Activity 8 -- Launching the Seed
Introduction: From the activity "Getting Acquainted with a Seed," students found that on brassica seeds the tip of the embryonic root points down toward the micropyle, near a darkened circular area on the seed coat associated with the attachment of the seed to the maternal ovary by way of the funiculus. Follow the steps below to begin to explore the interaction of germination and orientation.
Question: For a brassica or other seed, which way is down? Or up?
Sample Hypothesis: The dark spot near the micropyle is down - brown is down.
Sample Null Hypothesis: Brown is up.
- Germinate seeds oriented in different directions. Observe and record initial and later direction of root emergence from seed. Alter orientation of seedlings, predict, observe and record responses.
- Observations are recorded on the "Launching the Seed Student Sketch Sheet," (right).
- Design Tip: You may want to begin this activity on a Monday, so that your germination will occur during the week when students can make observations. Alternatively, students may set it up and observe it at home.
Students will be able to construct their seed germinator and place their seeds within one 50 minute class period. This activity is designed for the student to observe their germinators over a two to three day period. Data are collected as drawings and a written discussion. Time for observations should be five to ten minutes on each of three consecutive days.
In participating in this activity students will:
- determine whether their understanding of seed anatomy from Activity 7 is correct and their hypothesis regarding seed orientation is verified, namely that the micropyle is the down orientation in brassica seeds; and
- understand that plant roots reorient in the direction of their growth to conform with the direction of the gravitational force.
- two soda bottle caps (film can lids will also work)
- four brassica or other similar-sized seeds (lettuce, turnip, or alfalfa)
- forceps for handling seed
1. Cut two layers of paper towel into circles that will fit into the bottom of a soda bottle cap.
2. Place the towel in the cap and moisten it with water. Pour off any excess water.
3. Orient the four seeds in north-south-eastwest positions on the moist towel surface, making sure that the brown micropyle area (spot) is pointing toward the center of the cap (Figure 1).
4. Make a mark on the bottle cap that indicates the direction of north, or up.
5. Cover the open cap with plastic wrap and secure the wrap with an elastic band. Trim off excess wrap with scissors.
6. Position the "bottle cap seed germinator" so the seeds are in a vertical orientation by standing it in a second bottle cap (Figure 2, page 76).
7. Make a drawing of the seeds in the bottle cap germinator, including the orientation of the brown micropylar area toward the center of the circle. Use the "Circle 1" on the Launching the Seed Student Sketch Sheet.
8. If students carry their bottle cap seed germinator home with them, they can observe it every few hours. Be sure to keep it in the vertical position with the proper north-south-east-west orientation.
9. When the roots begin to emerge, record the direction of the emerging root from each seed with a second drawing on the Sketch Sheet (Circle 2). You may wish to use a hand lens.
10. Continue to observe the germinating seeds being sure that the paper towel is kept moist. Note the appearance of the fine, fuzzy root hairs and the extension of the hypocotyl.
11. After 24 to 48 hours, make a third drawing depicting the direction of the roots and hypocotyl and illustrating the root hairs, cotyledons and seed coat (Circle 3).
12. Then reorient your bottle cap seed germinator in some way that will give you more information on seed orientation. Make another drawing that predicts how the seedlings will look after 12 or 24 hours in this new orientation (Circle 4). Observe them from time to time - do you notice anything happening?
14. Twelve or 24 hours after reorientation, draw the seedlings on the Sketch Sheet (Circle 5). Has anything changed? Compare the outcome of the orientation with what you predicted in your last drawing.
15. Have students write about what they have learned on the Sketch Sheet.
Concluding Activities and Questions
In this activity students will have observed the effects of the Earth's gravity in reorienting the direction of roots in the direction of the gravitational force. They will also have observed that shoots orient against the direction of the gravitational force, bending and growing upward. This should raise a discussion around the questions:
- In microgravity which direction will roots and shoots grow?
- Is there a guiding force for root growth in the absence of gravity? Could you generate a hypothesis and experiment to carry this question farther? Hint: What would happen if you ran this experiment on a centrifuge?
- What might be the possible influence(s) of light in this experiment?
