Lesson from the Garden: Phototropism

What is Phototropism?

Tropism is a growth response between a plant and an external stimulus. The stimulus could be weather, touch, time, gravity or light. A positive response is indicated by growth toward a stimulus and a negative response is indicated by growth away from the stimulus. Light is a stimulus that plants respond to. This is called phototropism (photo= light).

Plants and Light

Plants usually display a positive phototropic response to light, which means they grow toward a light source. Plant hormones called auxins play a part in phototropism. Auxin is a plant growth hormone. When light is shined on one side of a plant the auxins move to the dark side of the plant. The hormones stimulate the cells on the dark side of the plant to elongate, while the cells on the light side of the plant remain the same. This elongation on one side and staying the same on the other causes the plant to bend in the direction of the light. This bending allows more light to reach more cells on the plant that are responsible for conducting photosynthesis. Wow!

Bend a Plant

You can create an experiment to see phototropism in action!

To see the effects of phototropism on a plant, make your own shoe-box maze.  You will need: a shoebox, extra cardboard, scissors, tape, and a small potted plan. Bean plants work well. Students should have an adult help with the following cutting and box building steps.

  1. Cut a large hole at one end of the shoebox. Hold the box up to the light and be sure to tape up any other spaces where light shines through.
  2. Cut two pieces of cardboard in the following sizes:
  3. First make both pieces half the width of the shoebox.
  4. Then make both pieces the same height of the shoebox.
  5. Now divide the box in thirds and tape one cut cardboard piece on the left side of the box at the one-third mark. Next, tape the other cardboard piece on the right side of the box at the two-thirds mark.

The box should look like the box shown below.

 

Place the small potted plant in the shoebox opposite the hole, make sure that it is well watered. (We started a bean in a plastic cup.)

 

 

 

Close the box, tape it, and place it in a sunny window.

 

In about 4 or 5 days open the box and notice how the plant grows in the direction of the light coming from the hole!!!

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