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Zigzag Seeds

Objectives:  Students will observe the effect of gravity on seeds.

Grade Level: 2-4

Groupings: Small groups

Materials:  (per group) At least 4-6 flat seeds (pumpkin, cucumber, squash); paper towels; spray water bottle; two panes of Plexiglas measuring about 5"x6"; two large rubber bands; rectangular pan filled with water; grease pencil.

Time Allotment:  15 minutes set-up, several weeks for results


Directions:

1. Seeds fall from the fruits of plants and can land in just about any orientation. Ask the students what they think happens to a seed that lands upside down. Does it germinate? If so, do the roots come out above the ground and the shoots grow down? Or does the plant know to turn itself around? How? Make a list of the students' ideas and theories on this subject.

2. Have the students work in small groups to set up an experiment to see whether seeds 'know' up from down. Give each group several seeds, two pieces of Plexiglas, and some paper towels. Have them fold the paper towels to fit on top of one piece of Plexiglas and use a spray bottle to thoroughly wet the paper towel.

3. Have them observe their seeds and make a detailed drawing of one. Ask them to guess what is the top of the seed and what is the bottom, and record their guesses. Ask them to arrange their seeds on the top of the paper towel in any orientation, with growing space between the seeds. They may choose to place all of their seeds in the same orientation or vary them. Have them make a diagram of their seed arrangement noting the orientation of each.

4. Have them place the second piece of Plexiglas on top of the seed layer, as if they were making a sandwich and the Plexiglas was the bread. Have at least two students work together to place two rubber bands around the whole assembly, holding the seeds tightly in place. Have them label the top, bottom, left and right side of the Plexiglas with a grease pencil.

children observing the experiment

5. Have them fill a basin with one inch of water and place the bottom of the Plexigals seed sandwich into the basin. Remind them to be sure that the paper towel is at or below water level so it can act as a wick and keep the seeds moist.

6. Ask the students to predict and record when their seeds will germinate and the orientation of the emerging roots and shoots on each.

7. Have the students observe their seeds daily and record the germination date, noting root and shoot orientations. (In all cases the shoot will grow up and the root will grow down.) Have the groups compare results.

8. Ask them to let the shoot and root grow until they are about 1/2 inch long. Have them turn the plexiglas sandwich so a different side is facing up. What will happen to the newly established seedling? Have them record their predictions. (The plant will turn until the shoot is facing up and the root down.)

9. Ask them to observe and record the seedlings' growth in response to this change in orientation. What would happen if they turned the Plexiglas seed sandwich again? And again? Have them make predictions and test their guesses. How do they explain their results?


Extension:

Ask the groups to draw a simple zigzag pattern and try to get a seedling to follow it. Ask them to predict the number and orientation of the turns it will take to copy the pattern. Then have them perform the experiment. Did the results match their predictions?


WHAT'S UP?

How does a little seed buried underground know which way it should grow? Believe it or not, seedlings are not completely 'in the dark.' They can sense up from down! This principle is called geotropism. Plants, like the rest of us, are under the influence of gravity. Gravity is a strong, constant force that exerts an influence on all life on Earth. That's why it takes energy to lift things off the ground, from something as small as a pin to something as large as an airplane. Gravity is also the reason why all things, including us, fall down. Certain growth controlling substances within plants are sensitive to gravity. Plants respond to this pull of gravity by orienting shoot growth up and root growth down. When a plant's vertical orientation changes, it causes an imbalance in these substances on the upper and lower sides of the shoot and root. The plant grows more on one side of the shoot or root (the lower side of the shoot and the upper side of the root) until the plant curves back to its normal vertical orientation and the growth controlling substances are in equilibrium once again.


* This material has been used with permission from Shelburne Farms, Copyright © 1995.

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