Materials Included In Kit

Additional Materials Required

Safety Precautions

This activity is considered non-hazardous. However, all normal laboratory safety guidelines should be followed. Safety goggles should be worn. Always remind students wash hands thoroughly with soap and water after working in the laboratory.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All materials can be disposed of in the trash according to Flinn Suggested Disposal Method #26a.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This activity can be introduced and set up in one class period. Brief observation times should be scheduled on successive days until final data are collected.

• Remind students how to label their dishes. They should place their initials, date and treatment on the cover.
• Students may wish to collect and try “fresh” pine needles in your area.
• This is an excellent activity for introducing hypothesis formation and formulation of if/then type logic.
• This activity can be easily extended into a student “research-type” project. Many variables can easily be tested and controlled with simple equipment and space. Many variables can affect seed germination and seedling growth. Each variable can be isolated and serve as a basis for another simple experiment.
• A “relevant” variable to consider is acid rain. What might acid rain be doing to the chemicals found in pine needles under trees? Can dilute acid solutions remove more toxic chemicals from pine needles than just plain water?
• This is also a great activity to teach about controls and their importance.

Answers to Questions

___X____ Ryan’s Hypothesis Tested

1. Why was a Petri dish with just seeds and water included in this experiment?

The Petri dish with just seeds and water was used as a control.

2. If ___Ryan___’s hypothesis is TRUE, then what should be observed in the two Petri dishes in this experiment?

The grass seed covered by the pine needles will not grow.

3. If ___Ryan___’s hypothesis is FALSE, then what should be observed in the two Petri dishes in this experiment?

The grass seeds covered by the pine needles will grow.

4. Describe the results of the experiment.

Almost all of the seeds in the Petri dish with just water germinated. About half of the seeds in the Petri dish with pine needles germinated. The results may vary.

4. What is the status of ___Ryan___’s hypothesis based upon your results?

Ryan’s hypothesis may hold some truth, but it does not explain why all of the grass under the pine tree is not growing.

6. Answer if Ryan’s hypothesis was tested—how well did this experiment test Ryan’s hypothesis? Could you test it better? Describe how. (Hint: Do all pine trees have the same thickness of pine needles under them? Does light get through the pine needles to the grass seed? If the temperature of the ground a factor?)

Answers will vary.

___X____ Emily’s Hypothesis Tested

1. Why was a Petri dish with just water included in this experiment?

The Petri dish with just water was used as a control.

2. If ___Emily___’s hypothesis is TRUE, then what should be observed in the two Petri dishes in this experiment?

The grass seed in the Petri dish containing the “pine needle juice” will not grow.

3. If ___Emily___’s hypothesis is FALSE, then what should be observed in the two Petri dishes in this experiment?

The grass seed in the Petri dish containing the “pine needle juice” will grow.

4. Describe the results of the experiment.

Almost all of the seeds in the Petri dish with just water germinated. Only a couple of the grass seeds in the Petri dish containing “pine needle juice” germinated. The results may vary.

5. What is the status of ___Emily___’s hypothesis based upon your results?

Emily’s hypothesis was correct. Only a couple of the grass seeds germinated in the Petri dish containing the “pine needle juice.”

6. Answer if Emily’s hypothesis was tested—how well did this experiment test Emily’s hypothesis? Could you test it better? Describe how. (Hint: Is the water that reaches grass seeds under a pine tree boiled? Is tap water the same as rain water?)

Answers will vary.

Discussion

The boiled “pine needle juice solution” will inhibit grass seed germination completely or nearly complete. Pine trees contain polyphenols and monoterpenes, both of which have been connected with the inhibition of growth in some plants. By “boiling” the pine needles in tap water, these chemicals are extracted and enter the tap water solution. The pine needle bed on top of the seed will likely be inhibitory to the grass seed growth, but depending upon its depth, it may or may not completely inhibit seed growth. The amount of water in the dish and the condition and thickness of the pine needles will affect the level of inhibition. Emily’s hypothesis would seem to have a valid foundation based upon typical results from this experiment.

Introduction

Emily: “Hey, look, there’s no grass growing under that pine tree.”

Ryan: “Yeah, you’re right. That’s because the pine needles on the ground are so thick the grass sprouts can’t push up through them. They can’t survive under all the needles.”

Emily: “Really? I heard that when it rains, chemicals soak out of the needles and kill the grass seeds. Then the seeds won’t sprout.”

