The cotyledon is the largest part of the inside of the bean. It stores a lot of the food for the growing bean. Like a chick embryo has a yolk and a baby has an umbilical cord, a bean seed has a cotyledon to act as a source of food.
At the top of the cotyledon is the epicotyl. This is the beginning of the bean’s shoot and will eventually form the leaves. Look closely. Can you see what will form the bean’s future leaves? Just under the epicotyl is the hypocotyl. This is the beginning of the bean’s stem. The radicle is under the hypocotyl. This is the beginning of the bean’s roots. A whole baby plant is nestled inside that tiny, growing bean seed.
A bean needs water to grow. At first, it absorbs this through a small hole called the micropyle that is found in the hilum, the scar on the side of the bean that shows where it was attached to its parent plant. When the bean germinates, or begins to grow, the baby bean plant starts to take shape inside the bean seed. It uses the starch that’s in the cotyledon as food.
What would happen if you cut away part of the cotyledon? To extend your experiment, get 5 new bean seeds and soak them overnight. Cut off the lower half of one of the cotyledons of one bean, the lower half of both cotyledons on another bean, ¾ of the cotyledon on another bean, and all of the cotyledon on the last bean, leaving only the embryo. Leave one unaltered bean plant as the control.
Label your pots like this:
- 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.
Plant these seeds in soil in five pots and see how each one grows. Mist the beans daily. Remember what the cotyledon gives to a plant. Which one will grow the best?
