Roots growing towards the pull of gravity are called geotropic. This is a specific type of tropism, which is a growth response to an external stimulus.

Here's a breakdown:

* Tropism: A growth response of a plant in a particular direction, either towards or away from a stimulus.

* Geotropism: A growth response to the force of gravity. Roots are positively geotropic, meaning they grow downwards towards the pull of gravity.

* Phototropism: A growth response to light. Shoots are positively phototropic, meaning they grow towards light.

So, in summary, roots growing towards the pull of gravity is a geotropic response.