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Mr. Cogbill’s Seventh Grade Science Class

Seed Germination: Experiment


Seed germination and seedling growth are influenced by many factors. Some of these are physical factors of the environment in which the seed exists, including climate (i.e., temperature, light quality and intensity, and moisture availability), soil factors (fertilizer or nutrient levels, acidity [pH], and salinity) and other physical stimulants or inhibitors (i.e., fire, freezing, or abrasion of the seed coat). Other factors which may influence seed germination are a result of the biotic (living) environment surrounding the seed. These include competition with other organisms for water, space, and light, as well as allelopathy, the inhibition of plant growth by chemicals produced by another species of plant.

At the beginning of this school year you were introduced to the basic experimental approach to problem-solving in the sciences (review the Scientific Method in your notes if necessary). This lab exercise involves designing, carrying out, and writing up the results of a simple experiment on seed germination. Think about and then select a physical or biotic factor that you will vary during the experiment. Some factors that are easily varied include pH (see the discussion that follows), salinity, temperature, light/dark cycles, chemical stimulants or inhibitors, or exposure to freezing or scorching conditions. Use your curiosity and creativity, and remember the best experiment is simple and straightforward, one that provides easily quantified results.

The pH Scale

In any given quantity of water, some water molecules will dissociate into hydrogen and hydroxyl ions:    H20          H+ + OH-. .   In pure water, the number of hydrogen ions (H+) is equal to the number of hydroxyl ions (OH-), and we say the water is neutral. Any substance that dissolves in water and increases the concentration of hydrogen ions is said to be acidic. A substance that dissolves in water and increases the concentration of hydroxyl ions is basic or alkaline. The pH scale, which has a range of 0-14, is based on the H+ concentration. The pH of pure water is 7; values lower than 7 are acidic and those greater than 7 are basic. Most natural waters have a pH range of  7 to 9.

Furthermore, the pH scale is logarithmic; a solution with a pH of 5, for example, is 10 times more acidic than a solution with a pH of 6. This information is useful when it comes to making solutions of varying pH. For example, if you had a starting solution of pH 2 (e.g., lemon juice) and wanted to make a solution of pH 3, you would  mix 1 part of lemon juice with 9 parts of water. If you wanted a solution of pH 4, you would then take 1 part of the pH 3 solution, and add 9 parts of water. (This technique is called serial dilution.)

Mature plants, as well as their seedlings, typically have specific tolerances for the range of pH in which they will grow. Be sure to look at the poster in the laboratory that shows the pH values of some common household items. A miniature copy is included here.


Supplies and Equipment

You will be provided with plastic ziplock baggies, paper towels, and seeds to conduct your experiment. The other supplies and equipment which are commonly used in carrying out a simple seed germination experiment are usually found in most homes or kitchens, or can be inexpensively purchased. For example, lemon juice, vinegar, or household ammonia can be used to make dilutions of varying pH. Table salt can be used to mix solutions of varying salinity. A dark cabinet is useful for varying the light/dark cycle to which seeds are exposed. Seeds can be placed in a freezer or microwave for varying periods of time to test the influence of freezing or scorching, respectively. Household spices (e.g., pepper, chili powder, paprika), flavorings (e.g., vanilla or mint extract), home-made extracts of plant debris, or liquid fertilizer are useful in examining allelopathic (inhibitory) or stimulatory effects. [Homemade extracts of plant debris can be made by blending a measured weight of plant matter (e.g., leaves, stems, and/or roots) in a measured volume of water, and then straining the resulting solution through a coffee filter or cheesecloth.]

A good general design, one which can be reasonably managed by a busy student, is to have 3 treatments and a control. For example, if you were interested in the effects of pH on corn germination, you might soak one paper towel in lemon juice (pH 2), a second towel with a 1:9 dilution of lemon juice (= pH 3), and a third towel with a 1:9 dilution of the pH 3 solution  (= pH 4). Your control would be corn seeds placed inside a paper towel soaked in water. (Note in this example that you would NOT soak the paper towels for the 3 treatments in solutions of varying pH prepared from different substances, e.g., lemon juice, vinegar, and tomato juice. Why? Because you have not only varied the pH, but you have also varied other chemical components.) You wish to manipulate only one variable (in this case, pH) and keep the other components of the seeds’ environment constant.

