Hands-on Activity: Light Plants and Dark Plants, Wet Plants and Dry Ones
Educational Standards :
Learning Objectives (Return to Contents)
After this activity, students should be able to:
Materials List (Return to Contents)
- Use a knife blade or scissors to make a small hole (about 1 cm in diameter) in the bottom of each plastic cup.
- Use a permanent-ink marker to draw a line around the inside circumference of each cup, about 1" below the top rim.
- Use the same marker to write each student's name on two of the cups, such that each student has two cups with his /her name on them.
- Use the extra 6 cups (with holes punched in the bottoms) to fill each with soil, tamping it down lightly as you go. Using a measuring cup, determine how much water is needed to soak the soil in each cup. Add the water slowly, about a quarter of a cup at a time, until excess water begins to run out of the bottom of the cup. Make note of the average amount. Finally, in each cup press a seed into the soil to the depth stated in the package instructions. Cover the seeds lightly with some of the surrounding soil.
Create and make copies of a simple data sheet for students to use. It should have lines where students can fill in the heights of their plants (in inch-cubes) on 8-10 different days. There should be enough for 2 sheets per student (one sheet per plant). Each sheet should also have a place where the experimental treatment for that plant (e.g., light and water, no water, no light) can be filled in.
Introduction/Motivation (Return to Contents)
What do animals need to live? What about cats, dogs, people? (Listen to student ideas.) That's right, they all need food to eat, water to drink and air to breath. And yes, people also need shelter and clothing to keep warm and dry.
What do you think plants need in order to live and grow? (Have handy a few house plants in the classroom to refer to. Perhaps students have seen you water them, or if houseplants are not available, suggest they think about lawns and gardens that they have seen. Listen to student ideas.) Do plants need light? Are gardens and farms planted in forests (dark, minimal light) or in open areas (bright, much sunshine)? Do plants need water?
Would you like to plant some seeds and then see if your plants grow better in light or in dark conditions? And aso investigate whether your plants need water or not? Let's conduct an experiment. Let's grow some plants from seeds, put them in different growing conditions, and measure their growth. This will tell us more about the needs of plants.
Vocabulary/Definitions (Return to Contents)
Procedure (Return to Contents)
Once the issues raised in the Introduction/Motivation section have been worked out, demonstrate how to fill the plastic cups with potting soil using scoops. This is much easier for students if the potting soil is first poured into plastic bins that are not too deep for them to reach into. Show them how to tamp the soil down lightly two or three times as they fill the cups. Demonstrate how to add water to soak the soil, using the amount you determined previously. It is easier for students if you use a permanent marker or tape to show target filling lines on the measuring cups. Also demonstrate how to place a seed in the center of the soil, at the depth stated in the seed planting directions.
When each student completes the planting process, place all cups in the plastic trays (to catch drips). Place the trays in a bright location for germination, explaining that once the small plants appear, some of them will be moved to the dark place, and some will not be watered. Point out that some of the seeds may be defective and won't germinate. Show students the extra cups you prepared earlier, and tell them that students can "adopt" these extras if their own seeds fail to germinate.
Keep the soil in the cups moist, but not soggy, by watering when necessary. Demonstrate how to add water, again a little at a time, until it just begins to drip from the holes in the bottom.
Part 1: Running the Experiment
Begin the experiment proper when enough seedling plants have developed so that each student can have two plants. The seedlings should be only an inch or two tall at this point. Begin by showing students the data sheets and explaining that every few days, they will measure the heights of their plants using inch-cubes. Demonstrate how to do this with one plant, and point out to record on the data sheet.
After all students have measured their plants and recorded the heights, explain that half of the class will use one of their two plants to test for the effects of watering or not watering. Thus, this half of the class will choose one of their two plants to be deprived of water. The other half of the class will test for the effects of light versus no light. These students will chose one of their two plants to be kept in a dark location, removing it only to measure it every few days and water it when needed.
Determine which students will test for water/no water, and which will test for light/no light. Use a permanent marker to identify which cups will receive water and which will not, by drawing a large water droplet on the cups of those plants that are to be watered. Draw a water droplet that has been crossed out on the cups of the non-watered plants. Likewise, draw a sun or light bulb on the cups for plants left in the light, and the same symbol crossed out on cups to be left in the dark. Make sure students who are testing for light/no light know that both of their plants should be kept moist. Also, make sure students label their data sheets appropriately, so that their plant measurements for each of their two plants match the treatment conditions for each.
