by wind, landslides or snow can gradually straighten themselves over years and decades. They do it by laying down special reaction wood. Broadleaf trees tend to form tension wood on the upper side of a leaning stem, which contracts and pulls the trunk upright. Conifers do the opposite, forming compression wood on the lower side, which expands and pushes. Walk through a forest on a slope and you may see trunks with a curve at the base, a record in wood of a tree slowly correcting a lean.

Gravity is not the only signal a plant listens to. Shoots bend towards light, a response called phototropism, which uses the same auxin machinery. Roots grow towards moisture and away from obstacles. Plants combine these signals, and gravity is one of the most dependable, the one that does not change with the weather or the time of day.

That makes space an interesting laboratory. In the nineteenth century the German botanist Julius Sachs used a slowly rotating device called a clinostat to average out gravity by constantly changing the direction a plant experienced. Modern experiments aboard the International Space Station have gone further. In near weightlessness, roots no longer have a clear down. Some grow in wandering, skewed patterns, while others orient themselves using light, moisture or contact instead. Remove gravity, and the plant falls back on its other senses.

It is tempting to describe this in the language of intention: the root wants to go down, the shoot seeks the sky. The plant does not want anything in the way that we do. But the behaviour is more than passive. A plant detects the state of the world, integrates several kinds of information, and changes its body in response, over and over again throughout its life. That is a form of intelligence, even if it is spread across cells and measured in hours and years rather than milliseconds.

There is something quietly moving in that. We usually think of trees as the most stationary things in the world, symbols of stillness and patience. But every trunk that rises straight from the ground is the visible outcome of an unending process of correction, one cell at a time, against the steady pull of the Earth.

The next time you walk beneath a forest canopy, look up the length of a trunk towards the light. Inside it, and in every root beneath your feet, minute grains of starch are settling in the dark, and the tree is listening to them.