Exterior and Interior Parts of a Bean Seed (page 2)

Design Your Own Experiment

1. Starch is a stored food supply for seeds. Is starch stored in seed parts other than the cotyledons? Using beans and corn seeds, determine the presence of starch by using iodine on each of the following seed parts: cotyledon, endosperm, 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. CAUTION: Keep the iodine out of reach of small children. It is poisonous and is for external use only. A color of blue to blue-black indicates that starch is present in the material that the iodine touches. 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.
2. Seeds vary in size, shape, and color. Collect and display samples of seeds. Organize the seeds to indicate which are angiosperms (flowering plants) and which are gymnosperms (evergreens or conifers). Further classifications, such as how the seeds are transported, can also be used.

Get the Facts

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. Some plants, such as milkweed, have a parachutelike structure attached to each seed so that it will float easily and be carried by even the lightest wind. Fmd out more about seed dispersal, which is the way that seeds are carried by wind, water, and animals.
3. The most abundant plants are those that reproduce by seeds. Find out about the reproductive cycle of seed-bearing plants. How does seed production differ between the two classes of seed-bearing plants (angiosperms and gymnosperms)? Diagram the reproductive cycle of different seed-bearing plants.
4. 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, 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?
5. 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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