What if plants could walk?

A dog can walk to its food. A bird can fly to its nest.

A plant stays in one spot its whole life. What if plants could walk?

Animals move around to find food, water and a safe place to live. Plants stay where they started growing, held in the ground by their roots, for their whole lives.

But staying put does not mean plants never travel or never move. In this thought experiment, imagine plants could get up and walk. Then explore the clever ways real plants already travel and move, and decide whether walking would really help them.

Most plants are sessile: once a seed germinates, the plant is fixed to that spot. Animals solve the problems of finding resources, escaping danger and spreading their young by moving; plants solve them by growth, chemistry and seeds.

This thought experiment imagines a walking plant and follows three threads: how plants already spread to new places, what roots do that a walking plant would lose, and how plants move without going anywhere. The outcomes are reasoned ideas built on real plant biology, not predictions.

A plant's seeds can travel far away.

The wind blows dandelion seeds. Water carries coconuts. Squirrels bury acorns, and some seeds stick to fur!

A plant can't walk, but its seeds can travel. Light seeds, like dandelion and maple seeds, ride on the wind. Seeds that float, like coconuts and mangrove seeds, drift on water. Animals help too: they eat fruit and leave the seeds behind in their droppings, squirrels bury acorns, and burrs hook onto an animal's fur.

Some plants even fling their seeds. Jewelweed bursts open and shoots its seeds outward. So each new plant can start life somewhere new.

Seed dispersal is how a sessile plant reaches new ground. Columbus State University's Oxbow Meadows center lists the main routes: small, light seeds such as dandelion and maple travel by wind; floating seeds such as coconut, sea bean and mangrove travel by water; animals eat fruit and pass seeds, and squirrels bury acorns; burrs cling to fur; and some plants, such as jewelweed, forcibly eject their seeds.

In effect, a plant moves between generations rather than within a lifetime. The parent stays rooted, and its offspring do the traveling.

A walking plant would have to pull up its roots.

But roots drink water and hold the plant still. Walking might make the plant thirsty and wobbly!

Imagine a plant that walked. It could stroll out of the shade into the sunshine, or away from a hungry deer. That sounds useful.

But roots do important jobs. They anchor the plant to the soil, they soak up water and minerals, and they store food the leaves have made. A plant that pulled up its roots to walk would lose its grip, its water supply and its food store all at once. It might have to stop and grow new roots every time it moved.

Roots anchor the plant, absorb water and minerals and transport them upward, and store the products of photosynthesis (OpenStax Biology 2e). Root hairs multiply the absorbing surface, and a taproot can reach deep into the soil while a fibrous system spreads near the surface.

A walking plant would have to trade this fixed, ever-growing absorbing network for mobility. It might gain access to light and escape from grazers, but it would face repeated loss of contact with water. Real plants do the opposite: they stay put and move by growing.

Plants do move, but slowly. Their stems bend toward the light.

Young sunflowers turn to follow the sun across the sky. Some leaves fold up when you touch them!

Plants move by growing. A stem on a windowsill bends toward the light, because the cells on its shady side grow longer. Roots grow down and shoots grow up. A pea plant's shoot winds around a stick to climb.

Young sunflowers turn from east to west during the day to follow the sun, then swing back east at night. Some plants move fast: when touched, the sensitive plant folds its leaves in seconds, and a Venus flytrap snaps shut on an insect.

Most plant movement is growth. In phototropism, blue light causes the hormone auxin (indole acetic acid) to accumulate on the shaded side, which elongates and bends the stem toward light; gravitropism sends roots down and shoots up; in thigmotropism, cells touching a support contract while the opposite side expands, so tendrils and pea shoots coil.

A UC Davis team (Science, 2016) found that young sunflowers track the sun because a circadian clock makes the east side of the stem grow faster by day and the west side by night; mature sunflowers stop and face east. Faster responses, such as the Venus flytrap's closing leaves, are touch responses called thigmonasty.

Mature sunflowers face the morning sun. Bees like the warm flowers.

You can watch a plant bend toward a window. Turn the pot and watch it bend back!

Once sunflowers grow up and bloom, they stop following the sun and face east. Scientists found that these east-facing flowers warm up faster in the morning and get five times as many visits from bees and other pollinating insects as flowers turned to face west.

The sensitive plant shows how much a quick move can cost. When touched, its leaves fold up and its branches droop in seconds, then it needs about 10 to 20 minutes to perk back up. Scientists think the folding may help protect it from animals that might eat it, but they aren't sure.

The UC Davis study found that east-facing mature sunflower heads warmed faster in the morning and attracted five times as many pollinating insects as west-facing ones; heating west-facing heads brought pollinators back. Here, a plant's orientation, not its location, is the adaptive 'move'.

The sensitive plant (Mimosa pudica) folds its leaves and droops within seconds of being touched and recovers over 10–20 minutes. AAAS notes that scientists aren't sure why it evolved the response; the leading idea is protection from herbivores. Rapid movement is possible for plants, but it is rare, and walking would be a far bigger step.