Concepts

• Scientific method

• Experimental data
• Hypothesis testing

Background

Who is right? When scientists are faced with questions like Ryan’s and Emily’s, they find answers using the scientific method. The scientific method is a way of solving problems using a systematic approach. A hypothesis is formulated based on a series of observations, and then the hypothesis is tested by means of experiments designed to either support or invalidate the hypothesis. Based on experimental evidence, a theory is proposed to account for initial observations. If the theory lacks the ability to explain further observations, a new hypothesis may be developed and tested, leading to a refined theory. In a sense, scientific knowledge is continually changing and becoming more reliable as we gather more information, perform tests and improve a hypothesis.

An organized strategy, such as the scientific method, is an effective way of approaching a problem. One such strategy is described in the following list of “typical” steps that scientists may use to solve a problem. Keep in mind, however, that the strategy and the order of steps may vary greatly from problem to problem.

Typical steps in the scientific method

• Define a problem or ask a question—A clear statement of the problem or question to be studied is a crucial step in beginning an investigation.
• Make observations about the problem—Gathering as much relevant information as possible about the problem is helpful in writing a plausible hypothesis and in designing a good experiment.
• Develop a hypothesis—This is a possible answer or tentative explanation to the problem or question. It should be based on the facts and observations and should be capable of being tested.
• Design and implement an experiment—Experimental testing will provide evidence which either supports or contradicts the hypothesis.

• Record and analyze data—Data, such as observations and measurements, are recorded and then analyzed. If the data support the hypothesis, then the conclusion would state that the hypothesis is correct. If the data contradict the hypothesis, then a new hypothesis must be made and tested.
• Draw a conclusion—Scientists base their conclusions on observations made during experimentation. When a hypothesis has been tested many times and has proven to be correct, it may become a theory. However, a theory is not a law. A theory usually leads to new predictions which must be tested by additional experiments.

Materials

Safety Precautions

Student should wear safety goggles when working in the laboratory. Follow all laboratory safety precautions. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Complete a preliminary experiment designed to test either Ryan’s or Emily’s hypothesis by following the test procedure outlined for each hypothesis.
2. Observe and record the experimental results.
3. Create and complete an appropriate worksheet for your experiment.
4. (Optional) Refine the experiment or design one of your own to further clarify why grass does not grow under pine trees.

Ryan’s Hypothesis

1. Place a piece of filter paper into the bottom of each Petri dish.
2. Pour a small amount of tap water into each dish. Wait approximately one minute for the water to soak into the filter paper. Pour off any excess water.
3. Sprinkle grass seed on top of the wet filter paper. Try to spread about 50–70 seeds evenly over the wet filter paper in each dish.
4. Break about a half-cup full of pine needles into small pieces. Cover the seed in only one dish with the pine needles. Be sure to cover the seeds with several layers of pine needles.
5. Do not cover the seeds in the second Petri dish.
6. Cover and label both Petri dishes and place them near a window, under a gro-light, in a greenhouse or in a place designated by your instructor.
7. It will take several days for the seeds to sprout. Check the grass seed every day and record all observations. Add tap water as needed.
8. Complete Questions 1–3 of The Grass Won’t Grow Worksheet. The remainder of the questions may be answered after data are collected.

Emily’s Hypothesis

1. Break about a half-cup full of pine needles into small pieces and place them in a 100- or 250-mL borosilicate glass beaker.
2. Add about 50 mL of tap water to the beaker.
3. Using a hot plate, Bunsen burner or microwave, heat the water in the beaker until it boils. Let the water boil for about five minutes.
4. Remove the beaker from the hot plate and let it cool to room temperature.
5. Place a piece of filter paper into the bottom of the two Petri dishes.
6. Pour some tap water into one of the dishes. Wait a minute for the water to soak into the filter paper. Pour off any excess water.
7. Sprinkle grass seed on top of the wet filter paper. Try to spread 50–70 seeds evenly over the wet filter paper.
8. Place the lid on the Petri dish, label the dish, and set it aside.
9. Pour the liquid from the boiled pine needles beaker into the second Petri dish. Wait a minute for the solution to soak into the filter paper. Pour off any excess liquid.
10. Sprinkle grass seed on top of the wet filter paper. Try to spread 50–70 seeds evenly over the wet filter paper.
11. Place the lid on the dish and label appropriately. Place the dishes near a window, under a gro-light, in a greenhouse or in a place designated by the instructor.
12. It will take several days for the seeds to sprout. Check the grass seed every day and record all observations. Add more tap water to dishes as necessary. Adding more pine needle water would increase the dose. Use only tap water.
13. Compete Questions 1–3 of The Grass Won’t Grow Worksheet. The remainder of the questions may be answered after data are collected.

Student Worksheet PDF

10574_Student1.pdf