Designing an Experiment

The design of your experiment involves 5 steps:

STEP 1:  Decide what problem you want to investigate. Some examples of experimental topics include “Effects of Temperature on Kidney Bean (Phaseolus vulgaris) Germination”; “Influence of Eucalyptus Extract on Seedling Growth in Mung Beans (Vigna radiata)”; “Simulated Acid Rain Effects on Germination in Corn (Zea mays)”; and “Effect of Fire Exposure on Lima Bean (Phaseolus lunatus) Germination”.  Be sure to include the common and scientific name of the type of seed you will use as your experimental subject.



 STEP 2:  Write down the hypothesis that you will test. For example, if your topic were “Simulated Acid Rain Effects on Germination in Corn (Zea mays)”, you might hypothesize that increasing acidity (relative to the acidity of tap water) inhibits growth of the radicle.




STEP 3:  Decide on the details of your procedure:

1.  Describe the experimental treatments for your experiment. Using the simulated acid rain example, you might soak one paper towel for germinating corn seeds with pH 6 water, a second towel with pH 5 water, and a third towel with pH 4 water. (Ask your parents or Teacher if you need help in how to mix solutions.)






2. Describe the control for your experiment. In our simulated acid rain example, a paper towel soaked with tap water (pH @ 7) for germinating corn seeds would serve as a control.






3. What will be the sample size (number of seeds) for each treatment and control?



4. List the steps involved in setting up and carrying out your experiment (your methods) and list all types and amounts of seeds, supplies, equipment, and solutions you are using (your materials). Be specific and descriptive so your Teacher can spot potential pitfalls and make suggestions.







5. State the type of measurements or observations you expect to make in your research. Will you be counting number of seeds germinated, measuring lengths of radicles, heights of shoots, weight of new growth, number of days until germination begins, or what? How often will you record observations? We suggest you conduct your experiment for 7 days. These measurements and observations will be your experimental data.






STEP 4. Make a prediction.

Write down your predicted results IF your hypothesis is correct. Using our simulated acid rain example, you would expect that the greater the water acidity to which corn seeds are exposed, the less is the mean length of the germinated radicle.
Now, construct the table in the space below that you will use to record your measurements and observations during your experiment.  Be sure to include all parts of a table that are pertinent for your observations and measurements.





Check it Out

STEP 5:  When you have finished the design of your experiment, have your Teacher check it over. When your Teacher says “OK,” then and only then start your experiment. Obtain the baggies, seeds, and paper towels you will need in order to conduct your experiment at home.

Writing Up Your Experiment

An experiment is not completed until the results have been property analyzed and reported.  Only then does it become useful information for other scientists to build upon. In writing up your experiment, an investigator would use the standard format of a scientific paper (Title & Author, Abstract, Introduction, Materials & Methods, Results, Discussion, Acknowledgements, References) that you learned about in the first part of this year. Because much of this information is already contained in your experimental design, however,  we will simplify the completion of your experiment by having you respond to the following:

STEP 6: Summarize your results, computing means for the quantitative variable you were measuring. On a separate sheet(s) of paper, use a table to summarize this quantitative data, and a graph to visually display your summarized data. Be sure your table and graph are properly labeled. Then, briefly describe your results in several sentences, including any important or unusual observations you may have noticed.

For example, if you were measuring radicle length in a simulated acid rain experiment with 3 different pH treatments, you would compute the mean and standard deviation for each of the treatments as well as for the control, and then present this information in the form of a table. You would also construct a bar graph showing the means of the control and 3 treatments. A verbal description might read something like, “the mean radicle length of all treatments and controls after 7 days were approximately the same (Figure 1). However, the controls showed substantial elongation of the shoot, while none of the treatments showed much shoot development.”



STEP 7: Discuss your results, by responding to the following questions:

a. Did your results support your original hypothesis? That is, did your actual results agree with the results you had predicted if your hypothesis were correct? (Be sure to restate your hypothesis when responding to this question; don’t simple answer “yes” or “no”.)




b. Briefly discuss why your treatment may have caused the results that it did. For example, a partial explanation of the results of our acid rain simulation experiment might include, “The enzymes that regulate shoot development might not function properly within the acidic range pH 4 to pH 6”.





c. What possible sources of error in your experiment might have affected your results?




d. If you were to conduct a similar experiment again, would you do anything differently? If so, what would you change?




e. New questions are usually raised in an investigator’s mind while conducting experiments and making observations. What new questions occurred to you while you were conducting this experiment?



Grading Structure for Seed Germination Experiment

Because your seed germination experiment reflects a substantial amount of work, your write-up will be worth TWICE as much as a normal lab summary. That is, when averaging your scores for the quarter, you will add in the value of this exercise two times, AS IF it were two separate summaries.