Food Storage In Seeds: Dissecting a Dicot
Bean plants are called dicotyledons because they have two food storage structures, cotyledons. The developing bean plant, the embryo, is found between the two cotyledons. The embryo has tiny leaves at one end. It also has a shoot called the epicotyl. The two halves are held together by an external seed coat. In this activity you will examine the structure of a lima bean.
Safety glasses; Lima bean (soaked in warm water overnight); Iodine; Medicine dropper; Paper towel; Magnifying glass; Black crayon Activity
- Dry the bean with a paper towel. Peel off the outer skin.
- Using your thumbs, open the bean so it falls into two halves.
- With the help of the magnifying lens, locate the parts of the bean discussed above. You will be asked to sketch these parts in the first follow-up question.
- Put on your safety glasses. To determine the part of the cotyledon that contains food, place a drop of iodine at several locations on the inside of the bean. Iodine turns black in the presence of starch (stored food).
- Draw a sketch of the inside of the dicot you examined. Label the cotyledon, embryo, and epicotyl.
- Using the crayon, color your sketch in areas where starch was located.
- The drawing should look something like the image shown here.
- The cotyledon is the food storage area, so those areas should be shaded.
Conduct an experiment in which you compare the growth of bean plants to the amount of cotyledon. You will need a control group of seeds that have normal cotyledons, a group with only one-half cotyledons, and a group with only one-quarter cotyledons.
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