Cactus Adaptations

Science, Language Arts

Grade Level: Third - April
Teacher of Lesson: Stacey Perri
Approximate Time:
Part One: Thirty Minutes Preparation and Discussion, Overnight Experiment
Part Two: Twenty Minutes Preparation and Discussion, Three to Four Day Experiment

PREINSTRUCTIONAL

Objectives:

Materials:

DURING INSTRUCTION

Procedures:

Part One: (What)
  1. Write the words photosynthesis and transpiration on the board. Briefly discuss the definition of photosynthesis, then discuss transpiration with the students (plants lose water through their leaves and stems when they photosynthesize. This water loss process is called transpiration).
  2. Ask the students how they think that they can test the transpiration in each plant. Ask them if there would be a difference.
  3. Discuss safety when handling the plants.
  4. Put a plastic bag over a stem or over the top of the cactus and tie the bag closed with the string. Put a plastic bag over a leaf of the woodland plant and tie closed with a string. Ask the students to sketch, or write about, this.
  5. Place the plants in a sunny spot. Leave for one day.
    day two:
  6. Droplets of water will be apparent on each bag.
  7. Ask the students what they are seeing (they should be able to recall the definition of transpiration).
  8. Ask the students which of the two plants lost the most water through transpiration (woodland plant).

Part Two: (Why)

  1. Ask the students why the cactus loses less water through transpiration, and why this is important in the desert.
  2. Give each group a dry sponge, two margarine tubs, and some petroleum jelly, and have them follow these steps:
    1. Pour one-fourth a cup (60ml) of water into each tub.
    2. Cut the sponge in half (measure).
    3. Cover one side and all four edges of one piece of sponge with petroleum jelly. Then lay it in one of the margarine tubs on its ungreased side.
    4. Lay the other piece of sponge in the other margarine tub.
    5. Watch as the sponges soak up water. The students keep track of how long it takes the sponges to dry out in their journals. (the ungreased side should dry out first.)
  3. After the three or four days of the experiment, have the students take out their journals. Ask them to explain how this information helps us understand how a cactus loses less water during transpiration.
  4. Discuss the coating on the cactus, and how this is a desert adaptation.
  5. Discuss the adaptations of desert plants that the students have experimented with and observed in the past few days.
POSTINSTRUCTIONAL:

Evaluation:

  1. Were the students able to define transporation?
  2. Were the students able to explain what accounted for the difference in the water loss during transpiration that occurred in the two plants?
  3. Look at the student's journals. Were their observations of the experiment accurate?
  4. Were they able to identify and explain the cactus' adaptation to the desert?

Reteaching Strategies and Follow-Up:

If a student does not understand the concept, allow him or her to feel the two plants (careful with the cactus!) in order to physically identify the difference (the cactus will have a waxy feel). Then, go over the experiment again with the student during study hall (by looking at students journals and results of the experiment).
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