BioEd Online

Teacher Background

Plants respond directly to Earth’s gravitational attraction, and also to light. Stems grow upward, or away from the center of Earth, and towards light. Roots grow downward, or towards the center of Earth, and away from light. These responses to external stimuli are called tropisms. Plants’ growth response to gravity is known as gravitropism; the growth response to light is phototropism. Both tropisms are controlled by plant growth hormones.

Indoleacetic acid, or auxin, is a plant hormone that, in high concentrations, stimulates growth and elongation of cells in stems, while retarding the growth of root cells. When auxin is distributed uniformly throughout a stem, all sides of the stem grow at the same rate, thereby enabling the plant to grow toward light and away from gravity (see illustration on page 5). If the plant is tipped over on its side, auxin concentrates on the lower side of the stem, causing the cells on the lower side of the stem to elongate. This process turns the stem so that it once again grows upward, presumably toward the light.

Roots also will change direction when a plant is tipped on its side. Auxin concentrates on the lower sides of the roots and inhibits the elongation of root cells. As a result, root cells on the upper side of the root grow longer, turning the roots downward into soil and away from the light. Roots also will change direction when they encounter a dense object, such as a rock. In these cases, auxin concentrates on the lower side of the roots, enabling the roots to change direction and find a way around the rock so that normal growth can resume.
investigate the effects of gravity

To learn the effects gravity has on growing plants, students create a simple germination chamber from a Zip-loc^®-type plastic bag and a moistened paper towel.

Note: For in-depth information regarding the role auxins play in plant growth and development, and about Brassica rapa, please download the Plants in Space Teacher's Guide.

Objectives and Standards

Inquiry

• Ask a question about objects, organisms and events in the environment.

• Plan and conduct a simple investigation.

• Use appropriate tools and techniques to gather data and extend the senses, and analyze and interpret data.

• Use data to construct a reasonable explanation.

• Think critically and logically to make the relationships between evidence and explanations.

• Use mathematics in all aspects of scientific inquiry.

• Communicate investigations and explanations.

Life Science

• Reproduction is a characteristic of all living systems; because no individual organism lives forever, reproduction is essential to the continuation of every species.

• All organisms must be able to obtain and use resources, grow, reproduce, and maintain stable internal conditions while living in a constantly changing external environment.

• Behavior is one kind of response an organism can make to an internal or external stimulus.

• An organism’s behavior evolves through adaptation to its environment. How a species moves, obtains food, reproduces, and responds to danger are based in the species’ evolutionary history.

Earth and Space Science

Gravity is the force that keeps planets in orbit around the sun and governs motion in the solar system. Gravity alone holds us to Earth’s surface and explains the phenomenon of the tides.

Materials and Setup

For complete list of materials, material options, safety issues and setup information, please download the PDF.

Materials per Student Group or Student

• 1–2 large seeds, such as corn or bean

• Resealable sandwich bag

• Cardboard square, cut slightly larger than the sandwich bag

• One sheet of white paper toweling

• Clear tape

• Metric ruler

• Pair of scissors

• Water

Procedure and Extensions

1. Fold a piece of paper towel to fit inside the sandwich bag.

2. Moisten the paper towel until it is uniformly damp. Empty any excess water from the towel and place the towel in the bag.

3. Position one or two seeds on top of and in the center of, the moistened towel. The seeds should be visible through the bag. Seal the bag.

4. Position the bag in the center of the cardboard, and secure the corners with cellophane tape. Stretch the bag tightly to prevent sagging, and to help hold the seeds in place. Stand the cardboard upright on its side and lean it against a wall.

5. Observe the seed and record its appearance over the next few days.

6. When the first root has formed and grown one to two centimeters long, turn the cardboard 90 degrees, as shown below.

7. Continue observing and recording the root growth for several days.

Handouts and Downloads

Download color images from the Plants in Space Teacher's Guide.