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Biology

Biology Lab: Phototropism

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Lab Exercise

Biology Lab: Plant Response

Goal: Determine rate of plant response to specific stimuli

LIGHT

Materials

Procedure

  1. Weigh all four houseplants with containers, if possible. (Otherwise, skip this step).
  2. Count the leaves on each plant.
  3. Placement:
    1. Put the first houseplant in a south-facing window which gets full sunlight much of the day.
    2. Put the second houseplant in an east or west-facing window which gets full sunlight less than half the day.
    3. Put the third houseplant in a north-facing window.
    4. Put the fourth houseplant in the middle of a room where it will get light from more than one direction, or in an interior room with indirect overhead lighting, preferably fluorescent.
  4. Arrange the leaves of each plant, if possible, to face up and outward around the plant somewhat evenly,
  5. If you can, take a picture of the plants; otherwise, draw a picture of each that shows the orientation of the leaves for each observation.
  6. For three days, count the leaves facing toward the current source of light at morning, midday (between noon and three), and evening. Do the leaves actually face directly toward the light, or to where the light was at some earlier point in the day?

Analysis

Create a table for your data that allows you to show trends and compare the behavior of the four plants.

Summarize your findings: did the plant leaves turn toward the light? How rapid was the response (first day? third day? was there a difference in response between morning and evening?

GRAVITY

Materials

Procedure

  1. Place 3 beans each on two pieces of filter paper. Lay the beans so that they are oriented the same direction on each paper (for kidney-shaped beans, put all the curved-in side to the left, for example).
  2. Lay a filter paper over the top of the seeds.
  3. Using staples, pins, or needle and thread, so around the seeds so that they cannot change position.
  4. Place one filter paper and seed “sandwich” inside each plastic bag.
  5. Add a tablespoon of water to the bag and make sure the filter paper touchs the water source and soaks up the water.
  6. Tape the sandwich bags to a window. Mostly close the bag to keep as much moisture in as possible, but leave a small hole open at the top.
  7. Allow the seeds to sprout. Check the water daily and make sure that the filter paper remains damp. Add the same amount of water to each bag.
  8. When all the root sprouts and new leaf sprouts reach at least 1/2 inch in length, take down all bags.
  9. Note the date and time.
  10. Lightly sketch with a crayon or pencil the position of the seeds, roots, and stems on the top sheet of filter paper.
  11. Put one bag back on the window in its original position.
  12. Take the filter sandwich out of the second bag and rotate it 90 degrees, and put it back into the plastic bag. It should now be oriented so that the roots point to the left and the stems to the right. Retape the bag on the window.
  13. Take the filter sandwich out of the third bag and rotate it 180 degrees, and put it back into its plastic bag. The plants should now be oriented so that the roots point up and the stems down. Retape the bag on the window.
  14. Let 3-5 days pass. Observe plant stem and root orientation three times a day (morning, noon, evening)
  15. Compare the position of the roots and stem now with the original position from step 10.

Analysis

Have the roots and stems of your plants changed direction between step 10 and step 16? How long did it take for the roots and stems to change directions, if they did? Was there any variation due to time of day?

Report

In your report, discuss whether sunlight and gravity can affect the direction of root and leaf stem growth. Justify the objectivity of your observations even though you did not make specific measurements.

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