Inside and Out: What's On the Outside of a Pinto Bean?

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Author: Janice VanCleave


What's on the outside of a pinto bean?


  • 4 to 6 dry pinto beans
  • coffee cup
  • ruler
  • tap water
  • timer
  • paper towel


  1. Place the beans in the cup and cover them with about 2 inches (5 cm) of water.
  2. Soak the beans for 24 hours.
  3. After 24 hours, take the beans out of the cup and place them on the paper towel to absorb the excess water.
  4. Use your fingernail to remove the outer covering from one of the beans. Observe the color and thickness of the covering.
  5. NOTE: Keep the remaining beans for the next two experiments.


The outside of the bean consists of a thin outer coat that is light brown with irregularly shaped dark spots.

Inside and Out


A bean is a seed, inside of which is an embryo (an organism in its earliest stage of development) surrounded by a stored food supply. The seed is covered by a protective outer covering called the seed coat, which protects the inside of the seed from insects, disease, and damage. The seed coat of the pinto bean has a light-colored, oval-shaped scar called the hilum and a small dot at one end of the hilum called the micropyle.

Let's Explore

  1. What's under the seed coat of a pinto bean? Use one of the soaked pinto beans from the experiment. Remove the seed coat to reveal a white structure with two separate halves connected at a single spot at the top. The two halves are cotyledons, or seed leaves, which are simple leaves that store food for the developing plant embryo. (plants with two cotyledons are called dicotyledons or dicots.) Extending from the connecting spot is a beak-shaped structure called the hypocotyl. The hypocotyl is the part of the plant embryo that develops into roots and, very often, the lower stem. The tip of the hypocotyl, called the radicle, develops into roots. Use a magnifying lens to examine these parts. Repeat the procedure, using 3 or 4 more beans.
  2. What's inside a pinto bean? Use the soaked beans from the original experiment. Remove the seed coat and gently pry the cotyledons open with your fingernail, then spread them apart. Be careful not to break the hypocotyl. Use a magnifying lens to study the parts of the embryo inside. Use the diagram to identify the following parts of the embryonic (undeveloped) shoot (the part of a plant that grows above ground):
    • epicotyl The part of a plant embryo, located above the hypocotyl, that develops into the plant's stem, leaves, flowers, and fruit.
    • plumule The part of a plant embryo, located at the tip of the embryonic shoot, that consists of several tiny, immature leaves that at maturity form the first true leaves. Science Fair Hint: Prepare and display a labeled drawing of the outside and inside of the bean.
  3. Inside and Out

  4. What's inside other beans? Repeat the previous experiment using different beans, such as lima and kidney.

Show Time!

    1. Use a magnifying lens to study the hilum and micropyle on a pinto bean. A biology text can be used to find out how the features formed during the development of the seed.
    2. Repeat the previous experiment, using other beans.
  2. During germination (the beginning of growth or development) of a pinto bean, which part of the embryo develops first? Soak 30 to 40 beans in 1 cup (250 ml) of water. NOTE: You'll need the extra beans in case some of the embryos are damaged. Fold a paper towel in half twice and place it on a piece of aluminum foil that is about 12 inches (30 cm) square. Wet the towel with water. Place all but 3 of the soaked beans on the wet paper towel. Fold the aluminum foil over the beans to keep them moist. Open the 3 reserved beans and use a magnifying lens to observe their embryo parts. Make a diagram showing the cotyledons and the attached embryo. The diagram should indicate the size of the cotyledon and the embryo parts. Each day for 7 days, remove 3 beans from the foil and observe their embryos. Make another diagram showing size and location of the embryo in the cotyledon. Create and display a poster using the drawings to show the development of the bean embryo.

Check it Oout!

The seed coats of different seeds vary in color, thickness, and texture. Sometimes the seed coat is smooth and paper-thin, like that of a pinto bean. A coconut's seed coat, however, is rough, thick, and hard. A seed cannot develop into a plant until the seed coat is broken. Find out how the seed coats of different seeds are broken. For more information about the breaking of seed coats, see Chapter 10, "Attractive," in Janice VanCleave's Plants (New York: Wiley, 1997).

Inside and Out

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