From Seed to Seed:
Plant Science for K-8 Educators

Activity 11: Grow, Baby, Grow

Grades: 5-8

Associated Lesson Topics:

• Phenomena controlled by auxin

National Standards:

• SCIENCE: Standard A, Standard C
• MATH: Measurement Standard

Planting the seed...

Present to your class two plants of the same species-one that is full and bushy and one that has only a single main stem. It is difficult to find commercially grown plants that have only a single stem, so you may need to produce one yourself-by not pinching it back-in advance of this lesson. Poinsettias or chrysanthemums work well. Which plant would you rather buy? Share with your students the fact that commercial growers manipulate plants to make them look bushier. What do you think growers do to their plants to change the way they grow?

Teacher Information:

The phenomenon that students will investigate in this activity is called apical dominance. When plants grow naturally, one shoot will become the "leader." At the growing tip of this shoot will be a high concentration of growth-promoting auxin. This results in a low concentration of auxin elsewhere in the plant-namely, the lateral buds located lower on the plant. This prevents the lower buds and branches from growing. When the apical bud is removed, the auxin is redistributed to the lower branches, allowing them to grow, giving the plant a bushy, full appearance desired by consumers.

Necessary Materials:

• Young plants suitable for pinching: chrysanthemum, poinsettia, petunia, impatiens, snapdragon, or geranium.

Procedure:

• Grow several plants of the same species.
• About once a week, pinch off the apical buds on half of the plants and leave the other half intact. (Note: Locating the apical bud is a great way to review plant anatomy with the students). Otherwise, these plants should be exposed to identical growing conditions: pot size, potting medium, amount of light, moisture, and nutrients.
• Weekly, students can draw the plants in their journals and record observations. How many stems are on each plant? If you were to hold a ruler up and measure the height and width of the aboveground growth, which would be wider? Taller? Which one produced more flower buds?
• After making these observations for an adequate amount of time-when differences between the two plants are obvious and being noted-launch into a discussion about the role of auxin in plant growth. The fact that the growth regulator auxin is responsible is not important. What is important is that students understand that plants have internal mechanisms that control their growth.

Harvesting the Crop...

How might this mechanism benefit the plant? How might plants get "pinched" in the wild? Do you think it matters when in the plant's life the pinching takes place? Students can experiment by allowing the plants to grow for different amounts of time before they begin the pinching regime. What is the best age to begin pinching a plant if you want to produce the most attractive plant possible?

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