What is the term used to describe why roots grow in the direction of earth's gravity pull

The specific term to characterized the downward growth of roots in response to earth’s gravity pull is called positive geotropism. The word geotropism was derived from two words, “geo” which means ground and “tropism” which describes a plant movement elicited by a gravitational stimulus.

In addition, the root cap makes the roots turn downward as they grow for they have specialized cells with sensors called statoliths that settle to the lowermost part of the root cap in response to the pull of gravity and help the cell multiply more rapidly in a downward direction.