Orientation and Guidance
Rooted in the ground, plants, unlike most animals, are unable to relocate themselves in their environment from their fixed position. They are, however, capable of movement and of orienting themselves so as to optimize their capacity to access the environmental components essential to their life. Plants orient themselves to light and gravity through tropism. Plants use guidance systems which sense and respond to gravity (gravitropism) ensuring that roots anchor plants and access water and that shoots emerge into the light. Plants then use light to activate energy-capturing photosynthesis. Light also guides the development of leaf expansion (photomorphogenesis), stem bending and elongation (phototropism), and pigment production (chlorophyll and anthocyanin).
- How does a seedling orient itself?
- How does a plant grow up?
- Why does the shoot grow up and the root down?
"...[in plants] we know that there is always movement in progress, and its amplitude, or direction, or both, have only to be modified for the good of the plant in relation with internal or external stimuli." - Charles Darwin, "The Power of Movement in Plants," 1880
Plants like many other organisms, including humans, use both gravity and light to provide them with orientation and guidance in their environments. Rooted in the ground, plants are unable to move, but they are able to grow and bend toward or away from various stimuli in what is known as tropism. Roots exhibit positive gravitropism and shoots negative gravitropism. Whereas shoots are also positively phototropic (bend or grow toward a source of light), plants and humans rely heavily on gravity for orientation. We are especially aware of the verticality of our orientation as we lean forward
or lift a heavy object.
The perception of the horizontal is mostly afforded us by light through sight. Our awareness of a horizon provides us with an essential sense of where we are in space. Just shut your eyes for a few moments and you come to realize how important 'horizontality' is to our orientation. The effects of the horizon on your sensory stability can be best experienced in movies of stunt aircraft flights or on roller coasters where the horizon is constantly changing. In the absence of sight, gravity and other sensory systems (sound, smell, touch) provide compensation in our orientation.
In the absence of light, plants rely on gravity for orientation. This leads to an interesting question for plant biologists: in the environment of microgravity and in the absence of light, are there other ways that a plant can become oriented? This and other questions will be under investigation in the CUE.
In the microgravity of the orbiting Space Shuttle, humans and plants do not perceive the force of Earth's gravity. Astronauts and cosmonauts orient visually on the many structures within the orbiter. In space, plants also must orient using light. Just how plants orient themselves to the light is still under intensive study by plant physiologists. Perhaps the best place to do such studies on phototropism is in the Space Shuttle and on the International Space Station, where the confounding influence of gravity as a guiding force is minimized.
Seedlings of various plants, including AstroPlants, are excellent model organisms with which to investigate the influence of light on tropic bending. As presented in the section on light in "Understanding the Environment" (page 13), visible light includes a spectral range of wavelengths from 400 nanometers to about 750 nanometers, ranging from ultraviolet through blue, green, yellow, orange to red. Plants use different colored molecules to capture various wavelengths and use that
energy for different functions, including photosynthesis, phototropism, and photomorphogenesis. Complex biochemical processes known as signal transduction pathways translate the energy transferred from light into various physical and physiological manifestations of growth and development.
The gravitropic response in plants is conditioned by a signal transduction pathway that is thought to be largely controlled by a group of growth promoting hormones called auxins, and by inhibitors of auxins. When a plant is turned on its side, auxins stimulate elongation in the cells on the lower side of the shoot, causing it to bend up (against gravity), but in the roots, cells on the upper side elongate causing the roots to grow down (with gravity).
In phototropism, an unequal distribution of auxins and inhibitors on the side of the plant away from the light stimulates cell enlargement on the "dark" side of the plant and results in the plant bending toward the light. Much has yet to be learned about how the force of gravity actually acts in the mechanism of tropism in plants.
In Activity 8 on brassica seed germination (page 67), students will have observed the strong combined influences of gravity and light on seedling orientation and growth. However, the relative roles of each of these environmental factors on the direction of seed growth were not clearly defined. To approach this challenge, it will be necessary to separate experimentally the influences of each factor, light and gravity, on tropic responses. The following activities on gravitropism and phototropism will help students to understand how plants orient themselves and also how complex the interpretation of experiments into tropism can become.