PURPOSE:  To determine the effect of winter weather on seed germination.

MATERIALS (groups of 2 students): 2 petri dishes, dish labels, scissors, tape or glue, filter paper, water, forceps, seeds of mullein and other weed species, graph paper.

INTRODUCTION:  Seeds are produced by plants during the growing season.  They contain the young plant embryos that are inactive or dormant in winter.  Under the right conditions, the seeds germinate and begin to grow.  Most seeds require water, oxygen, and warm temperatures to germinate.  But have you ever wondered why most wild seeds do not germinate during the warm days of autumn?  Does exposure to the cold temperatures of winter affect the ability of seeds to germinate? 

In this laboratory activity you will set up a controlled experiment to answer this question.  You will try to germinate seeds that were gathered last fall and kept outside exposed to the cold, wind, rain and snow. For comparison, you will try to germinate the same kinds of seeds that were gathered in the fall but kept inside at room temperature.  The species you will test are common weeds found throughout most of northern North America.

PREDICTIONS: Before you start your experiment, make some predictions. 

1.  Do you think that there will be a difference in the number of seeds that germinate between the ones kept inside and those left outside?  Explain your ideas.

2.  Do you think there will be any differences in the time it takes for the seeds to germinate between the ones kept inside and the ones left outside?  Explain your ideas.

PROCEDURE:

A. On Friday, line two petri dishes with two pieces of filter paper cut to fit.  Moisten the paper with distilled water and leave a thin layer of water on the bottom of the dish. Cut out the labels and stick one label on the top of each petri dish using tape or glue.

B.  Your teacher will give you plant seeds that have been stored indoors and outdoors since autumn. Count out 25 seeds from each group onto the moistened filter paper in the petri dishes.  Write the name of the plant on the label and the storage temperature. Put both dishes out of direct sunlight at room temperature over the weekend.

C.  Starting on Monday, check to see how many of the seeds have germinated each day.  You will see a white "root" called the hypocotyl if the seed has germinated.  Record the number of germinated seeds on the top of each dish each day, Monday through Friday.  If none has germinated record 0.  Use the forceps to remove and discard the germinated seeds.  Make sure the filter paper is kept moist.

D.  On Friday, count and record the number of seeds that did not germinate in each dish.  Add up the total number of seeds (germinated and ungerminated).  Calculate the percent germination for each sample, using this formula: 

                            # seeds germinated
            % germination = ------------------ x 100
                               total # seeds

E. Fill out the class data sheet, sharing your seed data with others in your class.

F.  Construct a bar graph from your data with percent germination on the vertical axis, and the seed species across the horizontal axis.

G.  Optional: Construct a line graph from your data showing the number of seeds that germinated on each day for your plant species.

CLASS DATA ON SEED GERMINATION:

CONCLUSIONS:

3. According to your bar graph, which seeds (those kept inside or outside) had the highest percent germination?  Was this true for each of the tested species?

4. According to your data sheet or line graph, which seeds (those kept inside or outside) germinated first?  Was this true for each of the tested species?

5. How do these results compare to your predictions?

DISCUSSION:

6. What was the experimental variable in this experiment?

7. Why was it necessary to test seeds kept inside as well as outside to determine the effect of winter weather on germination?

8. Which seed group do you think was the experimental group?  Which group was the control?  Explain your ideas.

9. What conditions are necessary for seed germination?

10.Why do most wild seeds fail to germinate in warm autumn weather?

11.Your conclusions are based on a sample size of 25 seeds per petri dish.  What could you do to improve this experiment so that you would have more confidence in your conclusions about each species?