Plant tropisms are growth responses that help plants survive in changing environments. A shoot may bend toward light, a root may grow downward, and a vine may coil around a support. These movements are not caused by muscles, but by unequal growth on different sides of plant organs.
Tropisms matter because they help plants capture sunlight, anchor themselves, absorb water, and compete for space.
The main control system behind many tropisms involves plant hormones, especially auxin. Auxin can collect more on one side of a stem or root, causing cells there to elongate differently. In shoots, more auxin usually increases cell elongation, so the shoot bends away from the auxin-rich side and toward light.
In roots, high auxin often slows elongation, which helps roots curve downward with gravity.
Key Facts
- Tropism = directional plant growth response to an external stimulus.
- Phototropism is growth in response to light, usually positive in shoots.
- Gravitropism is growth in response to gravity, with roots usually positive and shoots usually negative.
- Thigmotropism is growth in response to touch, such as tendrils coiling around a support.
- In shoots, higher auxin concentration usually increases cell elongation.
- Growth rate difference = elongation rate on one side minus elongation rate on the opposite side.
Vocabulary
- Tropism
- A tropism is a directional growth response of a plant toward or away from a stimulus.
- Phototropism
- Phototropism is plant growth in response to the direction of light.
- Gravitropism
- Gravitropism is plant growth in response to the direction of gravity.
- Thigmotropism
- Thigmotropism is plant growth in response to touch or physical contact.
- Auxin
- Auxin is a plant hormone that helps control cell elongation and bending during growth responses.
Common Mistakes to Avoid
- Thinking tropisms are fast movements, which is wrong because most tropisms happen through slow growth over hours or days.
- Saying plants move toward light because they can crawl, which is wrong because phototropism occurs when one side of the shoot grows faster than the other.
- Assuming auxin has the same effect everywhere, which is wrong because auxin often promotes elongation in shoots but can inhibit elongation in roots at higher concentrations.
- Confusing positive and negative tropism, which is wrong because positive means growth toward the stimulus and negative means growth away from it.
Practice Questions
- 1 A seedling shoot is lit from the left. The right side of the shoot elongates 6 mm while the left side elongates 3 mm over the same time. Which way will the shoot bend, and what is the growth difference between the two sides?
- 2 A root is placed horizontally. After 8 hours, the upper side has elongated 12 mm and the lower side has elongated 7 mm. Which way will the root curve, and how much greater was elongation on the upper side?
- 3 A climbing pea plant tendril touches a thin stick on one side. Explain how thigmotropism can help the plant reach better light without using muscles.