The growth of a plant part towards the earth is called geotropism, specifically positive geotropism.

Here's why:

* Tropism: A growth response of a plant to an external stimulus.

* Geo: Refers to the Earth or gravity.

* Positive: Indicates growth in the direction of the stimulus (in this case, gravity).

So, roots growing downwards into the soil exhibit positive geotropism, while stems typically grow upwards against gravity, demonstrating negative geotropism.