1. Plant Control Systems Response to Stimulus

2. Control systems ► Similar to animals, plants respond to stimuli ► We may respond to a loud noise by covering our ears jumping.  This is a stimulus response

3. Control systems ► Plants respond to different stimuli than animals do ► They respond to things that that will effect them such as sunlight, gravity, touch and temperature

4. Phototropism ► Photo – sunlight ► Tropism – the movement of plant is response to stimulus  Tropism can either be positive or negative ► Positive grows towards stimulus ► Negative grows away from stimulus ► The stem exhibits positive phototropism ► The root exhibits a weak negative phototropism

5. Darwin and son ► Which part of the plant detects and responds to phototropic stimulus?  Found that the tip of the grass controls the direction in which the plant grows  They found that with Avena sativa (oats) if the tip was not covered or removed the oats would grow towards the light

6. Boysen-Jensen ► What is the signal that initiates the phototropic response?  Found that on the dark side of the grass the cells were longer than on the sunny side, which caused the grass to bend.  These longer cells are in the area of elongation.

7. Boysen-Jensen ► Found that whatever was controlling the elongation could diffuse through gelatin but not mica.  It was therefore a chemical of some sort

8. Went ► What is the specific substance responsible for initiating the phototrophic response?  Isolated the chemical involved in Phototropism ► The hormone auxin

9. Gravitropism ► Movement in response to Gravity ► Scientist believe that plants rely on heavy Starch particles in specialized cells. ► These particles rest on the bottom of the cells (pulled down by gravity)

10. Gravitropism ► When a plant is tipped over the particles move and the plant responded by growing away (-) or towards (+) where the particles are resting ► The stem will generally grow away and the roots will grow towards the pull of gravity

11. Other Mechanisms ► Plants respond to other stimuli other than light and gravity. These stimuli include:  Touch  Temperature  Chemicals  Water  Etc

12. Links ► Light and Gravity Light and Gravity Light and Gravity ► Touch Touch