Exterior and Interior Parts of a Bean Seed

Given the right amount of water, oxygen, and warmth, most seeds germinate and develop into mature plants. Seeds vary in physical appearance both on the outside and on the inside.

In this project, you will have the opportunity to identify exterior and interior seed parts. The seed parts of two basic seed types, dicotyledons and monocotyledons, will be compared. You will determine why the cotyledon is necessary and investigate the presence of starch in each seed part

Getting Started

Purpose:   To dissect a bean seed and identify its parts.

Materials

• 8 to 10 pinto beans
• 1-pint (500-ml) jar
• Distilled water
• Refrigerator
• Paper towel
• Magnifying lens (handheld type)

Procedure

• Place the beans into the jar and cover them with distilled water.
• Put the jar of beans in the refrigerator to reduce bacterial contamination.
• Soak the beans for 24 hours.
• Remove the beans from the jar and place them on the paper towel to absorb the excess water.
• Inspect the outside of the beans and identify the seed coat, micropyle, and hilum (see Figure 1.1).
• Use your fingernails to carefully remove the seed coat from one of the beans.
• Very gently pry the rounded side of the bean open like a book with your fingernail.

• Spread open the two halves of the bean.
• Use the magnifying lens to study the inside of each bean half and identify the cotyledon, epicotyl, hypocotyl, and radicle (see Figure 1.1).
• Open several beans and compare their parts for differences in size, shape, and organization.

Results

The brown seed coat is thin and peels off easily to reveal a white structure with two separate halves connected at the top. Inside is a cylindrical structure with what appears to be folded leaves at the end.

Why?

The bean seed consists of three parts: a seed coat, an embryo, and two cotyledons (food-storage tissue). The surface covering of a seed is called the seed coat. This jacket around the seed protects the embryo (baby plant inside. the seed) from insects, disease, and damage. The scar on the seed coat is the hilum (point of attachment to the ovary wall). The small dot at one end of the hilum is the micropyle (small opening through which pollen grains enter).

The bean is a dicotyledon because it has two bean halves under the seed coat Inside the cotyledons is the embryo, the cylindrical structure within the seed that germinates (develops into a plant). The lower end of the peg-shaped hypocotyl which develops into the plant's first root is called the radicle. The section of the hypocotyl above the radicle becomes the plant's lower stem. The epicotyl on the end, which looks like folded leaves, forms the plant's first true leaves.

Try New Approaches

Note: Use a single-edge razor blade to carefully cut away the indicated parts of the cotyledons (see Figure 1.3). Make every effort not to disturb the embryo. Discard extra beans.

1.  

   CAUTION: Always be careful when cutting with a sharp instrument to cut in a direction away from your hands and fingers.

   1. While dicotyledons have two seed leaves, monocotyledons have only one seed leaf. How do the seed parts of a monocotyledon compare to those of a dicotyledon? Repeat the experiment replacing the bean seed with corn seed. Corn seed is much more difficult to open and may require the use of a single-edge razor blade. As in the original experiment, study and identify seed parts (see the corn seed diagram in Figure 1.2).
   2. Use a stereomicroscope (dissecting microscope that allows thick specimens to be observed) to study, identify, and compare the parts of bean seed and corn seed. Science Fair Hint: Diagram the parts of each seed type, label the parts, and display the diagrams.
2. How much of the cotyledon is necessary for development of the embryo? Soak 20 beans for 24. hours in a jar of water. Prepare four beans for each of the following five sections:
   • 100%—Do not cut away any part of the cotyledons.
   • 75%—Cut away the lower half of one cotyledon.
   • 50%—Cut away the lower half of both cotyledons.
   • 25%—Cut away all but one-fourth of the cotyledons, leaving the section attached to the embryo.
   • 0%—Cut away both cotyledons, leaving only the embryo.

Place the prepared beans on a moist paper towel. Set the towel on a sheet of aluminum foil and fold the foil around the beans. Open the foil package daily and make observations of any evidence of germination. Science Fair Hint: Use diagrams and photographs to represent results.

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