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Plants | Activity 3.2

Activity 3.2: Observing Plants in the Light and Dark (60 min over 2 days)

Students conduct an investigation about how plants affect BTB (an indicator for CO2) in the light and dark. They then compare their observations with other groups and work to identify patterns and reach consensus about their results.

Materials You Provide

  • Radish plants (either in tubes (GL) or containers (PT) from the Pre-Lesson. Note: 3-4 tubes or 1-2 containers will be plenty for each experiment replicate.
  • Dark box, thick black trash bag, or very dark closet (1 per class)
  • Fluorescent grow light or sunny windowsill (1 per class)
  • Label (1 per group student group)
  • Petri dish with blue BTB (1 per container/student group)
  • Petri dish with yellow BTB (1 per container/student group)
  • Sealable 6.8 liter (or 29 cup) container (1 per group)
  • (From previous lesson) 3.1 Predictions about Radish Plants Growing PPT (GL or PT) with student answerss

Resources Provided


Assign each group of students a treatment (light or dark). For each group, select growing radishes (3-4 GL tubes or 1-2 PT containers) that were planted in the Plants Unit Pre-Lesson. For each group, provide 1 sealable 6.8 liter (29 cup) container, one Petri dish of blue BTB, one Petri dish of yellow BTB, and a label. Prepare enough space under a grow light or in a sunny windowsill for the “plants in the light” containers, and enough room in a dark closet for the “plants in the dark” containers. Optionally, print a few copies of the BTB Color Handout for students to use as a color reference.


1. Use the instructional model to show students where they are in the course of the unit.

Show slide 2 of the 3.2 Observing Plants in the Light and Dark PPT..


  • Use the class discussion to interpret how successful your students are at identifying patterns in the class data. Use the 3.2 Grading the Observing Plants in the Light and Dark Worksheet to determine if your students had any trouble with data collection.
  • During this activity, note students’ success in observing changes in BTB. Also note students’ ability to reach a consensus about patterns in data and how they interpret results.
  • The discussion in step 4 can be helpful for informal assessment in two ways: 1) it can help you assess your students’ skills in identifying sources of error and finding patterns in data, and 2) it can help you assess how well students identify the limits of the evidence. Do they recognize that the investigation does not fully answer the Three Questions?


  • Aquatic plants, such as elodea, placed in test tubes with BTB, can also be used for this investigation.
  • Discuss threats to accuracy of measurement.
  • Check to see if students can identify unanswered questions from the investigation.


  • Conduct the investigation as a demonstration if you are short on time or resources.
  • Parts 1 and 2 of the Carbon TIME Growing Plants Video show the investigation and patterns of results. While we do not recommend this, if time is not available, students can watch the video instead of doing the investigation.
  • Have students develop the experimental design on their own using the tools provided. For example, students may choose to set up a control treatment as a container with BTB and no plants.
  • Put the “plants in the dark” container inside an opaque plastic bag instead of in a dark closet.

Extend the Learning

  • Have students investigate any answers on the pretest they were unsure about on their own.
  • Have students compare photosynthesis rates between plants in the sun and plants under a grow light.