Double: How Does a Pinto Bean Seedling Develop?

Problem

How does a pinto bean seedling develop?

Materials

• paper towels
• 10-ounce (300-ml) clear plastic cup
• 6 dry pinto beans
• tap water

Procedure

1. Fold a paper towel in half and use it to line the inside of the cup.
2. Crumple several paper towels together and stuff them into the cup. Use enough towels to hold the paper lining firmly in place around the inside of the cup.
3. Place the beans between the cup and the paper lining, spacing the beans evenly around the perimeter of the cup.



4. Moisten the paper towels in the cup with water. Keep the paper towels in the cup moist, but not dripping wet, during the entire experiment.
5. Observe the beans each day for 21 days.

Results

The seed coat breaks and the hypocotyl emerges. The free end of the hypocotyl grows down, and the end attached to the cotyledons grows up. The attached end is bent, but in time straightens and raises the cotyledons. Later, a stem and leaves grow above the cotyledons and the cotyledons wither and fall away.

Why?

As a pinto bean or any angiosperm seed germinates, two systems, root and shoot, develop. The root system is subterranean (below ground) and consists of many branching roots. The shoot system is aerial (above ground) and consists of a shoot.

During germination, the parts of the bean (described in chapter 9, "Inside and Out") begin to grow. The end of the hypocotyl, the radicle, is the first part to emerge, grows downward, and eventually develops into the root system.

The hypocotyllengthens, forming a hook at the end attached to the cotyledons. As the hooked hypocotyl grows upward, it would cut a passage through the soil if the bean were planted. This allows the attached cotyledons and epicotyl to be pulled through the soil without injury. After breaking through the soil, the hypocotyl straightens and raises the cotyledons and epicotyl upward. The epicotyl develops into the shoot, which at maturity consists of stems, leaves, flowers, and fruits. At first, two tiny leaves, which are the first true leaves, develop. Up to this point, the seedling (a young plant grown from a seed) has gotten its nutrients from the cotyledons, but now in the presence of light, chlorophyll is manufactured in the leaves and used to produce food. As the food supply in the cotyledons is used up, the cotyledons wither and fall away.

Let's Explore

What is the rate of growth for the hypocotyl? Repeat the experiment. At the first appearance of the hypocotyl, record the length of the hypocotyl as zero. Then, record the length of the hypocotyl at the same time each day for 7 days. Science Fair Hint: Prepare and display a graph of the growth rate.

Show Time!

1. Would planting the beans in soil affect the results? Fill a clear plastic cup three-fourths full with potting soil. Plant 6 beans about ~ inch (1.25 cm) beneath the surface of the soil so that they are visible through the side of the cup. Keep the soil moist with water. Observe the cup every day for 21 days. Science Fair Hint: When observing the beans, take photographs to use as part of a project display to represent the results.
2. Do other dicots develop the way pinto beans do? Repeat the previous experiment, using 2 cups. Plant 6 lima beans in one cup and 6 peas in the other cup. During development, observe and record the position of the cotyledons, the length of the hypocotyl, and the length of the epicotyl of each seed. Then, make a comparison of beans and peas.

Check It Out!

Plant hormones are chemicals that control plant growth and development. Use biology texts and encyclopedias to find out more about these chemicals. What effect do auxins have on seedling growth? How do gibberellins affect germination?

This article is brought to you by:

From this Book:

Janice VanCleave's Plants
Buy this book »