Auxin and geotropism
Geotropisms
Phototropism is a response to the stimulus of light, whereas geotropism (also called gravitropism) is a response to the stimulus of The force of attraction between all objects. The more mass an object has, the larger the force of gravity it exerts..
Plants responses to gravity:
- when the stem grows against the force of gravity, this is known as a negative geotropism
- when a root grows in the direction of the force of gravity, this is known as a positive geotropism
Just like phototropism, geotropism is also caused by an unequal distribution of auxin.
In a root placed horizontally, the bottom side contains more auxin and grows less - causing the root to grow in the direction of the force of gravity.
The opposite happens in a stem. When a stem placed horizontally, the bottom side contains more auxin and grows more - causing the stem to grow upwards against the force of gravity.