Final answer:
Geotropism is a plant's response to gravity, with roots showing positive geotropism (growth downward) and shoots negative geotropism (growth upward). Auxin, a plant hormone, regulates this movement by creating a concentration gradient that directs growth. Similarly, phototropins regulate phototropism, aiding in plants bending towards light sources.
Step-by-step explanation:
The geotropism response is a plant's growth movement in response to gravity. Roots grow downward, a process known as positive geotropism, because cells in root caps are specialized to respond to the gravitational pull. Shoots, on the other hand, exhibit negative geotropism by growing upward, away from gravity.
The plant hormone that regulates geotropism, as well as other growth movements like phototropism, is called auxin. Auxins are distributed unevenly in the plant, typically accumulating on the lower side of a root or the shaded side of a shoot. This uneven distribution results in a concentration gradient that causes cells to grow faster on one side, directing the growth of the plant toward or away from the stimulus, whether it is light or gravity.
Phototropism is a similar growth movement directed by the plant hormones known as phototropins, with plants bending towards a light source. For example, auxin stimulates elongation of cells on the darker side of a plant, causing it to bend towards light.