Why Do Seed Pods Burst Open?
Many seed pods split and twist when they dry. The two halves of the pod wall have fibres running in different directions, so they shrink by different amounts. In this worksheet students measure the curl of dried pods and build a paper model that bends the same way.
What happens
When a pod dries, the inner layer shrinks across its width while the outer layer shrinks along its length. The mismatch turns each half into a coil and throws the seeds up to several metres.
Activity
Glue two strips of paper with the grain at 90° to each other. Wet them, let them dry for 20 minutes and record the angle of the curl. Repeat with 45° and 0°.
Where it is used
Engineers use the same two-layer principle for shades that open with humidity and for small actuators that need no motor.
Introduction
- Level
- Middle school
- Time
- 2 hours
- Materials
- Dried pea or bean pods
- Printer paper
- Glue stick
- Spray bottle
- Protractor