Plant roots grow down, towards gravity. This effect is known as geotropism (it can also be called gravitropism). Geotropism happens whenever the direction that a plant grows in is in response to the direction of gravity. ("Tropism," which is part of the word "geotropism," refers to any organism that turns in response to something external, or around them in their environment.)

Here's an example of a fun science fair project you could do to investigate the effects of geotropism:

I hope this all helps!

Answer:

Geotropism literally means growth that is affected by the earth (you can see that geo is a word fragment that means earth: geology, geography, etc.).

See also tropism

Geotropism is the turning reaction of a plant or animal because of gravity. Plants are able to sense gravity and will grow against it. Even if you start a plant from a seed in complete darkness, it will grow up. You could easily perform this experiment with your lettuce seeds in a sideways sitting Petri dish by putting it in a box.