Whenever a seed is planted, its roots grow down into the ground and its stem and leaves grow up towards the sky. But seeds are planted underground where sunlight can’t reach, so how do they know which direction to grow?

Scientists have discovered that plants have an amazing ability to sense the pull of gravity. Gravity is the invisible force that holds everything on Earth down on the ground. Plants detect the direction of gravity and send their roots in the same downward direction, and their shoots (their stem and leaves) in the opposite direction, up towards the sky.

Many experiments have shown this phenomenon. In one experiment, plants were taken up into space, where there is no gravity. These plants had roots that grew in random directions, because there was no force helping them tell which way was down.

How do plants figure out which way gravity is pulling? Scientists aren’t entirely sure, but they do have an educated guess.

At the very bottom of their roots, plants have special cells. Inside those cells are dense particles called statoliths. You can think of statoliths inside these cells like a handful of chocolate chips inside of a big jar. Picture tipping the jar on its side or even upside down-- the chocolate chips will always stay on the bottom, or whatever surface of the jar is closest to the ground, because that’s the way that gravity is pulling them. Statoliths respond in the same way to gravity.

Scientists think plants use the location of statoliths to determine which direction is down, so they know to grow their roots in that direction. The opposite direction must be up towards the surface where there is sunlight, so plants grow their stem and leaves that way. This is the reason you can see trees on steep slopes of mountains still growing straight up and down.

Once the stem and leaves are above the surface, the direction of their growth is mostly determined by the direction of light. Plants use light to photosynthesize, or make their own food, which is essential for survival. Because of this, their shoots grow towards available light. This is why you can also see some plants bend or lean towards a source of light.

So, no matter how you put a seed into the ground, you can count on its roots going down and its shoots going up because of its ability to detect gravity!