1. Phototropism and Gravitropism!

2. Main Understandings
Explain phototropism and gravitropism as examples of control systems in plants
Explain how our understanding of phototropism and gravitropism has changed over time.

3. Cool Killer Parasitic Fungi

4. How do plants respond to different stimuli?
Tropism: a growth in response to a stimuli
Positive tropism: the plant grows TOWARDS the stimuli
Negative tropism: the plant grows AWAY from the stimuli

5. What is phototropism?
When light is the stimulus that the plant responds to
The plant has a hormone called auxin. This hormone is found in both the roots and the top of the plant (the shoot) and causes plant growth.
A high concentration of auxin results in the elongation/ growth of that part of the plant!

6. How does it work? Auxin
The auxin molecules moves away from the light and result in cell elongation wherever they move.
Light overhead: auxin moves down the stem causing the stem to grow
Light to the side: the auxin moves away from the light causing that side to grow and the plant to bend towards the light.

7. Darwin’s famous experiment
Cut off the tip it did not grow
Covering the tip, it did not bend.
Covering the base it bent.
Another scientist Boysen-Jensen studies plants after Darwin.
Inserted a disk on each side of the plant. Found when it was on the shady side- phototropism was affected.
Found when it was in the sunny side it did not affect bending.

8. Let’s see it in action!

9. gravitropism
Roots also have auxins that move based on the root’s position
Root horizontal- the auxin moves down and the BOTTOM SIDE GROWS LESS (this is opposite of auxin in the shoot).
Therefore, the top side grows more and it bends down.

10. Questions?!
We will now explore tropism through an interactive virtual lab!
MAX groups of two- but must work through the questions together and not split up the work.
Once you have shown us your completed sheet- you can work on your science pitch.