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Determine the Rate of Geotropism of Plant Stems

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Author: Janice VanCleave

What You Need to Know

Tropism is the growth of plants in response to stimuli, which are things that an organism responds to. If the stimulus is gravity, the response is called geotropism. Plant growth in the direction of the force of gravity, toward the center of Earth, is called positive geotropism. Plant growth in the opposite direction of the force of gravity is called negative geotropism. Auxin is a chemical in plants that affects plant growth.

How Does Geotropism Work?

If a plant were lying on its side, gravity would cause a buildup of auxin on the lower part of the plant's stems and roots. This high concentration of auxin causes plant stem cells to grow faster. Thus, the lower part of the stem would have longer cells, which would result in the stem bending upward as shown in the figure on page 56. Plant stem cell growth represents negative geotropism. Root cells have an opposite response to auxin. This represents positive geotropism. A high concentration of auxin causes root cells to grow more slowly. So the cells in the upper part of the root grow longer, resulting in the root bending down.

What Does This Have to Do with the Rate of Geotropism?

The rate of geotropism, or gravitropism, is a measure of the time it takes a plant stem or root to respond to gravity.

What Does This Have to Do with the Rate of Geotropism?

Fun Fact

The stems and roots of a plant placed on its side on a spinning turntable will grow in opposite directions. The stems grow toward the outside edge of the spinning table and the roots grow toward its center (the axis). How does the spinning table simulate gravity?

Real-Life Science Challenge

Prolonged spaceflights will require the growth of plants for water purification, atmospheric conditioning, nutrient recycling, and food production. In environments with zero or reduced gravity, the stems and roots of plants grow in random directions. Scientists are still investigating how to solve this problem. What solutions do you suggest?

Experiment

Now, start experimenting with plants and rates of geotropism.

Hints

  • Obtain a few samples of one kind of plant.
  • Plant plants at different angles.
  • Determine a method for measuring geotropism over time.
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