Plant Responses & Tropisms

What is a tropism?

A directional growth response to a stimulus.

What is phototropism?

A growth response to light.

What is gravitropism?

A growth response to gravity.

What is positive phototropism?

Growth towards a light source.

What is negative phototropism?

Growth away from a light source.

What is positive gravitropism?

Growth towards the direction of gravitational pull.

What is negative gravitropism?

Growth away from the direction of gravitational pull.

How do plant shoots usually respond to light?

They show positive phototropism.

How do plant shoots usually respond to gravity?

They show negative gravitropism.

How do plant roots usually respond to gravity?

They show positive gravitropism.

Why do shoots grow towards light?

To increase light absorption for photosynthesis.

Why do roots grow downwards?

To provide anchorage and access to water and mineral ions.

What controls many plant growth responses?

Plant hormones, particularly auxins.

What is an auxin?

A plant growth hormone that influences cell elongation and growth responses.

Where is auxin produced in shoots?

Mainly in the shoot tips.

How does auxin affect cell growth in shoots?

It generally stimulates cell elongation.

How does auxin affect cell elongation in roots?

At the concentrations involved in root gravitropism, it generally inhibits elongation.

Why can roots and shoots respond differently to the same auxin concentration?

Root and shoot cells have different sensitivities to auxin.

Why do tropisms involve growth rather than movement of the whole plant?

Cells on different sides grow at different rates, causing bending.

How is a tropism different from a reflex?

A tropism is a growth response, whereas a reflex is a rapid nervous response.