Here's why:
* Gravity is a constant force: Plants are always experiencing gravity. It's not something that changes or needs to be "switched on" like a light source.
* Tropic responses are directional: Tropic responses, like phototropism (growth towards light) or gravitropism (growth towards gravity), are about a plant's growth in response to a directional stimulus.
* Gravity's role is more about orientation: Plants use gravity as a reference point for orienting themselves. They have specialized cells called statoliths that sense gravity and help the plant determine which way is up and down. This information is essential for root growth (downwards) and shoot growth (upwards).
So, while gravity is crucial for plant growth and orientation, it's not a "stimulus" that induces tropic responses in the same way as light or touch. Instead, gravity acts as a constant reference point that plants use to guide their growth.
