Imagine a seed planted a few inches down in moist, dark soil. After a period of time the seed breaks open and sends roots downward and shoots towards the surface. Similarly, a plant in a pot tips over (like in the picture above) so it changes its growth so the branches curve upwards. How does the seed/plant know which way is up? It’s really quite remarkable and the ability of plants to sense and react to gravity is known as gravitropism.
Scientists understand mostly how plants sense gravity, but there are some open questions and the theory continues being refined. A short summary of how it works is: some plant cells act as mini snow globes to sense gravity and then produce hormones that direct branch and root growth. The more detailed explanation of the prevailing theory:
- Plants contain specialized cells called statocytes that have organelles within them called statoliths.
- Statoliths are slightly denser than the rest of the cell and thus they settle in the direction of gravity.
- When Statoliths settle to the bottom of the statocyte cells, they exert force on a membrane which causes a biochemical signal (the exact mechanism of which is not understood) which triggers the production of a hormone (called auxin) at that (bottom) part of the cell.
- Auxin promotes cell elongation in shoots and inhibits this process in roots.
- Differential cell elongation between one side the cells and the other causes the root to bend downwards and the shoot upwards.