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Tropic Responses Let Plants Grow Toward Light and Gravity Without a Nervous System
- Plants have no nervous system, so they rely on coordination by plant hormones to control growth.
- These hormones let a plant sense and respond to stimuli such as light and gravity.
- The directional growth responses that result are called tropic responses, controlled mainly by auxins.
Example
- Plant a seed on a windowsill and within days the shoot bends toward the light.
- This is phototropism, one type of tropic response.
Seedlings Show Three Main Tropic Responses
- A tropism is a directional growth response to an environmental stimulus.
- Phototropism
- Growth in response to light.
- Shoots grow toward the light because auxin builds up on the shaded side and makes those cells elongate more.
- Gravitropism (geotropism)
- Growth in response to gravity.
