Plants on the Move! Experiments with Phototropism

Important Notes Before You Begin

1. Because of the natural variability in plant growth, you may need to adjust this procedure in order to get good results. For example, you may need to expose the plants to light for more or less time than is recommended in the procedure. Adjust the procedure to find the conditions that work best for you.
2. The objective of this science fair project is to expose the plants to different amounts of light, with the light coming from the side, so that the plants will bend toward it. To begin, you'll label the milk cartons 1–5, add soil to them, and then plant four seeds in each carton. Note: The plants need to grow for a period of time before being exposed to light. This way, the stems will be growing vertically when they are exposed to light from one side.
3. Ultimately, you will need three trials of this project to make sure your results are consistent. You can either run them one at a time, or run them concurrently.

Preparing the Cartons

1. Using the permanent marker, label the sides of the milk cartons with the numbers 1–5.
2. For carton # 1, cut off the top of the carton with the scissors.
   1. The plants in this carton will grow in normal lighting.
3. For carton # 2, do not make any holes.
   1. The plants in this carton should germinate and start to grow, but will be kept in total darkness.
4. For the following cartons, use the hole punch to make holes in one side of each carton, about 6 cm from the bottom. You will need to open the top of the milk carton to make the holes.
   1. Carton #3: One hole
   2. Carton #4: Two holes; should be near each other, but not overlapping
   3. Carton #5: Four holes; should be near each other, in a cluster
5. Be sure to write in your lab notebook the number of holes in each carton, and what each carton's conditions will be once the experiment begins (lighting).
6. Now make some additional holes to allow air into the cartons and excess water to drain out. Punch 10 holes in the bottom of each carton using a pen. Each hole should be about 5 mm in diameter.

Planting the Seeds

1. Put about 3 centimeters (cm) of moist potting soil into each carton. Keep the amount of soil equal in each carton.
2. Moisten the potting soil with water. It should just be moist, not saturated.
3. Put four seeds in each carton. Bury the seeds to a depth of about 1 cm, or follow the directions on the seed package for the recommended depth. Plant all of the seeds at the same depth.
4. Use the permanent marker to put a letter on the side of the carton near each seed so that you can identify them as they grow. For example, label them A, B, C and D, with markings inside the carton.
5. Close cartons 2–5 and seal them with clear tape.
6. Wrap the tops and sides of cartons 2–5 in aluminum foil to block light from reaching the plants. Don't cover the holes in the bottom.
   1. Write the number of the carton onto the aluminum foil with the permanent marker.
7. Record the time and date that the seeds were planted in your lab notebook.

Making Your Observations

1. Check carton #1 each day. Wait until the seeds in carton #1 have germinated and grown about 3 cm. Then begin the exposure to light for the plants in cartons 3–5 (remember, carton #2 stays sealed). Note: Start the exposure to light in the morning so that the plants have a full day of light on the first day.
2. Using the scissors, remove the foil only from over the hole in carton #3. Leave the rest of the foil in place to keep the plant in darkness, except for the light from the one hole.
3. Remove the foil only from over the holes in cartons #4 and #5. Leave the rest of the foil in place to keep the plant in darkness, except for the light from the holes.
4. Record the time and date that the plants were exposed to light in your lab notebook.
   1. Use the clear tape to hold the aluminum foil in place around the holes, if desired.
5. Place the cartons in a well-lit room, but out of direct sunlight. Record the time and date in your lab notebook
   1. As an option, you can use a grow lamp as a light source.
   2. Expose the plants to light during the day. Keep them in darkness at night.
   3. Record how long the plants receive light from the holes in the carton.
6. The next morning (day 1), take the all of the cartons to a dimly lit room, open the cartons, and observe the plants.
   1. Use the protractor to measure the angle of the stems. If the protractor is too big to fit into the cartons, use a twist tie to obtain the angle for the stem. Position a twist tie parallel to the base of the stem to be measured. Bend the twist tie to match the angle of the bend in the stem. Use the protractor to record the angle of the twist tie.
   2. Record your observations of the plants from all five cartons in your lab notebook.
   3. As an option, check the plants more often than once per day in the morning. Keep the light exposure to a minimum while taking measurements.
   4. Do not let the soil dry out. If needed, add some water (equal amounts) to each plant, using an eye dropper.
7. Return the cartons to their original states (covered or uncovered), place them in the light again, and allow them to grow for another day.
8. The next morning (day 2), again take the cartons to a dimly lit room, open the cartons and observe the plants. Repeat steps 6–7. Note: Remember to treat all of the plants exactly the same. The only difference between the plants in the different cartons should be the amount of light to which they are exposed.
9. Repeat steps 6–7 until you feel you have enough data. Five days worth of data will probably be sufficient, but use your judgment about when to stop the experiment.
10. For each day you collected data on the angles of the plants, graph the angle of the stems vs. the amount of light (the number of holes) to which the plants were exposed.
11. You can graph the data for each plant individually, or graph the average of the angles within a carton.
12. Repeat the entire procedure at least two more times so that you have a minimum of three trials. The different trials can run concurrently. Graph the data for each trial separately.
    • Note: The goal is to determine how exposure to different amounts of light affects the growth of the plants. You may need to experiment with the amount of light (the number of holes), the duration of exposure, the height of the holes, or other factors to get a clear result.