How Do Plants Know Which Way Is Up And Which Way Is Down?

 "It's dark down there in the potting soil. There's no light, no sunshine. So how does it know which way is up and which way is down?

 It does know. 

Seeds routinely send shoots up toward the sky, and roots the other way. 

Darkness doesn't confuse them. Somehow, they get it right. 

More intriguing, if you turn a seedling (or a whole bunch of seedlings) upside down,

 as Thomas Andrew Knight of the British Royal Society did around 200 years ago, the tips and roots of the plant will sense, 

"Hey, I'm upside down," and will wiggle their way to the right direction, doing a double U-turn. 

 How do they know? "

Krulwich Wonders... : NPR / more