Answer:
When an organism turns all or part of it in response of an external stimulus towards certain direction it's called "tropism";
thus phototropism describes the organism's growth towards sunlight or light in general, and geotropism is described when the growth direction is related to the force of gravity (or towards the earth's center).
The "growth chemical" or plant's hormone responsible for controlling the direction is: auxin, which is essential for the plant's development.
Explanation:
