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SBI3U
Teacher Demonstration: Plant Tropisms
Topics
plant
tropism
auxin
asymmetric growth
Timing
preparation: 5min
demonstration: 10-15min
Specific Expectations: SBI3U
Introduction
This student activity illustrates how plants grow away from or towards a stimulus by continuing
cell growth and elongation on one side of the stem while ceasing cell growth and elongation on
the opposite side of the stem. This connects to the curriculum by allowing students to be able to
explain how this change in growth happens.
Materials
one slinky (2 - 4 cm ring diameter)
one sheet strong cardboard (30 cm x 30 cm)
one pair scissors
20 cm duct tape
one rubber balloon
one elastic band
one plastic straw (or 20 cm rubber tubing)
five wire twists (used for closing plastic bags)
safety glasses
Safety Considerations
Check that no one in the class has a latex allergy. Consider using non-latex balloons.
Safety glasses protect eyes from balloon bits or broken wire twists.
Procedure
Preparation
1. Insert one end of straw/rubber tubing into balloon.
2. Seal using elastic band.
3. Exhale 2 full breaths into balloon.
4. Cut cardboard into 2 squares/circles large enough to cover slinky ends.
5. Punch hole into center of one cardboard piece (big enough to squeeze balloon through).
6. Tape cardboard pieces to each end of slinky.
7. Stuff the balloon into slinky. Leave straw/tubing outside of slinky.
8. Place slinky vertically on flat surface on slinky end without straw.
Prediction/Explain
Organize students into pairs so that they can collaborate during the demonstration.
Instruct student pairs to make a sketch of the materials.
Ask the student pairs to make a sketch to predict what will happen when air is blown into the
balloon. They must also include an explanation for their prediction.
Observe
Inflate balloon about halfway by blowing into straw.
Keep the flat surface from moving while gently bending growing “plant”. Ask a student
volunteer to help with this.
Hook one wire twist around several slinky hoops to join them together.
Blow into balloon.
Explain
Instruct student pairs to revisit their predictions and explanations.
How did adding the wire twist change the process?
Challenge students to make parallels between this demonstration and a real growing plant. When
would this happen? How could this be tested?
Disposal
Broken materials can be added to the garbage waste.
What happens?
Inflating the balloon will result in the slinky stretching up and becoming stretched around the
balloon. This will make the slinky stand up rigidly on one end. Attempting to bend the slinky
will result in it bouncing back to position. Joining some slinky hoops together and then inflating
the balloon more will result in the slinky bending in the direction of the joined hoops.
How does it work?
The slinky expands due to the pressure of the air in the balloon pressing outwards from inside the
slinky. This models the effect of turgor pressure in plants, caused by water inside cell
membranes pushing outwards onto cell walls and causing the plant to become rigid.
Linking hoops together with the twist tie models growth away from or towards a stimulus by
showing how a plant will grow on one side of the plant stem but not the opposite side.
This results in the plant bending towards or away from a stimulus.
Teaching Suggestions/Hints
A slinky may be too small or too big for the balloon, so test balloons of various balloon sizes
before doing the activity to find the slinky/balloon combination that works best.
To explain turgor pressure (but not plant growth) use an example of wilted celery. Put some
celery in water overnight to show students how it becomes turgid again when it can absorb
water. Remember to keep some celery without water as a control.
This activity can be used together with Plant Tropisms Part 2.
Explain the role of auxins in plant growth.
This demonstration could be stopped before the use of a twist tie so that students can investigate
how adding a twist tie will change growth.
Next Steps
Challenge students to investigate their understanding of plant growth using auxin.
Additional Resources
This demo was modified from: http://www.cals.ncsu.edu/nscort/outreach_exp_tropism.html
Wikipedia article about phototropism: http://en.wikipedia.org/wiki/Phototropism
Video showing phototropism in tomato seedlings:
http://www.youtube.com/watch?v=Ze8NV7cvW8k
Specific Expectations
SBI3U
A1.1 formulate relevant scientific questions about observed relationships, ideas, problems, or
issues, make informed predictions, and/or formulate educated hypotheses to focus inquiries
or research
F3.4 describe the various factors that affect plant growth (e.g., growth regulators, sunlight,
water, nutrients, acidity, tropism)