Plants display two sorts of gravitropism simultaneously:
- Positive gravitropism: The roots grow in the direction of gravity.
- Negative gravitropism: The shoots grow against the direction of gravity.
Plants continuously sense which way is "up" and adjust their growth directions accordingly. A common experiment is to plant a seed in a pot and to place the pot on its side after some time. The result is that the roots and shoots will grow in a curve to adjust for the perceived directional change of gravity.
The exact mechanism that allows them to do so is still a matter of debate, with several theories having been voiced by scientists. One such theory argues that heavy starch grains in special cells will tell the plant the direction of gravity by sinking to the bottom of their cells. This theory is called "statolith theory".