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OCR for page 225
APPENDIX H-2
Example of A Traditional
Laboratory Exercise
LAB #2: THE EFFECT OF pH ON THE GROWTH OF DUCKWEED
NAME
INTRODUCTION
PER
Duckweed is a common aquatic plant that can be grown in the lab in a simple
nutrient culture medium. The growth of these plants occurs by mitosis and can
be quantified by counting the number of new buds found on the plant after a
period of time. The growth of a population of plants establishing itself in a new
area typically follows an S-shaped curve. This curve consists of three phases:
1. A period of slow growth, the lag phase, followed by;
2. Exponential growth and finally;
3. An equilibrium phase where the population levels off.
Time
225
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226
PROFESSIONAL DEVELOPMENT OF SCIENCE TEACHERS
Biotic and abiotic factors influence the growth of individuals and populations.
The pH of an aquatic environment is an abiotic factor which dramatically influ-
ences its inhabitants and their growth. In this experiment, you will determine the
optimal pH for the growth of duckweed, along with the effects of sub-optimal pH
values on the growth of duckweed.
MATERIALS
50 ml graduated cylinder
6 containers of culture media each with a specific pH (4, 5, 6, 7, 8, 9)
6 petri dishes
duckweed
marking pens
inoculation loops
PROCEDURE
1. Take 6 sterile petri dishes and label each with your initials and one pH
value starting with 4 for the first dish, continuing until the last dish is labeled 9.
Label on the bottom or side of the dish.
2. To the petri dishes add 25 ml of the culture media (decholorinated tap
water plus liquid fertilizer) at the given pH.
3. With an inoculating loop transfer 15 duckweed plant lobes to each of the
petri dishes. Record the total number of lobes per petri dish. Each lobe is actually
a plant, although you often consider a three-lobed structure a plant. For statistical
tests, it is IMPERATIVE that each group start with as close to 15 lobes as
possible.
4. Place your 6 labeled petri dishes on the counter under the lights and allow
them to remain undisturbed for one week or more, depending upon the success of
the experiment.
5. Observe the plants in the petri dishes each day and record the number of
pH
DAY 1
DAY 2
DAY 3
DAY 4
DAY 5
DAT 6
DAY 7
DAY 8
DAY 9
4 5 6
~7 8 9
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A TRADITIONAL LABORATORY EXERCISE
DAY 10
DAY 1 1
DAY 12
DAY 1 3
DAY 14
227
lobes in each dish. Count weekends as one day. Do this up until 14 class days
and record data below:
6. With all the growth data for each pH as one population, do an ANOVA F
test on your data to see if each group is similar. Your Null Hypothesis here would
be that all the populations of duckweed grew at the same rate despite the pH.
F test value =
Degrees of freedom for numerator =
Degrees of freedom for denominator
Critical F value on the ANOVA chart
Do you reject of fail to reject your Null Hypothesis?
Explain in complete sentences:
7. Collect class data on the following charts so you can compare growth per
pH for all of the 8 groups, using the statistical ANOVA F test.
Representative terms from entire chapter:
culture media