Have students observe and measure their plants every few days, recording their measurements on their corresponding data sheets. Be sure to ask them what they notice about the differences between their own two plants, as well as the differences in other students' plant pairs. Ask questions that cause them to be specific and detailed about the comparative observations, and also ask them if what they are seeing is what they expected to see.
Continue the experiment until the healthy plants are about a foot tall. Unhealthy plants can be discarded, but healthy ones can be given to students to take home or kept in the classroom for further observation.
Part 2: Graphing and Analyzing the Data
Direct each student to use the one-inch grid paper to make a bar graph comparing the final heights of his or her two plants. Show students how to do this by coloring in the number of squares corresponding to the number of inch cubes they counted for the final heights. It is best if all students use the same colors, for example, green for the plants that received both light and water, black for the plants grown in darkness, and brown for the plants that did not get watered.
Post the graphs on a bulletin board and then hold a brief discussion about them. Ask the students: What do you notice? How do the graphs show the differences between the plants grown under different conditions? Which plants grew tallest? How do the graphs show that? Do the graphs show anything else about what the plants looked like, for example, the color of the plants or the number of leaves they grew?
What can you conclude from your experiment about what plants need to live and grow? If you wanted to advise someone who was going to plant sunflower seeds in a garden, what would you say? Do you think sunflowers would grow well in a forest? In a field? In a dry place, like the desert or under a car port where they get no rain?
Finally, call their attention to the plants they see everyday outdoors. Ask : What are the weather conditions like for these plants? Is there lots of sunshine? Is the weather often wet, or are there sometimes long dry spells? What might happen to the plants that grow naturally in our region if there was a long drought?
Investigating Questions (Return to Contents)
As students move their seedlings to their experimental locations, or before water is withheld, ask:
Then, after a week or so, ask questions such as:
Assessment (Return to Contents)
Drawing: Provide students with a copy of a hand-drawn picture showing two plastic cups. On one cup include the water droplet symbol, and on the other include the water droplet symbol that has been crossed out. Ask students to draw a plant in each cup, so that the finished drawing shows which plant they think would grow taller. Do the same with another copy, but this time the cups should bear the light/no light symbols.
Measuring/Graphing Test: To assess the development of measurement and graphing skills, provide two or three objects of different heights, but whose heights can easily be measured with inch cubes. Ask students to measure these objects, and record their results on a provided data sheet. Finally, ask them to create a graph that shows the three heights, using paper cut-outs representing inch cubes and provided grid paper.
Activity Extensions (Return to Contents)
Ask students to remind you of what people and other animals need to live. They should be able to include food in their list. Ask if plants need food like people do. Students may or may not know that lawns, gardens and crop fields are often fertilized. Ask if they know how plants get their nutrition. Make sure they know that plants don't "eat," but their roots absorb nutrients from the moist soil. Show students the roots of plants by going outside and pulling up a clump of grass or some weeds. You can also gently pull out of a cup one the sunflower plants grown by the class to expose its roots.
Students can help design an experiment to see if plants grow more quickly in nutrient-rich soil compared to nutrient-poor soil. Again, grow sunflower plants from seed, using the same cups (after cleaning) that were used before. Garden supply stores typically sell large bags of "top soil" that are inexpensive and also poor in nutrients. Before planting the seeds, mix half of the soil with a good quality fertilizer, following the directions on the fertilizer package. Use this enriched soil for half of the seeds. Use the remainder of the soil, without fertilizer, for the rest of the seeds. To keep other conditions the same for both sets of plants, keep all the plants in the same location and water them all at the same time and with the same amounts of water. Depending on the type of fertilizer used, you may need to add more of it during the several weeks that the experiment runs.
ContributorsMary R. Hebrank, project and lesson/activity consultant
Copyright© 2004 by Engineering K-Ph.D. Program, Pratt School of Engineering, Duke University
including copyrighted works from other educational institutions and/or U.S. government agencies; all rights reserved.
Supporting Program (Return to Contents)Engineering K-Ph.D. Program, Pratt School of Engineering, Duke University
Last Modified: April 24, 2014