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Grant Wood Area Education Agency

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Experimenting with Plant

Experimenting with Plants: Lesson 14: Two Tropism Experiments (Days 12, 13, or 14)

Lesson Summary

This lesson and the next one offer students the opportunity to use all of their expertise in conducting two sets of experiments on an interesting phenomenon called tropism.

Teacher Background

Tropism is the growing or bending of a plant in response to an external source of stimulation.  This lesson considers two types of tropism only:  phototropism, which is the way a plant grows or bends in response to light, and geotropism, which is the way a plant grows or bends in response to gravity.  These are familiar occurrences in nature that we all witness but seldom think about very much.  Below are more detailed descriptions of these phenomena.

Phototropism
The geranium on the windowsill that becomes lopsided unless you remember to turn it once in a while is exhibiting phototropism.  Its leaves and stem have responded positively to light by turning toward the nearest and strongest source.

Geotropism
Generally, plants respond in two different ways to the pull of gravity:  the shoots go up and the roots go down.  Plants grow upward against gravity as well as downward toward gravity.

Set-up/Management Tips

  • This lesson assumes students can use protractors to measure angles. 
  • Lessons 14 and 15 are really hard concepts for students to grasp and the experiments are not always successful.  Depending on your class, you may opt not to do these lessons.
  • Duplicate Activity Sheet 9, Experiments in Plant Tropisms.
  • Make sure you have 4 or more quads of Brassica plants 12 to 14 days old.

Literacy Support

Scientific Vocabulary
The following words are key vocabulary words that will be introduced in this lesson and reinforced throughout the unit:

Tropism, phototropism, geotropism