Have you heard of gravitropism?
It's the unique ability of plants to sense gravity, and respond accordingly. In other words, it's how a plant knows to point its roots downward.
Turn a tiny, brand-new clover seedling that has started to grow roots on its side, and it immediately changes the direction of its roots. How does it know to do that?
Well, scientists aren't entirely sure, but the best theory is that it's the statocytes.
Statocytes
Statocytes are tiny sensors that contain grains of starch at the center which, when pulled by gravity, give off a signal. Statocytes are a simple way to determine which direction is down.
How does the root bend, even if the plant knows which way is down?
Probably the statocytes signal the cells in the root-tip to change their growth rate. If the ones on the top grow faster than the ones on bottom, the whole root will bend downward.