Control and Coordination · Lesson 8 of 11
Movement Due to Growth
“Roots, shoots and tendrils always seem to know which way to grow.”
- Explain how unequal growth produces directional plant movement. - Describe how tendrils coil around a support. - Distinguish positive and negative tropism. - Explain phototropism and geotropism in roots and shoots. - Describe hydrotropism and chemotropism with suitable examples. - Interpret an experiment showing a plant's response to directional light.
A pea tendril that touches a support slowly coils around it. A shoot placed near a window curves towards the light, while its root continues to grow into the soil. These movements look deliberate, but they result from controlled differences in growth on opposite sides of plant organs.
Tendril Movement
Tendrils are slender climbing structures sensitive to touch. When one side contacts a support, growth on that side slows relative to growth on the opposite side. The faster-growing outer side becomes longer, so the tendril bends around the support. Continued unequal growth produces coiling, allowing the plant to climb without investing in a thick supporting stem.
Tropic Movements
A directional growth response of a plant part to an external stimulus.
A tropic response is positive when growth is towards the stimulus and negative when growth is away from it. The same stimulus can produce opposite responses in different organs because those organs have different biological roles. Roots and shoots, for example, often respond oppositely to light and gravity.
Phototropism
Phototropism is directional growth in response to light. Shoots commonly bend towards a light source and therefore show positive phototropism. This places young leaves where they can receive more light. Roots commonly grow away from direct light and show negative phototropism. The bending occurs in growing regions; an older region that has completed elongation does not simply change direction when the light source moves.
Geotropism
Geotropism is directional growth in response to gravity. Roots usually grow in the direction of gravitational pull and are positively geotropic. Shoots grow away from the pull and are negatively geotropic. These opposite responses help establish the basic organisation of a plant: roots enter the soil for anchorage and absorption, while shoots grow into the air.
Hydrotropism
Hydrotropism is growth in response to water or a moisture gradient. Roots can grow towards a region with greater moisture, helping the plant reach water needed for transport, photosynthesis and other cellular activities. To demonstrate hydrotropism fairly, moisture should vary while light and gravity are kept as similar as possible on both sides.
Chemotropism
Chemotropism is directional growth in response to a chemical stimulus. During reproduction in flowering plants, chemical signals guide a pollen tube through floral tissue towards an ovule. The tube grows rather than travelling as a separate organism; therefore the response is a growth movement.
Activity
Purpose: To observe the effect of directional light on newly growing roots and shoots. Setup: Fill a conical flask with water, cover its neck with wire mesh, place freshly germinated bean seeds on the mesh, and keep the flask in a box open on the side facing a window.
- Leave the setup undisturbed until the shoots bend towards the opening and roots grow away from it.
- Turn the flask so the shoots point away from the light and the roots towards it.
- Observe the older portions and the direction taken by newly growing portions over the next few days.
- Keep water available and avoid disturbing the seedlings except when turning the flask.
Observation: Newly growing shoot regions curve towards the light, while new root growth tends away from it. Older regions change little because their elongation is largely complete. Conclusion: Directional light affects the direction of new growth rather than rotating already formed tissue.
Problem
A root grows towards a moist patch. Name and classify the response.
- 1.Water or moisture is the directional stimulus.
- 2.Growth is towards the stimulus.
- 3.The response is positive hydrotropism.
Problem
Classify the responses of a shoot and root to light.
- 1.The shoot grows towards light, so it shows positive phototropism.
- 2.The root grows away from light, so it shows negative phototropism.
- 3.The opposite responses suit the different functions of shoots and roots.
Problem
A root grows downward and towards a wet region. How could you investigate whether water rather than gravity caused the sideways bend?
- 1.Keep the direction of gravity the same for all roots.
- 2.Create a moisture gradient horizontally, with one side wetter than the other.
- 3.Use a control with equal moisture on both sides.
- 4.A consistent sideways bend towards moisture in the experimental setup, absent in the control, supports hydrotropism.
Positive does not mean beneficial and negative does not mean harmful. In tropism, positive means growth towards a stimulus and negative means growth away from it.
Quiz
What is a tropism?
Why does a tendril coil around a support?
Which response is shown by a shoot growing towards light?
Which is an example of chemotropism?
Why do old parts of a turned seedling change direction less than new parts?
Practice Problems
- Explain tendril coiling. Model answer: Contact slows growth on the touched side while the opposite side grows faster, so unequal growth curves the tendril around the support.
- Differentiate positive and negative tropism. Model answer: Positive tropism is growth towards a stimulus; negative tropism is growth away from it.
- Classify a root growing downward and away from light. Model answer: It is positively geotropic because it grows with gravity and negatively phototropic because it grows away from light.
- Give one example each of hydrotropism and chemotropism. Model answer: A root growing towards moist soil shows hydrotropism; a pollen tube growing towards an ovule shows chemotropism.
- Design the essential comparison in a hydrotropism investigation. Model answer: Compare roots exposed to a sideways moisture gradient with roots in equally moist conditions while keeping light and gravity alike; growth towards the wetter side supports hydrotropism.
Key Takeaways
• Growth movements result from different rates of cell growth on opposite sides of an organ. • Tendrils coil because the side away from contact grows faster. • A tropism is a directional growth response to a stimulus. • Positive tropism is towards a stimulus; negative tropism is away from it. • Shoots are commonly positively phototropic and negatively geotropic. • Roots commonly show positive geotropism and can show positive hydrotropism. • Pollen-tube growth towards an ovule is an example of chemotropism.