BioEd Online

Teacher Background

Only producers, such as green plants, are able to make the molecules needed for life from simple compounds in the air, soil, and water. Almost all producers use energy from the sun to make food through photosynthesis. During photosynthesis, light energy is trapped and transformed into chemical energy that can be used by cells. Very few raw materials are required. Green plants need only water (H₂O) and carbon dioxide (CO₂) in the presence of light to manufacture sugar molecules and other carbohydrates, such as starch. Plants use the energy held in carbohydrates to fuel chemical reactions and to make other molecules necessary for life. Other needed materials (such as nitrogen, phosphorous, or potassium) are taken in through plant roots. This activity allows students to learn about the needs of plants and the role of light in plant growth.

Growing plants in the classroom can be a simple and rewarding process for students. Elaborate equipment is not necessary for growing plants indoors. If you do not have a window with bright light, place plants under a fluorescent lamp. Allow only about five inches from the tops of the pots or growing plants to the light source. Inexpensive fluorescent lamps appropriate for growing plants often are sold in hardware stores as “shop lights.”

Objectives and Standards

Concepts

• Plants require light, water, air, and soil to grow.

• Light is necessary for the production of new plant material.

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Science, Health and Math Skills

• Observing

• Recording observations

• Measuring in centimeters

• Comparing measurements

• Graphing measurements

• Interpreting results

• Drawing conclusions

Materials and Setup

Materials per Student Group

• 12 soaked bean seeds

• 4 dry bean seeds

• 4 hand lenses (magnifiers)

• 4 peat pots (3-in. size) or disposable cups

• 4 pieces of string or yarn (each approx. 6-in. in length)

• 4 sheets of paper towels or paper plates

• 2 cups of moistened soil

• dispensing bottle (2-oz size) or dropper

• metric ruler

• copies of student sheets

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Setup

1. Soak enough bean seeds overnight in a container of water to give at least 12 soaked seeds to each group of students (4 seeds for observation). Each group also will need at least 4 dry bean seeds.

2. Moisten the soil before use. Place the soil in a plastic bag or container and add water until it is damp. Let the moistened soil in the unsealed bag sit for at least 1/2 hour before using.

3. As an alternative to peat pots, use disposable plastic or foam cups (punch one or more drainage holes in the bottom of each cup). Once students have planted their seeds, set the pots on plastic or foil trays near a light source.

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Safety

Have students wash hands before and after the activity. Clean work areas with disinfectant.

Procedure and Extensions

Time

30 minutes for each session; 10–20 minutes each day for 1–2 weeks

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Session 1: Observing dry seeds

Give each student a dry bean and a magnifier. Have students use their magnifiers to observe the bean seeds, then draw a seed on his or her “Seeds and Seedlings” sheet. Make sure that each student is able to observe the seed coat and the dark indentation on one side of the seed, corresponding to where the new plant will emerge.

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Session 2: Observing and planting soaked seeds

1. Before proceeding with planting, give each student a soaked seed (on a paper towel) for observation. The students should compare the soaked seed to a dry seed. Ask, How is the soaked seed similar to the dry seed? How is it different? Have students remove the “skin” (seed coat) and spread apart the pieces of the tiny plant inside. They will be able to identify the cotyledons (seed leaves), other tiny leaves, and the beginnings of what will become the plant root.

2. Have Materials Managers pick up 4 pots and 8 soaked seeds from a central location in the classroom. Direct the members of each group to pick a name for their group and to write it on the pots. They should number their pots: 1, 2, 3, and 4.

3. Have groups fill their pots about 3/4 full of soil.

4. Direct the students to make two indentations (about 1/2 cm deep) in the surface of the soil and to place one seed in each hole. Have them cover the seeds lightly with soil. Each group will have four pots, with two seeds in each pot.

5. Have students place the pots on trays near a bright, sunny window or under a fluorescent light. Over the next several days (once the seeds sprout) . . .

• Have students “mark” one of the two plants within each pot by loosely tying a piece of string around its base. If a plant dies, students should continue to measure the remaining plant.
• Have students measure both plants in each pot every day or every other day and record the length of the stems in cm on their “Seeds and Seedlings” sheets.
• Let students water the plants every day or two with a squirt bottle. The soil should be moist but not wet.

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Session 3: Light experiment

1. When most of the seedlings are approximately 10 cm tall, explain to the students that they will now investigate the effect of light on the growth of the bean plants. Ask, What do you think will happen if we give some of the plants less light?

2. Have each group move pots 3 and 4 to a new location that you have selected (in the back of the classroom or in a dark corner away from the windows or light source). Ask, Do you think that the plants in the new place will have as much light as the others? Why or why not? What do you think will happen to the plants receiving less light? Have students discuss possible outcomes and make predictions.

3. Students should continue to measure the plants for another 3–5 days and record their measurements on their “Just Growing Up” student sheets.

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Session 4: Looking at data

1. After making their final observations, have students complete the remaining questions on the their student sheets.

2. Discuss results as a class. They should be able to conclude that the difference in available light led to observed differences between the two groups of plants. Ask, Were the plants all about the same size before you moved pots 3 and 4 out of the bright light? Are all the plants still the same size? Why do you think that is so? Are there any differences other than size? Help students to conclude that the differences in growth (the plants with less light will have grown less or will have developed tall, narrow stems) and in color (the plants with less light will be lighter green) were caused by the differences in the availability of light. What is the only thing that was different about the two sets of pots? (Only the amount of light changed; all other aspects of the experiment—water, soil, seedlings, pots, planting method—were unchanged for both groups.)

3. Ask, Where do you think the plants in pots 1 and 2 got the materials and energy to produce more stems and leaves? What were the plants in pots 3 and 4 missing? What do you think would happen if we put the plants in pots 3 and 4 back in the light?

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Extensions

• Have students rinse away the soil and compare the final masses in gm of the plants in pots 1–2 vs. pots 3–4.

• Conduct the same activity with corn seeds (a monocot) and compare the results.

• Help students “see” chlorophyll, the pigments essential for converting light energy into chemical energy (food molecules), by placing a handful of crushed fresh leaves (any kind) into a clear container filled up to about 2 cm with rubbing alcohol. Stir the mixture briefly and insert the tip of a paper strip of coffee filter paper into the alcohol. The pigments will travel up the paper strip and form a green band that will be visible after about 1/2 hour. This method of separating chemicals in solution is known as paper chromatography.
  
  Safety Note. Make sure the area is well ventilated and have students wear protective eyewear. Do not use alcohol near an open flame.

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