Applying the Scientific Method:
Studying Plant Tropisms
NAME:_________________________________
DATE:__________________ PERIOD:______
Pre-Lab: you should complete ALL Sections through the procedure BEFORE the day of the lab!
*Define all italicized terms in the background information.
*you will be analyzing your data by calculating the percentage of corn seeds that germinated using
the formula below:
Number of corn seeds that germinated
X 100 =
% germination
Total number of corn seeds
Copy the formula into your lab book and solve the practice problem below.
Sally had 25 out of 40 seeds germinate, Sam had 12 out of 30 germinate. Calculate each students %
germination. Who was more successful at germinating their corn seeds?
Background Information: (Summarize in your own words your understanding of plant tropisms.)
When a seed is planted, two parts emerge from the seed. One part is the stem, or shoot, which
grows into a new plant, and the other is the root.
The movement of a part of a plant in response to environmental stimuli such as light, gravity, water
or touch is called a tropism. The growth of a plant stem toward a light stimulus is called
phototropism. The growth of a root downward toward the center of the Earth (gravity) is called
gravitropism. The growth of a plant toward water is called hydrotropism. A plant’s growth movement
in response to touch is called thigmotropism.
When a plant part responds to a stimulus by growing toward the stimulus, the response is known as
a positive tropism. When the plant part responds by growing away from the stimulus, the response
is known as a negative tropism. Thus, stems show a positive tropism to light and a negative tropism
to gravity. Roots, on the other hand, show a negative tropism to a light stimulus and a positive
tropism to gravity. In many tropisms certain plant hormones called auxins regulate plant growth and
help a plant respond to some stimuli.
Problem: How do corn seeds respond to certain environmental stimuli?
Hypothesis: (an “if-then” statement of your expectations)
Materials: (per group)
4 corn kernels
1 Petri dish
scissors
paper towels
marking pen
clear tape
water
Petri dish rack
Figure 1
Procedure: Sketch a diagram of the lab setup in your lab notebook and summarize the procedure
below.
1. Arrange four soaked corn seeds in a Petri dish as shown in “A” of Figure 1.
The pointed end of each seed should face the center of the Petri dish. One seed
should be placed at the 12 o’clock position, and the other seeds should be at the
3, 6, and 9 o’clock positions respectively.
2. Fold a piece of paper towel so that you have 4 layers. Use the bottom of the Petri
dish to trace a circle on the paper towels, then cut these circles out. Carefully
place the layers of paper towels over the corn kernels.
3. Wad the scraps of paper towels on top of the layers of paper towel so that the
corn will be held firmly in place when the other half of the Petri dish is put on.
See “B” of Figure 1.
4. Pour in enough water to completely soak the paper towel, but not so much that
water pools or drips from the Petri dish when placed on its side.
5. Cover the Petri dish and seal the two halves together with strips of masking tape.
6. With a marking pen, draw an arrow on the lid pointing toward 12 o’clock.
Label the lid with your class period and group number.
7. Place the Petri dish in the rack so that the arrow is pointing upward.
8. After 5 days, open the Petri dish and record your observations.
Results: Include at least three written observations. Neatly draw and label your results.
Calculations: What percentage of the corn seeds germinated? (Show your work!)
Source of Error: Identify and explain any unexpected results.
Analysis Questions:
1.
2.
3.
4.
5.
6.
7.
8.
How do you distinguish the roots from the stems?
In what direction did the roots tend to grow?
In what direction did the stems tend to grow?
Which part of the germinating corn seeds showed positive gravitropism?
Which part of the germinating corn seeds showed negative gravitropism?
What is the stimulus of the environment that causes roots and stems to germinate differently?
What other environmental stimuli might affect germination or growth of seeds?
When farmers plant large fields of crops, do all of the seeds have to be turned a certain way?
Explain your answer.
9. How is the plant’s positive gravitropism an advantage for its survival?
10. Which of the following drawings is accurate? Explain your answer using scientific terms.
Independent and Dependent Variables
A variable is a factor in an experiment that can change. In a controlled experiment, the experimenter
follows a procedure to test a hypothesis by collecting information under controlled conditions . That is, only
one variable is changed at a time. The condition that is deliberately changed by the experimenter is the
independent ( or manipulated) variable. The condition that is observed or measured in an experiment as a
result of this change is the dependent (or responding) variable.
11. Identify the independent variable in this experiment.
12. Identify the dependent variable in this experiment.
Conclusion: A summary of the lab procedure and results as well as how the results support or refute the
hypothesis. It should be written in the third person and must explain why the seeds germinate as they do.
Extension: What other tropisms could be investigated using similar materials? Briefly describe how
you would perform that experiment.