Materials Included In Kit

Additional Materials Required

Safety Precautions

This activity is relatively nonhazardous. Follow normal laboratory procedures and wear chemical splash goggles and chemical-resistant gloves where appropriate. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Materials may be disposed of following Flinn Suggested Disposal Method #26a.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs. The initial experiment can be setup easily during one class period. Observations will require only 5–10 minutes/week after the initial setup. Your curriculum goals, time and equipment will dictate the amount of time dedicated to Part II of the exercise.
• This activity can be done with minimal preparation. Results should be good and predictable.
• Keep the Petri dishes in a well-lighted area but not direct sunlight. Use gro-lights if bright natural light is not available.
• Pooling class data for Part I of the activity is much more likely to yield the expected growth curves when they are graphed.

Teacher Tips

• Let your students invent ways to increase the rate of duckweed growth. Anticipate obvious variables and have materials available for use. Light, heat, fertilizer, soil and aeration are just a few of the likely variables students will suggest.

• Raise the issue of number of individuals versus the biomass being produced by the duckweeds. How much biomass can a Petri dish support? Would removing some individuals produce more biomass? Can selective harvesting increase production?
• If some groups are unsuccessful after several weeks, give them a new duckweed culture and have them start again and run the experiment for a shorter period of time.
• You can maintain a colony of duckweed indefinitely. Grow them in a large aerated tank and thin the population regularly.

Sample Data

The data presented here serve only to illustrate the possible trend in the experiment. Actual class data could vary considerably from these numbers. The number of fronds that are produced may be less depending upon the growing conditions.

Graph Results

Answers to Questions

1. Describe the trend of the population growth of duckweed.
   

   The growth will usually follow the classic s-shaped population curve. A rapid increase early in the growth pattern is followed by a slower rate later in the population history.
   

2. Compare the rate of growth of duckweed during weeks 0–4 compared to weeks 9–12. What might account for any differences?
   

   The slower rate in the later weeks might be due to crowding and limited resources like minerals, light and space.

Discussion

Alignment with AP^® Environmental Science Topics and Scoring Components

Topic: Population. Population Biology Concepts (Population ecology; carrying capacity; reproductive strategies; survivorship).

Scoring Component: 4-Population, Population Ecology

Introduction

What affects the size of a population? How fast can a population grow? What limits the growth? Duckweed is an excellent organism for studying some of these questions.

Concepts

• Population growth rate

• Limiting factors

Background

Duckweed is an easily-grown, floating water plant found on the surface of lakes and ponds throughout the United States. There are four genera in Lemnaceae, the duckweed family. All four genera are minute, floating aquatic plants. One genus, Wolffia is about the size of a sand grain and is the smallest of the flowering plants. Other species range from 2 to 4 millimeters in size.
The oval or elongated, flattened bodies of duckweed are made up of leaf-like stems called fronds. The fronds float on the water’s surface while thread-like rootlets trail in the water (see Figure 1).

The following simplified key can help identify the duckweed in this laboratory:

1A. Plant with rootlets and several reproductive pockets or joints . . . . . Go to 2

1B. Plants without rootlets and only one reproductive pocket or joint . . . . . Go to 3

2A. Two or more circular joints up to 3.5 mm wide; purple on the underside of the joints; several rootlets from each joint . . . . . Spirodela

2B. Oval, circular or elongated-forked joints; 2–4 mm across; not purple on underside; only one rootlet from each joint . . . . . Lemna

3A. Plant composed of minute globules or subspherical, granular bodies; 2 mmor less in diameter . . . . . Wolffia

3B. Plant formed of one or more minute, elongated or flattened structures; 4–8 mm in diameter; often arranged in a star-shaped cluster . . . . . Wolffiella

Duckweed is widely distributed and found on the surface of bodies of quiet fresh water—ponds, swamps and slow-moving streams. Duckweed is eaten by ducks, geese, fish and some snails. In cold regions, clusters of fronds sink to the bottom of the body of water in the fall. In the spring, the plants float to the surface and begin reproducing. Duckweed grows best in well-lighted areas but not direct sunlight and can reproduce rapidly when conditions are ideal.

Even though duckweeds are among the smallest flowering plants, reproduction by flowering is rare in indoor conditions. Reproduction is usually by vegetative means. New fronds arise from growth regions—meristems—located in one or two pockets near the sides at one end of the parent frond. As new fronds grow, they may remain attached to the parent frond for a short time before separating. This growth pattern makes duckweed look like it is growing in clusters. Depending upon the nature of the body of water and its motion, duckweed can form large mats covering the entire surface of the body of water.

Materials

Safety Precautions

The materials used in this laboratory exercise are considered relatively nonhazardous. However, chemical splash goggles and chemical-resistant gloves should be worn. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part I. Duckweed Population Growth

1. Label a Petri dish appropriately with your name and date.
2. Place 15 mL of distilled water or spring water in the Petri dish.
3. Add one drop of fertilizer to the water in the Petri dish.
4. Add a pinch of potting soil to the water in the Petri dish.
5. Let the Petri dish sit undisturbed for several minutes to be sure the soil and fertilizer mix into the water.
6. Use a dissecting needle or other pointed object to carefully transfer three separate duckweed plants to the surface of the water in the Petri dish.
7. Count the number of total fronds making up the three duckweeds in your Petri dish. Record this number on the chart on the Duckweed Population Worksheet.
8. Place the top on the Petri dish and place it in a well-lighted area designated by your teacher.
9. Use only distilled or spring water to replace any lost water volume during the rest of the experiment.
10. Count each week’s data for the next 10–12 weeks. Try to count on the same day of each week.
11. Record your numbers each week in the data chart on the Duckweed Population Worksheet. Graph results and answer the Post-Lab Questions.

1. How might the growth rate of duckweed be increased? What might be done to get a “bumper crop” in a hurry? Form a research group as directed by your teacher. Brainstorm hypotheses and discuss variables to be tested. Review the list of possibilities. Discuss possible experiments that might test each hypothesis. Be sure to discuss any “controls.”
2. Pick the hypothesis that seems to be most viable and interesting to your group. Design an experiment to test the hypothesis. Write up the experimental design in detail and discuss it with your teacher.
3. Secure materials you need from your teacher.
4. Conduct your experiment. Create appropriate data tables and record the results from your experimental work.
5. Write a complete laboratory report for your experiment. Be sure to include the following:

1. State your hypothesis.
2. Describe the experimental design including any controls.
3. Document the results of your experiment.
4. Write an interpretation of the experimental results.
5. Do you reject or accept your hypothesis?
6. What further experiments are suggested by your results?

6. Conduct a class seminar and share all of the experimental designs and results.
7. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

10252_Student1.pdf