TEACHER GUIDE

TOPICS:  Plant ecology, physiology and reproduction; seed structure, dormancy and germination.

LEVEL:  High school and advanced placement

TIME:  30 min to introduce activity and set up experiment.  10-week chilling period to stratify the seeds.  A few minutes per day to count germinated seeds over one week.  One class period to analyze the data and complete the activity.

ADVANCE PREPARATION:  Collect seed samples before they are subjected to the low temperatures of autumn nights.  Any temperate zone species can be tested; see Dormancy and Seed Germination Teacher Background for suggestions.  The seeds should be stored dry at room temperature until used .

PREDICTION: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students:  In northern regions with long cold winters, native plant seeds may require chilling periods of one or two months for maximum germination.  In areas with mild winters, native species may germinate well with shorter chilling periods, or be more sensitive to light or moisture conditions than temperature .

PROCEDURE: The time required to begin germination and the chilling period needed for maximum germination will vary with the plant species selected.  Since most temperate zone plants for which data are available require 5-10 weeks of chilling at 1-5^oC, the recommended stratification periods (0-10 weeks) should supply reliable data .

STEP B. Prepare labels for the petri dishes like the one at the right. Make one each for 0, 2, 4, 6, 8, and 10 weeks .

STEPS E-F. Check refrigerated dishes every two weeks to make sure moist conditions are maintained in each during the chilling period

STEP K. These results were obtained using mullein (Verbascum thapsis).  Maximum germination occurred at 8 weeks of stratification.  A chilling period of 4 weeks or more was sufficient to produce good germination.

QUESTIONS 5-9 require higher order thinking skills.  We recommend that you discuss these questions in class before having students write out their answers .

2. Answers will vary.  Many temperate zone plants require an extended period of chilling (6-10 weeks) in order to break dormancy and achieve maximum percent germination.  Among these, the lowest percent germination should occur among the unchilled seeds .

3. Answers will vary, depending on species tested .

4. For species requiring a chilling period, seeds may begin germination earlier as the stratification period increases .

5. In addition to water, oxygen, and proper temperature for germination, temperature zone plants may require a chilling period to break dormancy .

6. A chilling period requirement may be a survival advantage by ensuring that the seeds will germinate in spring, and not during the warm days of autumn.  Decreased germination time may give those seeds a competitive edge in establishing a growing space .

7. The seeds may have been dead before the experiment began due to a variety of reasons including genetic disfunction, environmental stress, and chance.  The seeds may require other environmental conditions to break dormancy (light, physical or chemical action on seed coat, or dryness for example) .

8. Arctic plants probably require a longer exposure to periods of cold before germination .

9. Tropical plants do not require exposure to cold in order to germinate.  In fact, germination rates might decrease with cold stratification .

10. Most crop seeds will germinate without exposure to cold.  A criteria for development of most crop plants is maximum germination under a wide variety of environmental conditions.