Science Puzzle
How Does a Seed Know Which Way Is Up?
Plant a bean upside down and it doesn't matter. The root curves round and grows downwards, and the shoot turns and grows up to the surface. Plant it on its side and the same thing happens.
It's dark in the soil, so the seed can't see the light. How does it know which way is up?
The Answer
It feels gravity. Cells near the tip of each root contain heavy grains that sink to the bottom of the cell, and wherever they settle tells the plant which way is down.
The grains are packed with starch, which makes them denser than the rest of the cell, so they settle like sand in a snow globe. Turn a root on its side and within minutes the grains slide to the new bottom. The plant responds by moving a growth hormone called auxin towards the lower side. In a root, extra auxin slows growth, so the top side grows faster and the root bends downwards. In a shoot, extra auxin speeds growth, so the lower side grows faster and the shoot bends up.
Charles Darwin and his son Francis tested this in 1880. When they cut the very tip off a root, it carried on growing but no longer turned downwards, which showed the sensing happens at the tip.
Above ground, shoots also steer towards light. On the International Space Station, with no gravity to feel, plant roots wander off in all directions unless light or water guides them.
The principle: Gravitropism. Plants sense gravity with heavy grains that settle inside cells in their root tips, then grow roots down and shoots up by changing how fast each side grows.