The response of the plant, where the stem grows upward and the roots grow downward, can be described as "positive phototropism" for the stem and "positive gravitropism" for the roots.
- Positive phototropism is the growth of the plant towards light, which causes the stem to grow upward.
- Positive gravitropism (or negative phototropism for the roots) is the growth of plant parts in response to gravity, leading the roots to grow downward.
So overall, the stimuli describing the plant's response would be light (for the stem) and gravity (for the roots).