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Parts of a Bean Seed (page 2)

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Author: Janice VanCleave

Design Your Own Experiment

Note: Use a scalpel to carefully cut away the indicated parts of the cotyledons (see Figure 14.3).

Make every effort not to disturb the embryo. Discard extra beans.

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.

  1. Bean seeds have two cotyledons. Are both cotyledon's necessary for their germination (process by which a seed grows)? One way to determine this is to grow bean seeds with different amounts of the two cotyledons. Soak 30 beans for 24 hours in a jar of water. Prepare five or more beans for each of the following five groups:
    • 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.
  2. Starch is a stored food supply for seeds. Is starch stored in seed parts other than the cotyledons? One way to determine this is to test for the presence of starch using iodine. CAUTION: Keep iodine out of the reach of small children. It is poisonous and is for external use only. It can stain skin, fabric, and furniture. A positive test for the presence of starch is a dark blue-black color on the material where the iodine is placed. Using bean seeds, determine the presence of starch by using iodine on each of the following seed parts: cotyledon, epicotyl, hypocotyl, radicle, and seed coat. Separate the parts from one another and place them in shallow containers. Add one or two drops of tincture of iodine (found at a pharmacy) to each part. Prepare and display a data table to indicate the presence of starch. Use a plus sign (+) to indicate the presence of starch and a minus sign (–) to indicate its absence.

Get the Facts

Exterior and Interior

  1. Endosperm (the food-storage tissue of a seed) is present at some stage during the development of all seeds. It is present in mature corn seed but not in bean seed. What happens to the endosperm? Why is it present in some mature seeds but not in others?
  2. The seed coat of different seeds varies in color, thickness, and texture. Sometimes, it is smooth and paper thin, as on a bean. A coconut's seed coat is rough, thick, and hard. This outer coat protects the embryo from drying out against injury from falls or being struck by objects, and from attacks by insects, some animals, bacteria, and fungi. It also insulates the embryo from extreme temperatures. Germination cannot take place unless the seed coat is cracked. The seed coats of some plants are cracked by alternate freezing and thawing. Not all seeds are affected by temperature changes, and many plants do not live where the temperature drastically rises and falls. How else might a seed coat crack, allowing water and oxygen in and the developing embryo to emerge?
  3. Many types of seeds do not germinate regardless of the environmental conditions. They germinate only after a period of rest called the dormancy period. Dormancy may be the result of many factors; for example, the seed coat may contain chemical inhibitors that prevent germination. During a period of rest soil moisture leaches out the chemicals or they break down as they react with other chemicals in the soil. When the inhibitors are gone, the seed germinates. Find out more about seed dormancy and the events that result during a rest period.
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