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Water, air, food, or soil — these controlled experiments discover which is the most needed for plant growth




9, 10, 11, 12  

Title – How Do Plants Grow?
By – S. Neaderhiser
Primary Subject – Science
Grade Level – 9-12


      Define terms: control group, experimental group, independent variable, dependent variable.

Control group

      — group that does not receive treatment in a controlled experiment.

Experimental group

      — group that receives treatment in a controlled experiment.

Independent variable

      — the variable that the experimenter controls.

Dependent variable

    — the variable the experimenter measures (results).


    Show an acorn and a log to the class. Ask the class how the acorn “becomes” the log. Establish prior knowledge of how photosynthesizers acquire mass. Record responses on a large sheet of paper. The responses will be reviewed at the end of the lesson. Prompt students by asking them “What do plants need to grow?”. Expected responses might include water, air, soil, and food. After the list is compiled, ask each student to pick one item that they believe is the most important in plant growth.


      Students will be divided into groups of 3 to 4. Each group will be given three small radish plants (4-6 inches in height). One plant will be the control group and will receive no treatment. Control plants will be placed in the same place in the classroom where they have adequate light. Each control plant will receive 25 mL of water daily. Each group will decide what variable to test, light, water, soil and absence/presence of air being the parameters. Each group will write a hypothesis and the procedure for the experiment. Depending on the level of the class, you may want to provide an outline or skeleton of an experimental write-up.
      Students should take height measurements, count leaves, describe color of plant, rigidness of plant, etc.
    Students will begin conducting their experiments after each receiving teacher approval. The teacher will provide all the materials necessary, but students are in charge of executing the experiment. The experiments will begin Monday and results will be collected that Friday.


    During the week of experiments, a general introduction to photosynthesis should be given. A general equation of carbon dioxide and water fueled by sunlight yield glucose (sugar) and oxygen should be used. Depending on the level of the class, you may want to include more detail. Ask students to revisit the variables they chose and their hypotheses. Students should not amend their hypotheses, but encourage groups to discuss whether they still believe they are correct.


      Students should collect their data that Friday. They should measure height of plant, count leaves, describe the color of the plant, and describe the rigidness of the plant. Note any changes from Monday.
    Students should answer analysis questions that try to draw out an explanation for their observations. How does the variable chosen relate to the photosynthesis equation given? How does increasing or decreasing that variable enhance or slow the growth of the plant? How does increasing or decreasing that variable affect the production of glucose (sugar)? Revisit the acorn and log question from the “Engage” section of the lesson. Show the list you created during the “Engage” activity. Have your students cross of items that are not needed for a plant to grow. The goal is for the students to understand that the carbon dioxide in air undergoes a process to become glucose. The glucose is what makes the plant grow.


      Show the “

From Thin Air

      ” clip from the Annenberg site

    . You will need to create a free account to view the file. The clip will show MIT graduates trying to answer the acorn and log question. Now your students know what many geniuses do not!

E-Mail S. Neaderhiser !

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