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Tricked: How Does the Absence of Light Affect Bean Seedlings?

based on 4 ratings
Author: Janice VanCleave

Problem

How does the absence of light affect bean seedlings?

Materials

  • masking tape
  • marking pen
  • two 9-ounce (270-ml) paper cups
  • potting soil
  • pencil
  • 2 saucers
  • 6 pinto beans
  • ruler
  • tap water
  • 2 cardboard boxes at least 18 inches (45 cm) tall
  • scissors
  • plastic food wrap
  • adult helper

Procedure

  1. Use the tape and marking pen to label one cup light and the other Darkness.
  2. Fill each cup three-fourths full with potting soil.
  3. Use the pencil to punch two opposite holes in the bottom edge of each cup and set a cup in each saucer.
  4. Plant 3 beans about 1/2 inch (1.25 cm) deep in the soil of each cup, placing the beans as far apart as possible.
  5. Moisten the soil with water.
  6. Place the cup labeled Darkness in one of the boxes. Close the box and seal all cracks in the box with tape to prevent light from entering.
  7. Close the second box and seal the cracks with tape.
  8. Ask an adult to make a hinged lid in the second box by cutting across the front and two sides about 4 inches (10 cm) from the top.
  9. Tricked

  10. Ask the adult to make a window in the hinged lid by cutting away the top of the lid, leaving 2 inches (5 cm) intact around the edges.
  11. Cover the window in the lid with plastic wrap and secure with tape.
  12. Place the cup labeled Light inside the second box.
  13. Place the boxes side by side near a window.
  14. Once a week for 3 weeks, open the boxes for a short time and observe the plants. Record a description of both plants, including stem length, stem shape, leaf size, and color of the stem and leaves. Keep the soil in both boxes moist.
  15. At the end of 3 weeks, remove the plants from the boxes. Again, record a description of both plants.

Results

All the plants have a hook-shaped hypocotyl that breaks through the soil. The bean plants grown in darkness generally have tall, pale, spindly stems and small, pale, undeveloped leaves. The bean plants grown in light generally have greener, shorter stems that are straighter at the top, and their leaves are bigger and greener.

Why?

In nature, germination begins underground without light. The hypocotyl of a germinating bean lengthens rapidly, and a hook forms at its tip. In time, the hypocotyl breaks through the soil into the sunlight. In the presence of light, the hypocotyl straightens, raising the cotyledons and epicotyl. The plumule spreads, forming the first true leaves, which become green as chlorophyll forms. This is what happens to the beans grown in light.

However, the beans grown in darkness are tricked into behaving as though they are still underground. When each hypocotyl breaks through the soil's surface, there is no light. Lack of light causes a plant condition called etiolation. The hypocotyl continues to elongate rapidly, and its hook remains at the tip for a longer period of time. The leaves fail to become green and do not develop properly. Without light, the plant cannot produce chlorophyll, and its shoot is long, thin, and pale.

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