How Do We Grow? Light vs. Dark

Dr. Hedi Baxter Lauffer
Director of the Fast Plants® Program
University of Wisconsin-Madison

May/June 2016

Are you an elementary teacher balancing a busy end-of-the-year schedule with time for science? Let Carolina’s new Wisconsin Fast Plants® seed disks simplify prep and teaching time for instruction on structure, function, matter, and energy flow in plants. In this activity, student groups germinate and grow Wisconsin Fast Plants® without soil—some plants with and some without light—for 5 days to gather evidence that plants require light to develop leaves and other structures necessary to live.

Growing Fast Plants® through their full life cycle requires soil, 24-hour fluorescent light, and nutrients. However, those 3 requirements are not needed for this quick 5-day activity to compare seedlings grown in light and dark. Seed disks containing Wisconsin Fast Plants® seeds are simply placed in a deli container, kept moist, and grown for 5 days in a location that receives a moderate to high amount of light (optimal conditions are fluorescent light 24-7, but for this activity a sunny window will work). The seedlings grown in light are then used as a comparison to the seedlings grown in the dark.

Grade level

Elementary

Next Generation Science Standards®

Considerations

We recommend beginning this lesson on a Monday so students can make their observations daily for 1 week. This lesson is designed for 8 groups of 4 students and takes place over 5 days as follows:

Materials (per student group)

• 2 Wisconsin Fast Plants® seed disks (item #158870 contains 8 seed disks)
• 2 small deli containers (8- or 12-oz deli containers, either new or recycled and washed)
• 1 watering bottle containing ~1/4 cup water (make watering bottle by poking a small hole in the lid of a recycled water bottle; depending on the type of plastic the lid is made from, teacher can use a ball point pen or sharp scissors in advance to make the hole)
• Aluminum foil (for wrapping deli container that will be kept in the dark)
• Plastic food wrap (to cover deli container that will be kept in the light)
  

Procedure

1. Set the stage for this lesson.
   
   
   1. Pose the class question, “Do plants need light to grow and develop?”
   2. Discuss how to design an experiment to gather evidence that could answer that question.
   3. Explain the reason why all conditions other than light must be kept the same for a fair test.
2. Distribute 2 seed disks and 2 deli containers per group.
3. Instruct students how to prepare their seed disks to germinate and grow—1 in the light and 1 in the dark.
   
   
   
   
   1. Place 1 seed disk in the bottom of 1 deli container.
   2. Use a watering bottle to gently cover the seed disk with water.
   3. Count to 30 after the seed disk has been covered with water, then gently pour off whatever water remains, leaving the seed disk lying flat across the bottom of the deli container.
   4. Loosely cover 1 deli container with plastic wrap (plants need air—the purpose of the plastic wrap is to reduce evaporation while allowing light to reach the seedlings).
   5. Completely wrap 1 deli container in aluminum foil.
   6. Put the plastic wrap-covered deli container in the light and the aluminum foil-wrapped deli container in the dark.
   7. In 2 days, the teacher will need to quickly unwrap groups’ aluminum foil-wrapped deli containers to see if water needs to be added to keep the seed disks moist. Do this one at a time to keep light exposure to a minimum.
4. Daily for the next 4 days, students work in groups to observe their plants growing in the light. Integrate art into this activity by having students draw their observations. Integrate reading/writing by building a word wall with the structures that emerge as the plants grow (e.g., stem, seeds, roots) and descriptions of their functions.
5. Five days after starting the seeds, bring out of the dark the seed disks that have had no light so students can compare them with those growing in the light.

Conclusion

1. During the day 5 observation and comparison time, have students refer specifically to the structures and functions already identified and discuss how those structures (and their functions) have been affected by the presence or absence of light.
   
   
   1. Facilitate guided discussions for students to learn the causes of the effects that they observe. For example:
      
      
      1. Stems on plants grown without light are thin, weak, and much longer (reaching to try and locate light) than stems grown in light. This is because—with light—plants are able to build strong stems, using energy from the light and matter from air and water.
      2. Leaves on plants grown without light are tiny and pale green or yellow compared to the larger, darker green leaves on those plants grown in light. This is because—with light—plants are able to build leaf tissue, using energy from the light and matter from air and water, and they can conduct the life processes that make plants green (photosynthesis).
      3. Students may also be able to observe that greater root growth occurred when plants had light energy because those plants were able to build roots—similar to building stems and leaves—using energy from the light and matter from air and water.
2. Note: Plants are able to germinate and grow a seedling in the dark because every seed contains a tiny embryo and sufficient stored food for the seedling to emerge (usually from under a layer of soil) and unfurl its seed leaves to begin photosynthesizing. Once the stored food is exhausted, the seedling cannot survive long without light.
3. You may choose to now expose to light the plants that had been in the dark (they will already be getting light while students make their observations). In under an hour, leaves will start to turn darker green as the tiny seedlings begin to photosynthesize.