Table of Contents

Plants and Light

Teacher's Notes

This unit is an introduction to Phototropism.

Things to Emphasize

1. Almost all green plants are able to detect light, even when the source is exceptionally dim;

2. Most green plants will tend to grow in a direction that will provide them with the maximum exposure to light;

3. Phototropism is a highly evolved survival mechanism;

4. Science investigation involves a multi-step inquiry process: ask an initial question, plan the investigation, record observations and collect data, analyze data to draw a conclusion, and communicate the findings.

Experiment

Shoe Box Potato Maze

This experiment requires a sturdy shoe box with a tight fitting lid.

You will also need a sprouting potato planted in a tea cup. Use toothpicks to hold the potato approximately one centimetre above the bottom of the tea cup and add water.

Try not to spill or splash water in the shoe box since water will eventually cause the cardboard to disintegrate.

Transparency Master

Installing the light baffles

Open the box and install three light baffles as shown in the diagram to the left.

It is essential that these baffles be absolutely light-tight along all edges and against the lid when it is fitted. If the baffles are glued in place their edges should be light-sealed with a couple of layers of black duct tape.

Finally, cut a circular hole in the end of the box to admit light into the interior.

Transparency Master

Installing the cover

It is important to prevent light from leaking over the top of the baffles where they come in contact with the lid. Two methods have proved useful.

1. Cut a thin sheet of foam padding to fit exactly inside the lid.

2. Cut three or four thickness of heavy felt to fit exactly inside the lid.

Carefully install the lid and gently weight it down so that it compresses the padding against the baffles and edges of the box.

Transparency Master

Running the experiment

If the box is properly assembled, the only opportunity for light to enter the box is through the hole in the end of the box. Covering the box (leaving the hole exposed) with a black cloth provides additional protection from the effects of unwanted light entering the box.

Transparency Master

Observations

This response is known as a phototropic response. The effect is called phototropism .

Observation Blackline Master

 

Transparency Master

Explanation

The green leaves of some plants are very sensitive to light. These plants have evolved to spontaneously grow towards a source of light.

Activity

How They've Grown, Student Activity Master

Topics for Class Discussion

1. What are the survival advantages for plants which have evolved to become phototropic?

2. Plants grown indoors on window sills during the winter should be rotated every week or two so that they grow straight. Why is this?

3. Suppose you wanted to grow plants on Mars or in a spaceship. There are very few, and very small, windows in either a spaceship or a Mars habitat. What are some of the difficulties of growing plants in such places and how might these difficulties be overcome?