How Do Seeds Know Which Way is Up?

A plant grows away from the ground up into the air, right? Of course it does, but how does a plant know to do this while it is a seed in the ground? It’s not like a seed has the sun to use as a guide. Why doesn’t a seed grow farther down in the ground or go sideways? Amazingly, plants have a unique sense that tells them which way they need to go to break into the sunshine and which way the roots need to grow to keep the plant alive.

When a plant has pushed out of the soil, its direction of growth is governed by the effect of light. This is called phototropism. When a seed is in the ground, however, there is no light. How then does a seed know which way is up? It all comes down to gravity. Plants are affected by gravity which dictates the direction roots will grow. This is called gravitropism. Phototropism and gravitropism are the responses that enable a plant to grow toward the light and the roots to grow toward the pull of gravity.

It has been a straightforward observation that plants grow toward the light, but until around 200 years ago, the question of how the plant got to that point was unknown. A botanist of the British Royal Society named Thomas Andrew Knight was the first to understand that there was some other stimulus that allowed a plant to know which way was up.