14.5 Tropic Responses

Plants are able to detect and respond to environmental stimuli such as light and gravity. These responses involve growth movements that help the plant survive in its environment. Unlike animals, plants cannot move from place to place, so they respond by growing in particular directions.

Gravitropism (also called geotropism) is a growth response to gravity.

Phototropism is a growth response to light.

These growth responses are examples of tropisms - directional growth movements in response to environmental stimuli.

Shoots generally grow away from the pull of gravity, so they are described as negatively gravitropic.

This response ensures that shoots grow upwards, away from the ground and towards the light. This positioning is essential because:

• Leaves are held out into the sunlight where they can photosynthesize effectively
• The more light the leaves receive, the better they can photosynthesize
• Flowers are held up in the air where they can be pollinated by insects, birds, or wind

Roots generally grow towards the pull of gravity, so they are described as positively gravitropic.

This response ensures that roots grow downwards into the soil. This is crucial because roots need to:

• Anchor the plant firmly in the soil
• Absorb water from between soil particles
• Absorb mineral ions from the soil