How do roots know which way is down?

Roots do not think their way downward. In the growing tip, heavy grains sink with gravity, like sand settling in a jar. When those grains rest on one side of a cell, they start signals that tell the root where Earth is pulling. Those signals make the tip turn downward.

The neat part is that shoots use a related sensor but answer it differently. A root curves into soil, while the stem bends toward light. One seed can sort out up and down before it has leaves, simply by letting tiny weights settle inside living cells.