TOPICS:  Plant reproduction, seed structure and germination

LEVEL:  Middle school and high school

TIME:  10-15 minutes on Friday to count out seeds; a few minutes Monday-Friday for data collection; 1 class period to complete the activity.

ADVANCE PREPARATION:  The teacher must collect seeds from mullein and several other plant species in the fall, after they are mature but before they have been exposed to low temperatures.  Keep half the seeds from each species in dry storage at room temperature.  Store the rest of the seeds outside exposed to winter weather.  The seeds can be left in the flower heads, put in a mesh bag and hung from a support such as a tree or post.  This will insure an adequate supply of seeds when needed .

MATERIALS: Mullein seeds are very small, but very abundant; they show a high germination percent, and delayed germination if not subjected to cold. Larger seeds (dock, milkweed, wild carrot, evening primrose) are easier for students to handle, but may not yield clear differences.  Apple seeds, while not weeds, work well but may be cumbersome to collect .

PREDICTIONS: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students: Some students may predict that there will be no difference in the number of seeds that germinate because seeds are unaffected by winter weather, while others may choose the indoor seeds because they are protected.  Still others may correctly predict that the outdoor seeds will show the greatest germination since weed seeds are naturally exposed to winter weather before they germinate in the spring.  The time required to begin germination is conjecture at this point, but it alerts students to look for differences as the experiment progresses .

STEP A:Prepare labels for the petri dishes like one at the right. Label them Mullein Inside, Mullein Outside and Inside and Outside plus the seed for the other dishes .

STEP E: Students need not record more than one data set per species.  For older students, you may want to add all the seeds of a given species that germinated on a particular day and calculate a class average.

QUESTIONS 6-11 require higher order thinking skills.  We recommend that you discuss these questions in class before students write out their answers .

3. The seeds kept outside should show the highest percent germination, although the seeds kept inside may show some germination.  This is usually true for each of the tested species .

4. The seeds kept outside should germinate before those kept inside by about two days.  This is usually true for these species .

5. Answers will vary among students .

6. The experimental variable in this experiment was exposure of the seeds to winter weather .

7. You must compare at least two groups exposed to different environmental conditions to determine if the experimental variable (winter weather) had an effect on germination .

8. Some students may correctly say that the inside seed group is the experimental group because it is the group with the manipulated temperature conditions while the outside seeds are exposed to natural conditions.  Others may argue that the experimental group is the outside group because the temperature changes, while the temperature of the inside group (control) is kept constant. Either answer is acceptable if supported by a reasonable explanation .

9. Seeds will break dormancy and begin germination under the "right conditions" which include moisture, oxygen, warm temperature, and in some species, exposure to a period of cold .

10. The seeds of most wild seed in northern areas do not germinate in the warm weather of autumn because they need exposure to cold temperatures before they can germinate .

11. To improve this experiment, you could increase the number of seeds in each petri dish, increase the number of petri dishes, or combine data collected from other classes.