Chapter 25 25-2 Plant Responses

a. Positive tropism – movement towards a stimuli. I. Tropisms A. Tropism is a slow plant movement, towards or away from a stimuli, that is determined by the direction of an environmental stimulus. 1. 2 Types a. Positive tropism – movement towards a stimuli. b. Negative tropism – movement away from a stimulus.

2. Phototropism A. A plant movement in response to light. 1. Light causes hormone Auxin to move to the shaded side of the shoot, elongating those cells causing the plant to lean towards the light. a. Solar tracking – leaves tracking the sun’s light and moving with the sun across the sky.

3. Thigmotropism A. A plant’s growth response to touch or touching a solid object. Some plants coil when they touch an object like the growth of vines.

A plants growth response to gravity. 4. Gravitropism A plants growth response to gravity. 1. Positive gravitropism – roots grow down with the gravitational pull. 2. Negative gravitropism – stem growth up and away from the gravitational pull.

5. Nastic Movement A. A. A plants rapid movement that occur in response to environmental stimuli that are independent of the direction of the stimuli. These movements are regulated by changes in the water pressure of certain plants.

6. Thigmonastic a. Venus Fly Trap- a combination of osmotic pressure and cell wall expansion causes the leaf to snap shut.

7. Nictinastic A. Nictinastic – plants movement in response to the daily cycle of light and dark. Turgur pressure is more gradual.

1. Short-day plants – flower when the days are short. 8. Photoperiodism a. Timing of seasonal activity such as flowering and growth. Phytochrome, a plant hormone is said to be responsible for photoperiodism. 1. Short-day plants – flower when the days are short. 2. Long-day plants – flower when the days are long.