Positive and Negative Geotropism
We all know that plants can’t walk from place to place, but they do move their stems, leaves, and roots in response to their environment. It just happens too slowly for us to see. These plant movements are called tropisms. In a positive tropism, the plant moves towards the stimulus. One familiar example is positive phototropism, in which a plant moves towards sunlight. Time lapse photography allows us to appreciate the tropisms of plants. This video shows the positive phototropism of sprouting snow peas. Note how the pea sprouts move back and forth with each day.
Plants can also move away from stimulus; this is called a negative tropism. Some plants move away from bright light, this tropism is called negative phototropism
Other physical phenomena can stimulate plant movement. Geotropism is the movement of a plant in response to gravity. This is especially evident in the early sprouting of bean seeds.
In this investigation, you will investigate how gravity affects the sprouting of radish seeds. Try to determine if gravity causes a positive or negative response in the plant.
Problem: How does gravity affect the growth of sprouting seeds?
- Aluminum foil
- 3 Paper towels
- Spray bottle
- Radish seeds
- Labels or masking tape
- Permanent marker
- Box big enough to fit a cookie sheet on the bottom
- Cookie sheet or other large pan
- Masking tape
- Dark room
- Using the ruler and scissors, measure and cut three 12”x12” squares of aluminum foil.
- Take a paper towel and fold it in half twice.
- Set the folded paper tower in the middle of one piece of aluminum foil.
- Using the spray bottle, spray the paper towel until all of it is moist, but not dripping wet.
- Make a line of about twenty radish seeds through the center of the paper towel.
- Carefully fold each side of the paper towel over the seeds in the center, covering them. What do you think the paper towel is for?
- Next, fold the foil around the paper towel. Why is the foil needed along with the paper towel?
- Place a label on the foil packet.
- Write “1: Bottom” on the label.
- Rest one end of the cookie sheet on top of the cardboard box so that the cookie sheet creates a ramp.
- Place the labeled foil envelope on top of the cookie sheet.
- Make two more foil packets filled with the same amount of seeds.
- Label the second packet “2: Side”.
- Tape this packet on the side of the box.
- Label the third packet “3: Turn”.
- Tape this packet to another side of the box.
- Every other day, rotate the “3: Turn” foil packet 90 degrees on the side of box.
- Make sure the paper towels remain moist inside the foil packets. Use a dropper to add a bit more water if necessary.
- After five days, carefully observe the sprouting of the seeds, paying careful attention to the direction of stem and root growth in relationship to where the seeds were positioned in the box.
- If little growth has occurred or you want to continue your experiment, rewrap your seeds and put them in the same position.