 Understanding A Seed's Response to Gravity
It is that time of the year when migrating birds return, breezes turn warm, temperatures rise, and plants come to life. Spring is in the air and everywhere! It is the time to get out in the garden to plant our seeds and to hope for a plentiful harvest! However, people don't plant all seeds. Sometimes, birds and other animals drop seeds on the ground and other times the wind carries and drops seeds. These seeds are dispersed randomly landing upside down and downside up. So, how do the seeds know to grow roots that go down, and stems that grow up? |
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Concept:
Although our lives depend on plants for virtually everything that keeps us alive (oxygen, food, fibers, lumber, fuel, etc), their lives remain a secret to most of us. The reason is simple - plants live on a different time-scale from ours. Although not usually obvious in the relatively hyperactive activities of humans, plants are in constant motion as they develop, search for light and nutrients, avoid predators, exploit neighbors, and reproduce. For example, seed growth response to gravity moves plant growth toward and away from gravity.
This response is called gravitropism. It is regulated by chemicals, called hormones, that allow them to sense gravity regardless of the orientation or location of the plant. One type of hormone accumulates in plant cells on the side of the cell that faces gravity, allowing the plant parts to sense the directional pull of gravity. Roots always grow in the direction of, or with, gravity, while stems always grow away from, or against, gravity. This is referred to as positive gravitropism (towards gravity) and negative gravitropism (against gravity).
Roots exhibit positive gravitropism, while shoots exhibit negative gravitropism. Gardeners and farmers don't have to worry about the orientation of their seeds when they plant. No matter which way the seed lands in the soil, the roots will always grow down and the stem will always grow up, producing a healthy, hearty plant. This also applies to seeds that are naturally dispersed by wind, rain, or animals. No matter how the seeds land, the roots grow towards gravity and the stems grow away from gravity. If the seeds had to land in a precise orientation, it would be very difficult for new plants to grow.
It is important to note that the stems and roots of bulbs will also follow the rules of gravitropism. However, the orientation of a bulb in the ground can be vital to its survival because bulbs expend large amounts of energy to redirect their roots and stems. Sometimes, they use all their energy in redirecting the roots and stems and they do not survive. For this reason, directions for planting bulbs will indicate how to place the bulb in the ground.
Objective:
To create a seed clock to observe how gravity affects the growth of roots and stems.
 Materials:
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Four Seeds
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Clear Tape
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Wax pencil or fine-tip permanent marker
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Colored Pencils or markers
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Supply of loose cotton or cotton balls
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Spray Bottle
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Empty plastic petri dishes (small clear jars [like baby food containers with the labels removed] will also work) - one per student
Procedure:
PART I
 STEP 1: Collect one empty plastic petri dish, four sunflower seeds, and enough loose cotton or cotton balls to almost fill the petri dish. Pack cotton loosely into the petri-dish - Use the spray bottle to moisten the cotton until it is thoroughly dampened.
STEP 2: Place the four sunflower seeds on top of the cotton according to the diagram to the right. Make sure that the narrow end of the seeds points to the center of the petri dish approximately 1/2 " to 3/4" away from the edge of the dish. Tape the lid to the petri dish with two pieces of clear tape. Make a legible mark on the lid directly above one seed (see the picture to the right) using the wax pencil or permanent marker.
 STEP 3: Place the petri dish vertically on its edge with the mark at the top (so that the four seeds are in the 12:00, 3:00, 6:00, and 9:00 o'clock positions). Place it in a warm, bright place, preferably on a windowsill. The seeds will/may need 7 days to germinate and sprout. After 3 to 4 days, open the lids to make sure that the cotton is still moist. If necessary, re-moisten the cotton. If the cotton dries out completely, the seeds may die.
3-7 DAYS LATER: PART II
 Pick up the Seed Clock. Without opening it, observe how the sprouted roots of each
seed grow towards the ground.
Questions:
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What effect did gravity have on the direction of stem growth?
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What effect did gravity have on the direction of root growth?
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What do you think will happen to seeds planted in space? Why?
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