What is Phototropism and Geotropism?
Background Information:
· Photo- Light
· Geo- Earth
· Tropism- A response to a stimulus, such as gravity or light.
· Plants use a variety of hormones (chemicals) to control their growth and development. A group of hormones called auxins is found in plants. These auxins typically promote stem and root growth.
Phototropism
As a young plant develops most of the new plant material is grown at the tip of the stem. Auxins are responsible for the cell getting bigger and the cells dividing into new cells. Auxins are evenly distributed in the stem tip of a plant under ordinary circumstances. However, if the plant is illuminated from one side then the tip containing most of the auxins demonstrates an interesting response. When the stem is exposed to light indirectly (from a direction to the side of the plant) then auxin production is slowed on the side of the stem tip nearest the light, and the auxins are pumped away from the illuminated side to the darker side of the stem. As the cells grow in size, the cells on the dark side will grow longer. As the stem cells divide, the cells on the dark side will also divide more frequently. This causes the stem to push towards the light or appear to bend toward the light. The figure on the next page diagrams the changes in plant cells that occur as a result of auxin imbalances caused by light.
Geotropism
The hormone, auxin, also contributes to geotropism in a variety of ways. Positive geotropism is when a plant grows towards gravity, or downward. This is most commonly seen in root growth. Negative geotropism is when a plant grows away from gravity, or upwards. This is most commonly seen in the growth of the stem.
In figure 1 the plant is growing in the traditional manor. The stem is growing upwards (negative geotropism) and the roots are growing downward (positive geotropism). In figure two the plant was tipped over onto its side and starting growing again. In this illustration it shows how the plant will appear to bend so that it now grows upwards instead of the stem growing in a straight line. The roots will continue to grow downward as well and also appear to bend. This is because gravity does affect the way the plants grow.
Discussion
Complete sentences required
1. What is the difference between positive geotropism and negative geotropism?
2. What are auxins?
3. What would be a good definition for phototropism?
4. What happens to a plant if it gets blown over in a storm, but does not die?
5. What causes a plant to bend towards the light?
6. What is it that helps cells to grow and divide that was discuss in the reading?
7. A sunflower will always face the sun as the sun appears to move across the sky. Describe a process that might contribute to this happening that takes place within the sunflower.