Hands-on Activity: Light Plants and Dark Plants, Wet Plants and Dry Ones

Contributed by: Engineering K-PhD Program, Pratt School of Engineering, Duke University

A photograph shows the faces of many big sunflowers and their leaves.
Students experiment with growing conditions.
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Copyright © http://www.wsu.edu/~lohr/wcl/

Summary

Students plant sunflower seeds in plastic cups, and once germinated, expose them to varying light or soil moisture conditions. They measure the seedlings' growth every few days using non-standard measurement (inch cubes). After a few weeks, they compare the growth of plants exposed to the different conditions and make comparative bar graphs that they analyze to draw conclusions about plant needs.

Engineering Connection

Determining the optimal environments for growing crops and other plants used to produce products is a big part of agricultural engineering.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

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Learning Objectives

After this activity, students should be able to:

  • Identify water, light and nutrients as needs of plants.
  • Measure heights of objects using inch-cubes.
  • Display data in the form of pictorial bar graphs.
  • State conclusions drawn from bar graphs.

Materials List

  • 20 quarts (~22 liters) of good quality potting soil
  • (optional) shallow plastic bins in which to place the potting soil for easier access
  • sunflower seeds, enough for 2 per student plus 6 extras
  • 20-oz disposable plastic drink cups, 2 per student plus 6 extras; punch holes in the cup bottoms in advance, as described in the Procedure section
  • knife or scissors, to punch drainage holes in the cups
  • permanent-ink marker
  • water
  • plastic trays, enough to hold all the plastic cups
  • 1" stacking cubes (Unifix cubes), 100 or more
  • several liquid measuring cups with spouts
  • several medium scoops, ~1 cup volume
  • 1" grid paper, 3 sheets per student (1 for data graph, 1 for post-activity assessment graph, 1 cut up into squares)
  • Drawing Handout, 1 per student
  • data sheet, 2 per student (= 1 per plant); prepare in advance as described in the Procedure section
  • data sheet, 1 per student, for a post-activity assessment activity

Introduction/Motivation

What do animals need to live? What about cats, dogs and 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

germination: The process by which a seed begins to develop into a plant.

Procedure

Before the Activity

  • Gather materials and make copies of the Drawing Handout.
  • Create and make copies of a simple datasheet for students to use. Provide lines for students to fill in the heights of their plants (in inch-cubes) on 8-10 different days. Also provide space for students to describe the experimental treatment for that plant, such as light and water, no water, no light.
  • Prepare the cups for students: Use a knife blade or scissors to make a small hole (about 1 cm in diameter) in the bottom of each plastic cup, including the extra six cups 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 cups, such that each student has two cups with his/her name on them.
  • Prepare the extra six cups, which serve as backup plants that students can "adopt" later in the experiment if their own seeds fail to germinate. Fill each cup 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.
  • (optional) To make it easier for students, use a marker or tape to show the target filling lines on the measuring cups.
  • (optional) Place the potting soil into shallow bins for easier scooping access.

With the Students: Part 1---Introduction and Experiment Setup

  1. Administer the pre-activity drawing activity, as described in the Assessment section.
  2. Present the Introduction/Motivation section content to the class.
  3. 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. Show students how to lightly tamp down the soil two or three times as they fill the cups. Demonstrate how to add water to soak the soil, using the amount you determined in advance. Also demonstrate how to place a seed in the center of the soil, at the depth stated in the seed planting instructions.
  4. After students finish 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 will be moved to the dark place and others will not be watered. Point out that some 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.
  5. 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 cup bottoms.

With the Students: Part 2---Running the Experiment

  1. 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 plant heights using inch-cubes. Demonstrate how to do this with one plant and demonstrate how to record the measurement on the data sheet.
  2. 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 chooses 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 choose 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.
  3. 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 not-to-be-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 need to be kept moist. Also, make sure students label their data sheets appropriately, so that the plant measurements for each of their two plants match the treatment conditions for each.
  4. Ask students for predictions, as described in the Investigating Questions section.
  5. Have students observe and measure their plants every few days, recording their measurements on their corresponding data sheets. Prompt them to carefully observe the plants and document on their data sheets what they notice about the differences between their 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. Also ask them if what they are seeing is what they expected to see.
  6. Continue the experiment until the healthy plants are about a foot tall. Discard unhealthy plants and give the healthy ones to students to take home, or keep them in the classroom for further observation.

With the Students: Part 3---Graphing the Data

Direct students to each use the one-inch grid paper to make a bar graph comparing the final heights of their 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.

With the Students: Part 4---Analyzing the Data

Post the graphs on a bulletin board and then lead a brief class discussion about them. Ask:

  • 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 look 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 want to advise someone who is 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?
  • Think about some of the plants we see outdoors every day: What are the weather conditions like for these plants? Lots of sunshine? Is the weather often wet, or do long dry spells sometimes occur? What might happen to the plants that grow naturally in our region if we experienced a long drought?

Attachments

Investigating Questions

Predictions: As students move their seedlings to their experimental locations, or before water is withheld, ask:

  • What differences do you expect to see in your two plants when you observe and measure them in a few days?
  • Why do you think that will happen?

Prediction Follow-Up: Then, after a week or so, ask questions such as:

  • Do you see any differences between your two plants?
  • Are any differences you see a surprise to you? In other words, are you seeing what you expected to see?

Assessment

Pre-Activity Drawings: Provide each student with a copy of the Drawing Handout, which shows four cups with symbols on their sides representing watered, not water, light, no light as growing conditions. Ask students to draw a plant in each cup, so that the finished drawing shows which plant they think would grow taller. Take a look at students' drawings to gauge their base understanding of the activity subject matter.

Post-Activity 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 cutouts representing inch cubes and provided grid paper. Review their data sheets and graphs to assess their depth of comprehension.

Activity Extensions

Ask students to remind you of what people and other animals need to live. Expect them to include food on the 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 taking them outside and pulling up a clump of grass or weeds. You can also show students the roots of one of the sunflower plants by gently pulling the plant out of its cup.

Ask students to help design an experiment to see if plants grow more quickly in nutrient-rich soil or 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 instructions 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.

Contributors

Mary R. Hebrank, project and lesson/activity consultant

Copyright

© 2013 by Regents of the University of Colorado; original © 2004 Duke University

Supporting Program

Engineering K-PhD Program, Pratt School of Engineering, Duke University

Acknowledgements

This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.

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